Perspecve
Separate Molecular Determinants in Amyloidogenic and Antimicrobial Peptides
Michael Landreh 1 , Jan Johansson 2, 3 and Hans Jörnvall 1 1 - Department of Medical Biochemistry and Biophysics, Karolinska Institutet, S-171 77 Stockholm, Sweden 2 - KI Alzheimer's Disease Research Center, Department of Neurobiology, Care Sciences and Society, Karolinska Institutet, S-141 86 Stockholm, Sweden 3 - Department of Anatomy, Physiology and Biochemistry, Swedish University of Agricultural Sciences, S-751 23 Uppsala, Sweden
Correspondence to Hans Jörnvall: [email protected] http://dx.doi.org/10.1016/j.jmb.2014.03.005 Edited by R. Wetzel
Abstract Several amyloid-forming and antimicrobial peptides (AMYs and AMPs) have the ability to bind to and damage cell membranes. In addition, some AMYs possess antimicrobial activity and some AMPs form amyloid-like fibrils, relating the two peptide types and their properties. However, a comparison of their sequence characteristics reveals important differences. The high β-strand and aggregation propensities typical of AMYs are largely absent in α-helix-forming AMPs, which are instead marked by a strong amphipathic moment not generally found in AMYs. Although a few peptides, for example, islet amyloid polypeptide and dermaseptin S9, combine some determinants of both groups, the structural distinctions suggest that antimicrobial activity and amyloid formation are separate features not generally associated. © 2014 Elsevier Ltd. All rights reserved.
Introduction Amyloidogenic polypeptides (AMYs) are associated with depository disorders in which they accumulate in highly ordered structures composed of stacked β-strands or β-hairpins. Such amyloid fibrils have been observed in several diseases, including Alzheimer's disease, prion diseases, and familial Danish and British dementias [1]. Additionally, several disease-related proteins, like α-synuclein, display a high propensity to form amyloid-like fibrils under physiological conditions [2]. However, a growing body of evidence shows that the cytotoxic effects underlying amyloid diseases are caused by a range of different forms of misfolded proteins with the ability to damage membranes [3]. Depending on the protein, the toxic form can be unfolded, small oligomers [4], compact β-strand structures [5] or mature fibrils [6,7]. In some cases, even multiple forms of the same protein, ranging from dimers to entire fibrils, can cause membrane damage [8,9]. Detailed NMR studies have revealed that some AMYs induce membrane damage in a two-step fashion, with oligomers causing leakage and fibrils leading to fragmentation [10,11].
Toxicity cannot be linked to a common conformation for all AMYs, and membrane disruption can be caused by several types of interactions. The only requirement appears to be an ability to alter the energetic landscape of the membrane enough to stabilize leak formation that normally occurs during transient fluctuations [12]. It is therefore possible that oligomer or fibril formation can in some cases even have beneficial effects by preventing detrimental membrane interactions exerted by other conformational states. Antimicrobial peptides (AMPs) are polypeptides of 12–50 residues that also have the ability to bind to and damage membranes of many pathogens (and at higher concentrations also of the host), thus functioning as a first barrier in innate immunity. Due to their amphipathic nature, binding of AMPs produces a detergent-like strain on the membrane, and leakage may be induced by toroidal pore formation, membrane thinning, barrel-stave insertion, or detergent-like interactions that solubilize large membrane pieces [13]. The toroidal pore hypothesis is supported by the observation that the minimum inhibitory concentrations at which most AMPs effectively kill bacteria are usually too low for effective carpet
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J. Mol. Biol. (2014) 426, 2159–2166
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formation. Instead, local accumulation of membranebound peptides may cumulate in energetically favorable pore formation [14]. Examples of all alternative mechanisms, including membrane thinning in the case of LL-37 [15], detergent-like interactions in the case of pardaxin [16], and barrel-stave insertion in the case of alamethicin [17], have been reported. Some AMYs not only do bind to and damage membranes but also are potent antimicrobial agents, which appears to link antimicrobial activity and amyloid formation. The Alzheimer's disease-associated amyloid β 1–42 peptide (Aβ42) efficiently kills fungi and bacteria despite its negative charge unusual for AMPs [18]. Serum amyloid A (SAA), which forms aggregates in systemic amyloidosis, human islet amyloid polypeptide (hIAPP), which deposits in the pancreatic β-cells of diabetes mellitus type 2 patients, and the N-terminal segment of the human prion protein have been shown to have antimicrobial action by inducing membrane leakage [19–21]. Some peptides that were identified to be pro-helical AMPs can participate in the formation of fibril-like aggregates in vitro, such as dermaseptin S9 and temporin B and L [22,23]. The non-helical AMPs protegrin-1 and H6 defensin have even been found to form functional amyloid-like structures in vivo, although significantly less efficiently than for most AMYs [24,25]. While these examples may appear to link amyloid and antimicrobial characteristics and raise the question whether the two properties may be associated, only a small subset of membranebinding peptides forms amyloid deposits in vivo. To clarify similarities and differences, we here compare the structural preferences of both peptide groups and relate them to their proposed mechanisms of membrane damage.
Membrane Interactions of Pro-Helical AMPs Based on their secondary structure, AMPs can be subdivided into α-helical, β-hairpin, cyclic β-turns, and random-coil peptides, all of which contain about 50% hydrophobic residues that are carefully counterbalanced by positive charges [26]. While the β-hairpin AMPs are often stabilized by disulfide bridges and thus maintain a stable amphipathic structure, α-helical AMPs can fold with the help of membrane interactions. These peptides often have a random-coil conformation in solution and are termed pro-helical. Accordingly, membrane adsorption drives the formation of an amphipathic α-helix via attraction between the positively charged peptide and the negatively charged lipid headgroups followed by insertion of the hydrophobic residues into the membrane [26]. This insertion of the apolar and neighboring polar residues increases membrane strain and distorts the permeability barrier
imposed by the hydrophobic membrane core [27]. For pro-helical AMPs, computational analysis of the energetics of membrane binding suggests that pore formation is the preferred mode of action [14].
Membrane Interactions of AMYs A number of AMYs also adapt a helical structure during the early stages of membrane interactions. A major difference, however, is that the helical conformations of AMYs are transient and quickly proceed to form fibrillar aggregates [3], while AMPs are able to maintain their helical structure. Amphipathic helices that resemble those found in AMPs have been identified in hIAPP [28], α-synuclein [29], SAA [30], and the arterial amyloid peptide medin [31]. It is likely that this conformation is important for the antimicrobial activity of hIAPP [21]. Membrane interactions of helical Aβ42 and the pulmonary surfactant protein C (which forms amyloid deposits in interstitial lung disease and has been implicated in the pulmonary defense against pathogens [32,33]) seem to be driven mainly by the burial of the hydrophobic residues [34,35]. Similar to the membrane-assisted helix formation in AMPs, the helical conformations in AMYs have to be induced by interactions with a binding partner. However, unlike in AMPs, the transitions from random coil to α-helix of AMYs can also occur during selfassociation. This leads to the formation of helical intermediates that shift to a β-strand-rich conformation during aggregation. Such a mechanism has been demonstrated in detailed structural investigations of the N-terminal segment of huntingtin, which has a random-coil structure, but becomes α-helical during oligomerization in solution and on membranes. The helical segments do not encompass the aggregation-prone polyglutamine stretch itself, but their self-association nucleates fibril formation [36–38].
AMPs do not Share Sequence Determinants of Amyloid Formation Computational studies have identified sequence determinants of both antimicrobial activity and amyloid formation. Common to pro-helical AMPs is the ability to adapt an amphipathic conformation that facilitates the interactions of their charged and hydrophobic residues with the membrane, which is crucial for their antimicrobial action [39]. Table 1 and Supplementary Table 1 summarize the overlap of conformational preferences, amphipathic moments, and experimentally determined structures of the AMPs deposited in the PDB database, as well as their predicted aggregation propensities. As expected, the observed locations of the helical segments generally
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Table 1. All helical AMPs filed in the AMP database [40] with a high-resolution structure deposited in the PDB database Name and PDB ID Alyteserin-1c, 2L5R Aurein 1.2, 1VM5 BMAP-27, 2KET Bombinin H4, 2AP8 CAP-18, 1LYP Carnobacteriocin B2, 1CW5 Cupiennin 1A, 2K38 Dermadistinctin K, 2JX6 Dermaseptin S4 1–13, 2DD6 Dermaseptin S9 [41]a Dermaseptin SDI1, 2K9B Enterocin 7B, 2M60 Fowlicidin 1, 2AMN Fowlicidin 2, 2GDL Fowlicidin 3, 2HFR Gaegurin 4, 2G9L IsCT1, 1T51 Lactococcin G-a, 2JPJ Latarcin 1, 2PCO Latarcin 2, 2G9P LL-37, 2K6O Magainin 2, 2LSA Mastoparan X, 2CZP and 2D7N Mellittin, 1BH1 Moricin, 1KV4 MS Moricin, 2JR8 Ovispirin, 1HU5 Oxyopinin 4a, 2L3I Pardaxin, 2KNS Phylloseptin-H1, 2JQ0 Piscidin 1, 2JOS Plantarcin A, 1YTR Pleurocidin, 1Z64 Ranatuerin-2Csa, 2K10 Spinigerin, 1ZRV Stomoxyn, 1ZRX Temporin 1A, 2MAA Temporin B [42]a Temporin L [42]a
Sequence and structural preferences GLKEIFKAGLGSLVKGIAAHVAS GLFDIIKKIAESF GRFKRFRKKFKKLFKKLSPVIPLLHLG IIGPVLGLVGSALGGLLKKIG GLRKRLRKFRNKIKEKLKKIGQKIQGFVPKLAPRTDY YGNGVSCSKTKCSVNWGQAFQERYTAGINSFVSGVASGAGSIGRRP GFGALFKFLAKKVAKTVAKQAAKQGAKYVVNKQME GLWSKIKAAGKEAAKAAGKAALNAVSEAV ALWMTLLKKVLKA GLRSKIWLWVLLMIWQESNKFKKM GLWSKIKAAGKEAAKAAAKAAGKAALNAVSEAV MGAIAKLVAKFGWPFIKKFYKQIMQFIGQGWTIDQIEKWLKRH RVKRVWPLVIRTVIAGYNLYRAIKKK LVQRGRFGRFLRKIRRFRPKVTITIQGSARF RVKRFWPLVPVAINTVAAGINLYKAIRRK GILDTLKQFAKGVGKDLVKGAAQGVLSTVSCKLAKTC ILGKIWEGIKSLF GTWDDIGQGIGRVAYWVGKALGNLSDVNQASRINRKKKH SMWSGMWRRKLKKLRNALKKKLKGE GLFGKLIKKFGRKAISYAVKKARGKH LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES GIGKFLHSAKKFGKAFVGEIMNS INWKGIAAMAKKLL GIGAVLKVLTTGLPALISWIKRKRQQ AKIPIKAIKTVGKAVGKGLRAINIASTANDVFNFLKPKKRKA GKIPVKAIKQAGKVIGKGLRAINIAGTTHDVVSFFRPKKKKH KNLRRIIRKIIHIIKKYG GIRCPKSWKCKAFKQRVLKRLLAMLRQHAF GFFALIPKIISSPLFKTLLSAVGSALSSSGGQE FLSLIPHAINAVSAIAKHN FFHHIFRGIVHVGKTIHRLVTG KSSAYSLQMGATAIKQVKKLFKKWGW GWGSFFKKAAHVGKHVGKAALTHYL GILSSFKGVAKGVAKDLAGKLLETLKCKITGC HVDKKVADKVLLLKQLRIMRLLTRL RGFRKHFNKLVKKVKHTISETAHVAKDTAVIAGSGAAVVAAT FLPLIGRVLSGIL LLPIVGNLLKSLL FVQWFSKFLGRIL
Underlined letters indicate the location of α-helices in experimentally determined peptide structures. Those in italics indicate discordant segments with high aggregation propensity. See Supplementary Table 1 for details. a For AMPs temporin B and L as well as dermaseptin 9, for which amyloid-like behavior has been reported, literature references for high-resolution structures are given.
coincide well with computational predictions [39]. It should be noted that the high content of hydrophobic amino acids and the requirement for membrane interactions to reach a stable conformation suggest that a subset of pro-helical AMPs contains regions that are predicted to be aggregation prone [43]. However, many antimicrobial peptides are water soluble even at high concentrations and can be produced recombinantly in large quantities. Significantly, common ground among all AMYs is the ability to self-assemble through aggregation “hot spots”, that is, amino acid sequences that form β-strand-rich fibrils. These regions can be predicted based on their amino acid composition and sequence [44,45]. The natively amyloidogenic peptides SP-C, Aβ42, hIAPP, and α-synuclein, (as
opposed to those that become amyloidogenic through mutations) contain aggregation “hot spots” in segments that are helical in the membrane-bound state but are predicted to strongly favor β-strand conformation (Fig. 1). The locations of experimentally observed helical segments with high β-strand and aggregation propensities are summarized in Table 2 and Supplementary Table 2 for membrane-binding AMYs with high-resolution structures available. Such discordant helices with a strong tendency to self-assemble into β-strands can be found in many amyloid-forming proteins and may act as starting points for aggregation [46]. Similarly, the N-terminal amphipathic helix of SAA [30,47] and the helical, membrane-binding C-terminus of the medin peptide [31] also contain aggregation-prone
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Fig. 1. Locations of amphipathic and discordant helices in membrane-bound AMPs and AMYs. Amphipathic segments are shown in green, and discordant helices are indicated in red. The membrane is indicated by a gray area. LL-37 is composed of a single, amphipathic helix, while the helical segments of Aβ40 and SP-C are discordant without well-defined amphipathicity. IAPP and α-synuclein contain spatially separated amphipathic and discordant helices.
segments with high β-strand propensity, although no detailed structures are available in these cases. These observations suggest that membrane-binding AMYs contain regions prone to adopting an energetically preferred β-strand structure, which distinguish them from the majority of pro-helical, amphipathic AMPs.
Refolding, Oligomerization, and Membrane Activity How can the different structural preferences of AMPs and AMYs be integrated with their ability to damage membranes? A possible answer is that AMYs alter their membrane-binding capabilities through refolding and self-assembly, while prohelical AMPs exert their membrane activity in their native conformation [21]. The top and bottom rows of Fig. 2 show how the similar starting structures of membrane-bound AMPs and AMYs may evolve to induce membrane damage in different manners depending on their respective structural preferences and oligomerization status. Clues to the role of refolding and oligomer formation in membrane binding of AMYs come from molecular dynamics studies of Aβ42. Even though Aβ42 can bind laterally to membranes, it does not form a well-defined amphipathic helix, and helical Aβ42 monomers are unlikely to cause membrane damage [48]. β-Hairpin Aβ42 oligomers, however, display a strong tendency to insert into membranes [49]. This is in good agreement with the suggestion that such oligomers may form pores and subsequently fibers, both of which can cause membrane damage associated with amyloid toxicity [50]. The strong propensity to self-associate makes it particularly easy for amyloid-forming peptides to accumulate membrane-active species locally, which can increase the efficiency of membrane disruption even at low peptide concentrations [48,51]. The observation of membrane channels composed of amyloid peptides [50], crystallographic identification of toxic β-strand-rich oligomers with
pore-like structures [5], engineered toxic β-hairpin variants of Aβ42 [52], and mitigation of Aβ42 toxicity by stabilization of the α-helical state [53] all suggest that refolding and self-association mechanisms are important for membrane disruption by AMYs. It is therefore reasonable to conclude that these unique structural features are responsible for their detrimental effects on membranes.
The Combination of AMP- and AMY-like Properties Distinguishes a Special Class of Peptides While antimicrobial activity and amyloid formation have separate sequence determinants (Tables 1 and 2 and Supplementary Tables 1 and 2), some peptides actually combine segments with these properties into potent AMPs that exhibit robust fibril formation. The best-studied examples are the hIAPP and the frog peptide dermaseptin S9, but several more may exist and comprise a special class of AMPs/AMYs. The middle panel in Fig. 2 shows how these peptides, exemplified by hIAPP, can interconvert between AMP- and AMY-like behaviors to induce membrane damage in different manners. The ability of hIAPP to induce membrane damage in an AMP-like fashion is well documented and can be attributed to the highly amphipathic helix at the N-terminal side [21]. This segment is located at the surface when binding to detergent micelles or phospholipid bilayers [54,55] and mediates the insertion into phospholipid monolayers [56]. However, besides its amphipathic helix, hIAPP also harbors an aggregation-prone sequence with high β-strand propensity in its C-terminal half [57]. Parallel orientation of helical hIAPP molecules triggers the assembly into fibrils that can cause additional membrane damage [28]. The reported helical or β-barrel-like pore structures may be short-lived intermediates of hIAPP aggregation in a membrane environment and illustrate how helical
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Table 2. Helical amyloid peptides with an experimentally determined high-resolution structure in a membrane environment deposited in the PDB database Name and PDB ID
Sequence and structural preferences
α-Synuclein, 2KKW
MDVFMKGLSKAKEGVVAAAEKTKQGVAEAAGKTKEGVLYVG SKTKEGVVHGVATVAEKTKEQVTNVGGAVVTGVTAVAQKTVE GAGSIAAATGFVKKDQLGKNEEGAPQEGILEDMPVDPDNEA YEMPSEEGYQDYEPEA DAEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGLMVGGVV MATLEKLMKAFESLKSFQQQQQQQQQQQQQQQQ KCNTATCATQRLANFLVHSSNNFGAILSSTNVGSNTY MVKSHIGSWILVLFVAMWSDVGLCKKRPKP FGIPCCPVHLKRLLIVVVVVVLIVVVIVGALLMGL
Amyloid β40, 1BA4 Huntingtin, 2LD0 IAPP, 2KB8 PrP 1–30, 2SKH SP-C, 1SPF
Underlined letters indicate the location of α-helices in experimentally determined peptide structures. Those in italics indicate discordant segments with high aggregation propensity. See Supplementary Table 2 for details.
and fibrillar species are both related to hIAPP toxicity [50,58]. In the light of these findings, it appears possible that the highly amphipathic and amyloidogenic regions, which do not overlap significantly in hIAPP (Fig. 1), each contributes to
toxicity. Separate or cooperative effects can in this case dominate membrane disruption, and their individual contributions may depend on the external conditions. The balance between both features is demonstrated by the observation that hIAPP can
Fig. 2. Possible stages of membrane disruption by AMPs and AMYs. Both helical AMPs and amyloids can bind laterally to membranes. Bound AMPs may cause damage by exerting strain on the membrane or by local accumulation and pore formation. AMYs require refolding and oligomerization to cause membrane damage and proceed to form fibrillar aggregates. Similarities to both mechanisms have been reported for hIAPP, which can damage membranes in its amphipathic helical state, but also assembles into β-strand-rich aggregates. In addition, hIAPP may be able to convert from an AMP-like state (top) to an AMY-like state (bottom) at any stage.
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remain α-helical and monomeric in micelles, can bind to zwitterionic membranes in random-coil conformation, and can aggregate rapidly [59]. This may explain why no clear structural preference is associated with hIAPP-mediated membrane disruption [60] (for an in-depth review of hIAPP aggregation and membrane binding, see Ref. [61]). The frog peptide dermaseptin S9 is an unusual AMP in the respect that its middle segment is largely hydrophobic with some β-strand propensity. However, it can adopt an α-helical conformation, possibly with the aid of its cationic N- and C-termini [41], when bound to anionic membranes. In agreement with the ambiguous sequence determinants, its aggregation kinetics are slower than those of “classical” AMYs such as Aβ42 [23]. Consequently, membrane disruption by β-strand-rich oligomers formed selectively on prokaryotic membranes has been suggested [23]. Our comparison shows that some other disease-associated proteins may also harbor both amphipathic and aggregation-prone segments. Membrane-bound α-synuclein contains both an amphipathic helix and an aggregation-prone helix with high β-sheet propensity [29,62]. A possible antimicrobial activity has not yet been investigated, but α-synuclein toxicity is known to be associated with refolding and oligomerization [2]. Huntingtin-1 also contains an N-terminal amphipathic helix. While it is spatially separated from the aggregation-prone polyglutamine segment, its molecular interactions are crucial in the control of aggregation [36,37,63]. While the aggregating and amphipathic segments are separated in hIAPP, α-synuclein, and huntingtin-1, computational analysis of the antimicrobial protein SAA suggests that its amphipathic N-terminal helix itself may trigger aggregation by refolding into a β-hairpin [47]. In all these cases, the presence of pro-helical segments not only does convey the ability to bind to membranes but also exacerbates the effects of their most aggregation-prone regions. As shown for huntingtin-1 (see above), unfolded AMYs may oligomerize and act as folding partners for each other's pro-helical segments [36]. The aggregation-prone sequences are aligned in the resulting helical homomolecular complexes and can nucleate amyloid formation [37]. A similar mechanism has been suggested for IAPP [58], and short-lived helical intermediates have been observed during Aβ aggregation despite its low helical propensity in solution [64].
membrane binding ability that governs their target selectivity and is safely contained by unambiguous structural preferences. In contrast, these features are distorted in the uncontrolled membrane interactions of AMYs and give rise to toxicity. In the same way, any modulation of peptide charge, structure, length, hydrophobicity, or self-association propensity can dramatically alter cytotoxicity of AMPs [25]. However, since most naturally occurring helical AMPs are not amyloidogenic, the targeted induction of AMP expression to battle antimicrobial infections does not convey a risk for protein aggregation disease. Some amyloids, however, may include antimicrobial properties as part of their biological function, as suggested for hIAPP and dermaseptin S9 [21,23]. In these cases, it is tempting to speculate that amyloid formation and cell membrane damage can represent aberrant activities of a special class of AMPs. This implies that the separation between biological function and disease depends on chaperoning mechanisms to keep the amyloidogenic peptides in a nontoxic state or careful dosing to keep the concentration below the toxicity threshold. Different chaperoning mechanisms for hIAPP have been described [65], and functional amyloids are part of the host defense system [25]. hIAPP is likely antimicrobial in the helical state [21], but the antimicrobial activity of Aβ42 from brain homogenates correlates with Alzheimer's disease pathology from toxic Aβ oligomers [18]. In the light of this, it appears possible that the antimicrobial activity of a few AMYs is not their primary biological function but may in some cases be the manifestation of an exceptionally broad toxicity. However, select properties of AMYs could be harnessed to generate functional amyloid-like antimicrobial agents that would, for example, selectively assemble on the surface of specific pathogens. In conclusion, we find that amyloid formation and antimicrobial activity generally have distinct determinants but may co-exist in special cases with mixed properties, often from separate regions, and sometimes with functional consequences.
AMYs are Not Generally AMPs by Design
Appendix A. Supplementary data
The impact of peptide folding on toxicity depends on the ability of a given structure to induce membrane strain. Helical AMPs possess a carefully balanced
Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.jmb.2014.03. 005.
Acknowledgements This work as supported by the Swedish Research Council.
Molecular determinants in antimicrobial and amyloid-forming peptides
Received 3 December 2013; Received in revised form 17 February 2014; Accepted 6 March 2014 Available online 17 March 2014 Keywords: antimicrobial peptides; amyloid formation; peptide folding; membrane binding; discordant helices Abbreviations used: AMP, antimicrobial peptide; AMY, amyloidogenic peptide; SAA, serum amyloid A; hIAPP, human islet amyloid polypeptide; Aβ42, amyloid β 1–42 peptide; MD, molecular dynamics.
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formation of medin via an alpha-helical intermediate. J Mol Biol 2007;374:186–94. Glasser SW, Senft AP, Whitsett JA, Maxfield MD, Ross GF, Richardson TR, et al. Macrophage dysfunction and susceptibility to pulmonary Pseudomonas aeruginosa infection in surfactant protein C-deficient mice. J Immunol 2008;181:621–8. Willander H, Askarieh G, Landreh M, Westermark P, Nordling K, Keranen H, et al. High-resolution structure of a BRICHOS domain and its implications for anti-amyloid chaperone activity on lung surfactant protein C. Proc Natl Acad Sci U S A 2012;109:2325–9. Johansson J, Szyperski T, Wuthrich K. Pulmonary surfactant-associated polypeptide SP-C in lipid micelles: CD studies of intact SP-C and NMR secondary structure determination of depalmitoyl-SP-C(1–17). FEBS Lett 1995;362:261–5. Shao H, Jao S, Ma K, Zagorski MG. Solution structures of micelle-bound amyloid beta-(1–40) and beta-(1–42) peptides of Alzheimer's disease. J Mol Biol 1999;285:755–73. Thakur AK, Jayaraman M, Mishra R, Thakur M, Chellgren VM, Byeon IJ, et al. Polyglutamine disruption of the huntingtin exon 1 N terminus triggers a complex aggregation mechanism. Nat Struct Mol Biol 2009;16:380–9. Jayaraman M, Kodali R, Sahoo B, Thakur AK, Mayasundari A, Mishra R, et al. Slow amyloid nucleation via alpha-helixrich oligomeric intermediates in short polyglutamine-containing huntingtin fragments. J Mol Biol 2012;415:881–99. Michalek M, Salnikov ES, Werten S, Bechinger B. Membrane interactions of the amphipathic amino terminus of huntingtin. Biochemistry 2013;52:847–58. Tossi A, Sandri L, Giangaspero A. Amphipathic, alpha-helical antimicrobial peptides. Biopolymers 2000;55:4–30. Wang G, Li X, Wang Z. APD2: the updated antimicrobial peptide database and its application in peptide design. Nucleic Acids Res 2009;37:D933–7. Lequin O, Ladram A, Chabbert L, Bruston F, Convert O, Vanhoye D, et al. Dermaseptin S9, an alpha-helical antimicrobial peptide with a hydrophobic core and cationic termini. Biochemistry 2006;45:468–80. Bhunia A, Saravanan R, Mohanram H, Mangoni ML, Bhattacharjya S. NMR structures and interactions of temporin-1Tl and temporin-1Tb with lipopolysaccharide micelles: mechanistic insights into outer membrane permeabilization and synergistic activity. J Biol Chem 2011;286:24394–406. Mahalka AK, Kinnunen PK. Binding of amphipathic alphahelical antimicrobial peptides to lipid membranes: lessons from temporins B and L. Biochim Biophys Acta 2009;1788:1600–9. Maurer-Stroh S, Debulpaep M, Kuemmerer N, de la Paz ML, Martins IC, Reumers J, et al. Exploring the sequence determinants of amyloid structure using position-specific scoring matrices. Nat Methods 2010;7:855–7. Fernandez-Escamilla AM, Rousseau F, Schymkowitz J, Serrano L. Prediction of sequence-dependent and mutational effects on the aggregation of peptides and proteins. Nat Biotechnol 2004;22:1302–6. Kallberg Y, Gustafsson M, Persson B, Thyberg J, Johansson J. Prediction of amyloid fibril-forming proteins. J Biol Chem 2001;276:12945–50. Nordling E, Abraham-Nordling M. Colonic amyloidosis, computational analysis of the major amyloidogenic species, serum amyloid A. Comput Biol Chem 2012;39:29–34. Poojari C, Kukol A, Strodel B. How the amyloid-beta peptide and membranes affect each other: an extensive simulation study. Biochim Biophys Acta 2013;1828:327–39.
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Separate Molecular Determinants in Amyloidogenic and Antimicrobial Peptides
Michael Landreh 1 , Jan Johansson 2, 3 and Hans Jörnvall 1 1 - Department of Medical Biochemistry and Biophysics, Karolinska Institutet, S-171 77 Stockholm, Sweden 2 - KI Alzheimer's Disease Research Center, Department of Neurobiology, Care Sciences and Society, Karolinska Institutet, S-141 86 Stockholm, Sweden 3 - Department of Anatomy, Physiology and Biochemistry, Swedish University of Agricultural Sciences, S-751 23 Uppsala, Sweden
Correspondence to Hans Jörnvall: [email protected] http://dx.doi.org/10.1016/j.jmb.2014.03.005 Edited by R. Wetzel
Abstract Several amyloid-forming and antimicrobial peptides (AMYs and AMPs) have the ability to bind to and damage cell membranes. In addition, some AMYs possess antimicrobial activity and some AMPs form amyloid-like fibrils, relating the two peptide types and their properties. However, a comparison of their sequence characteristics reveals important differences. The high β-strand and aggregation propensities typical of AMYs are largely absent in α-helix-forming AMPs, which are instead marked by a strong amphipathic moment not generally found in AMYs. Although a few peptides, for example, islet amyloid polypeptide and dermaseptin S9, combine some determinants of both groups, the structural distinctions suggest that antimicrobial activity and amyloid formation are separate features not generally associated. © 2014 Elsevier Ltd. All rights reserved.
Introduction Amyloidogenic polypeptides (AMYs) are associated with depository disorders in which they accumulate in highly ordered structures composed of stacked β-strands or β-hairpins. Such amyloid fibrils have been observed in several diseases, including Alzheimer's disease, prion diseases, and familial Danish and British dementias [1]. Additionally, several disease-related proteins, like α-synuclein, display a high propensity to form amyloid-like fibrils under physiological conditions [2]. However, a growing body of evidence shows that the cytotoxic effects underlying amyloid diseases are caused by a range of different forms of misfolded proteins with the ability to damage membranes [3]. Depending on the protein, the toxic form can be unfolded, small oligomers [4], compact β-strand structures [5] or mature fibrils [6,7]. In some cases, even multiple forms of the same protein, ranging from dimers to entire fibrils, can cause membrane damage [8,9]. Detailed NMR studies have revealed that some AMYs induce membrane damage in a two-step fashion, with oligomers causing leakage and fibrils leading to fragmentation [10,11].
Toxicity cannot be linked to a common conformation for all AMYs, and membrane disruption can be caused by several types of interactions. The only requirement appears to be an ability to alter the energetic landscape of the membrane enough to stabilize leak formation that normally occurs during transient fluctuations [12]. It is therefore possible that oligomer or fibril formation can in some cases even have beneficial effects by preventing detrimental membrane interactions exerted by other conformational states. Antimicrobial peptides (AMPs) are polypeptides of 12–50 residues that also have the ability to bind to and damage membranes of many pathogens (and at higher concentrations also of the host), thus functioning as a first barrier in innate immunity. Due to their amphipathic nature, binding of AMPs produces a detergent-like strain on the membrane, and leakage may be induced by toroidal pore formation, membrane thinning, barrel-stave insertion, or detergent-like interactions that solubilize large membrane pieces [13]. The toroidal pore hypothesis is supported by the observation that the minimum inhibitory concentrations at which most AMPs effectively kill bacteria are usually too low for effective carpet
0022-2836/$ - see front matter © 2014 Elsevier Ltd. All rights reserved.
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formation. Instead, local accumulation of membranebound peptides may cumulate in energetically favorable pore formation [14]. Examples of all alternative mechanisms, including membrane thinning in the case of LL-37 [15], detergent-like interactions in the case of pardaxin [16], and barrel-stave insertion in the case of alamethicin [17], have been reported. Some AMYs not only do bind to and damage membranes but also are potent antimicrobial agents, which appears to link antimicrobial activity and amyloid formation. The Alzheimer's disease-associated amyloid β 1–42 peptide (Aβ42) efficiently kills fungi and bacteria despite its negative charge unusual for AMPs [18]. Serum amyloid A (SAA), which forms aggregates in systemic amyloidosis, human islet amyloid polypeptide (hIAPP), which deposits in the pancreatic β-cells of diabetes mellitus type 2 patients, and the N-terminal segment of the human prion protein have been shown to have antimicrobial action by inducing membrane leakage [19–21]. Some peptides that were identified to be pro-helical AMPs can participate in the formation of fibril-like aggregates in vitro, such as dermaseptin S9 and temporin B and L [22,23]. The non-helical AMPs protegrin-1 and H6 defensin have even been found to form functional amyloid-like structures in vivo, although significantly less efficiently than for most AMYs [24,25]. While these examples may appear to link amyloid and antimicrobial characteristics and raise the question whether the two properties may be associated, only a small subset of membranebinding peptides forms amyloid deposits in vivo. To clarify similarities and differences, we here compare the structural preferences of both peptide groups and relate them to their proposed mechanisms of membrane damage.
Membrane Interactions of Pro-Helical AMPs Based on their secondary structure, AMPs can be subdivided into α-helical, β-hairpin, cyclic β-turns, and random-coil peptides, all of which contain about 50% hydrophobic residues that are carefully counterbalanced by positive charges [26]. While the β-hairpin AMPs are often stabilized by disulfide bridges and thus maintain a stable amphipathic structure, α-helical AMPs can fold with the help of membrane interactions. These peptides often have a random-coil conformation in solution and are termed pro-helical. Accordingly, membrane adsorption drives the formation of an amphipathic α-helix via attraction between the positively charged peptide and the negatively charged lipid headgroups followed by insertion of the hydrophobic residues into the membrane [26]. This insertion of the apolar and neighboring polar residues increases membrane strain and distorts the permeability barrier
imposed by the hydrophobic membrane core [27]. For pro-helical AMPs, computational analysis of the energetics of membrane binding suggests that pore formation is the preferred mode of action [14].
Membrane Interactions of AMYs A number of AMYs also adapt a helical structure during the early stages of membrane interactions. A major difference, however, is that the helical conformations of AMYs are transient and quickly proceed to form fibrillar aggregates [3], while AMPs are able to maintain their helical structure. Amphipathic helices that resemble those found in AMPs have been identified in hIAPP [28], α-synuclein [29], SAA [30], and the arterial amyloid peptide medin [31]. It is likely that this conformation is important for the antimicrobial activity of hIAPP [21]. Membrane interactions of helical Aβ42 and the pulmonary surfactant protein C (which forms amyloid deposits in interstitial lung disease and has been implicated in the pulmonary defense against pathogens [32,33]) seem to be driven mainly by the burial of the hydrophobic residues [34,35]. Similar to the membrane-assisted helix formation in AMPs, the helical conformations in AMYs have to be induced by interactions with a binding partner. However, unlike in AMPs, the transitions from random coil to α-helix of AMYs can also occur during selfassociation. This leads to the formation of helical intermediates that shift to a β-strand-rich conformation during aggregation. Such a mechanism has been demonstrated in detailed structural investigations of the N-terminal segment of huntingtin, which has a random-coil structure, but becomes α-helical during oligomerization in solution and on membranes. The helical segments do not encompass the aggregation-prone polyglutamine stretch itself, but their self-association nucleates fibril formation [36–38].
AMPs do not Share Sequence Determinants of Amyloid Formation Computational studies have identified sequence determinants of both antimicrobial activity and amyloid formation. Common to pro-helical AMPs is the ability to adapt an amphipathic conformation that facilitates the interactions of their charged and hydrophobic residues with the membrane, which is crucial for their antimicrobial action [39]. Table 1 and Supplementary Table 1 summarize the overlap of conformational preferences, amphipathic moments, and experimentally determined structures of the AMPs deposited in the PDB database, as well as their predicted aggregation propensities. As expected, the observed locations of the helical segments generally
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Table 1. All helical AMPs filed in the AMP database [40] with a high-resolution structure deposited in the PDB database Name and PDB ID Alyteserin-1c, 2L5R Aurein 1.2, 1VM5 BMAP-27, 2KET Bombinin H4, 2AP8 CAP-18, 1LYP Carnobacteriocin B2, 1CW5 Cupiennin 1A, 2K38 Dermadistinctin K, 2JX6 Dermaseptin S4 1–13, 2DD6 Dermaseptin S9 [41]a Dermaseptin SDI1, 2K9B Enterocin 7B, 2M60 Fowlicidin 1, 2AMN Fowlicidin 2, 2GDL Fowlicidin 3, 2HFR Gaegurin 4, 2G9L IsCT1, 1T51 Lactococcin G-a, 2JPJ Latarcin 1, 2PCO Latarcin 2, 2G9P LL-37, 2K6O Magainin 2, 2LSA Mastoparan X, 2CZP and 2D7N Mellittin, 1BH1 Moricin, 1KV4 MS Moricin, 2JR8 Ovispirin, 1HU5 Oxyopinin 4a, 2L3I Pardaxin, 2KNS Phylloseptin-H1, 2JQ0 Piscidin 1, 2JOS Plantarcin A, 1YTR Pleurocidin, 1Z64 Ranatuerin-2Csa, 2K10 Spinigerin, 1ZRV Stomoxyn, 1ZRX Temporin 1A, 2MAA Temporin B [42]a Temporin L [42]a
Sequence and structural preferences GLKEIFKAGLGSLVKGIAAHVAS GLFDIIKKIAESF GRFKRFRKKFKKLFKKLSPVIPLLHLG IIGPVLGLVGSALGGLLKKIG GLRKRLRKFRNKIKEKLKKIGQKIQGFVPKLAPRTDY YGNGVSCSKTKCSVNWGQAFQERYTAGINSFVSGVASGAGSIGRRP GFGALFKFLAKKVAKTVAKQAAKQGAKYVVNKQME GLWSKIKAAGKEAAKAAGKAALNAVSEAV ALWMTLLKKVLKA GLRSKIWLWVLLMIWQESNKFKKM GLWSKIKAAGKEAAKAAAKAAGKAALNAVSEAV MGAIAKLVAKFGWPFIKKFYKQIMQFIGQGWTIDQIEKWLKRH RVKRVWPLVIRTVIAGYNLYRAIKKK LVQRGRFGRFLRKIRRFRPKVTITIQGSARF RVKRFWPLVPVAINTVAAGINLYKAIRRK GILDTLKQFAKGVGKDLVKGAAQGVLSTVSCKLAKTC ILGKIWEGIKSLF GTWDDIGQGIGRVAYWVGKALGNLSDVNQASRINRKKKH SMWSGMWRRKLKKLRNALKKKLKGE GLFGKLIKKFGRKAISYAVKKARGKH LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES GIGKFLHSAKKFGKAFVGEIMNS INWKGIAAMAKKLL GIGAVLKVLTTGLPALISWIKRKRQQ AKIPIKAIKTVGKAVGKGLRAINIASTANDVFNFLKPKKRKA GKIPVKAIKQAGKVIGKGLRAINIAGTTHDVVSFFRPKKKKH KNLRRIIRKIIHIIKKYG GIRCPKSWKCKAFKQRVLKRLLAMLRQHAF GFFALIPKIISSPLFKTLLSAVGSALSSSGGQE FLSLIPHAINAVSAIAKHN FFHHIFRGIVHVGKTIHRLVTG KSSAYSLQMGATAIKQVKKLFKKWGW GWGSFFKKAAHVGKHVGKAALTHYL GILSSFKGVAKGVAKDLAGKLLETLKCKITGC HVDKKVADKVLLLKQLRIMRLLTRL RGFRKHFNKLVKKVKHTISETAHVAKDTAVIAGSGAAVVAAT FLPLIGRVLSGIL LLPIVGNLLKSLL FVQWFSKFLGRIL
Underlined letters indicate the location of α-helices in experimentally determined peptide structures. Those in italics indicate discordant segments with high aggregation propensity. See Supplementary Table 1 for details. a For AMPs temporin B and L as well as dermaseptin 9, for which amyloid-like behavior has been reported, literature references for high-resolution structures are given.
coincide well with computational predictions [39]. It should be noted that the high content of hydrophobic amino acids and the requirement for membrane interactions to reach a stable conformation suggest that a subset of pro-helical AMPs contains regions that are predicted to be aggregation prone [43]. However, many antimicrobial peptides are water soluble even at high concentrations and can be produced recombinantly in large quantities. Significantly, common ground among all AMYs is the ability to self-assemble through aggregation “hot spots”, that is, amino acid sequences that form β-strand-rich fibrils. These regions can be predicted based on their amino acid composition and sequence [44,45]. The natively amyloidogenic peptides SP-C, Aβ42, hIAPP, and α-synuclein, (as
opposed to those that become amyloidogenic through mutations) contain aggregation “hot spots” in segments that are helical in the membrane-bound state but are predicted to strongly favor β-strand conformation (Fig. 1). The locations of experimentally observed helical segments with high β-strand and aggregation propensities are summarized in Table 2 and Supplementary Table 2 for membrane-binding AMYs with high-resolution structures available. Such discordant helices with a strong tendency to self-assemble into β-strands can be found in many amyloid-forming proteins and may act as starting points for aggregation [46]. Similarly, the N-terminal amphipathic helix of SAA [30,47] and the helical, membrane-binding C-terminus of the medin peptide [31] also contain aggregation-prone
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Fig. 1. Locations of amphipathic and discordant helices in membrane-bound AMPs and AMYs. Amphipathic segments are shown in green, and discordant helices are indicated in red. The membrane is indicated by a gray area. LL-37 is composed of a single, amphipathic helix, while the helical segments of Aβ40 and SP-C are discordant without well-defined amphipathicity. IAPP and α-synuclein contain spatially separated amphipathic and discordant helices.
segments with high β-strand propensity, although no detailed structures are available in these cases. These observations suggest that membrane-binding AMYs contain regions prone to adopting an energetically preferred β-strand structure, which distinguish them from the majority of pro-helical, amphipathic AMPs.
Refolding, Oligomerization, and Membrane Activity How can the different structural preferences of AMPs and AMYs be integrated with their ability to damage membranes? A possible answer is that AMYs alter their membrane-binding capabilities through refolding and self-assembly, while prohelical AMPs exert their membrane activity in their native conformation [21]. The top and bottom rows of Fig. 2 show how the similar starting structures of membrane-bound AMPs and AMYs may evolve to induce membrane damage in different manners depending on their respective structural preferences and oligomerization status. Clues to the role of refolding and oligomer formation in membrane binding of AMYs come from molecular dynamics studies of Aβ42. Even though Aβ42 can bind laterally to membranes, it does not form a well-defined amphipathic helix, and helical Aβ42 monomers are unlikely to cause membrane damage [48]. β-Hairpin Aβ42 oligomers, however, display a strong tendency to insert into membranes [49]. This is in good agreement with the suggestion that such oligomers may form pores and subsequently fibers, both of which can cause membrane damage associated with amyloid toxicity [50]. The strong propensity to self-associate makes it particularly easy for amyloid-forming peptides to accumulate membrane-active species locally, which can increase the efficiency of membrane disruption even at low peptide concentrations [48,51]. The observation of membrane channels composed of amyloid peptides [50], crystallographic identification of toxic β-strand-rich oligomers with
pore-like structures [5], engineered toxic β-hairpin variants of Aβ42 [52], and mitigation of Aβ42 toxicity by stabilization of the α-helical state [53] all suggest that refolding and self-association mechanisms are important for membrane disruption by AMYs. It is therefore reasonable to conclude that these unique structural features are responsible for their detrimental effects on membranes.
The Combination of AMP- and AMY-like Properties Distinguishes a Special Class of Peptides While antimicrobial activity and amyloid formation have separate sequence determinants (Tables 1 and 2 and Supplementary Tables 1 and 2), some peptides actually combine segments with these properties into potent AMPs that exhibit robust fibril formation. The best-studied examples are the hIAPP and the frog peptide dermaseptin S9, but several more may exist and comprise a special class of AMPs/AMYs. The middle panel in Fig. 2 shows how these peptides, exemplified by hIAPP, can interconvert between AMP- and AMY-like behaviors to induce membrane damage in different manners. The ability of hIAPP to induce membrane damage in an AMP-like fashion is well documented and can be attributed to the highly amphipathic helix at the N-terminal side [21]. This segment is located at the surface when binding to detergent micelles or phospholipid bilayers [54,55] and mediates the insertion into phospholipid monolayers [56]. However, besides its amphipathic helix, hIAPP also harbors an aggregation-prone sequence with high β-strand propensity in its C-terminal half [57]. Parallel orientation of helical hIAPP molecules triggers the assembly into fibrils that can cause additional membrane damage [28]. The reported helical or β-barrel-like pore structures may be short-lived intermediates of hIAPP aggregation in a membrane environment and illustrate how helical
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Table 2. Helical amyloid peptides with an experimentally determined high-resolution structure in a membrane environment deposited in the PDB database Name and PDB ID
Sequence and structural preferences
α-Synuclein, 2KKW
MDVFMKGLSKAKEGVVAAAEKTKQGVAEAAGKTKEGVLYVG SKTKEGVVHGVATVAEKTKEQVTNVGGAVVTGVTAVAQKTVE GAGSIAAATGFVKKDQLGKNEEGAPQEGILEDMPVDPDNEA YEMPSEEGYQDYEPEA DAEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGLMVGGVV MATLEKLMKAFESLKSFQQQQQQQQQQQQQQQQ KCNTATCATQRLANFLVHSSNNFGAILSSTNVGSNTY MVKSHIGSWILVLFVAMWSDVGLCKKRPKP FGIPCCPVHLKRLLIVVVVVVLIVVVIVGALLMGL
Amyloid β40, 1BA4 Huntingtin, 2LD0 IAPP, 2KB8 PrP 1–30, 2SKH SP-C, 1SPF
Underlined letters indicate the location of α-helices in experimentally determined peptide structures. Those in italics indicate discordant segments with high aggregation propensity. See Supplementary Table 2 for details.
and fibrillar species are both related to hIAPP toxicity [50,58]. In the light of these findings, it appears possible that the highly amphipathic and amyloidogenic regions, which do not overlap significantly in hIAPP (Fig. 1), each contributes to
toxicity. Separate or cooperative effects can in this case dominate membrane disruption, and their individual contributions may depend on the external conditions. The balance between both features is demonstrated by the observation that hIAPP can
Fig. 2. Possible stages of membrane disruption by AMPs and AMYs. Both helical AMPs and amyloids can bind laterally to membranes. Bound AMPs may cause damage by exerting strain on the membrane or by local accumulation and pore formation. AMYs require refolding and oligomerization to cause membrane damage and proceed to form fibrillar aggregates. Similarities to both mechanisms have been reported for hIAPP, which can damage membranes in its amphipathic helical state, but also assembles into β-strand-rich aggregates. In addition, hIAPP may be able to convert from an AMP-like state (top) to an AMY-like state (bottom) at any stage.
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remain α-helical and monomeric in micelles, can bind to zwitterionic membranes in random-coil conformation, and can aggregate rapidly [59]. This may explain why no clear structural preference is associated with hIAPP-mediated membrane disruption [60] (for an in-depth review of hIAPP aggregation and membrane binding, see Ref. [61]). The frog peptide dermaseptin S9 is an unusual AMP in the respect that its middle segment is largely hydrophobic with some β-strand propensity. However, it can adopt an α-helical conformation, possibly with the aid of its cationic N- and C-termini [41], when bound to anionic membranes. In agreement with the ambiguous sequence determinants, its aggregation kinetics are slower than those of “classical” AMYs such as Aβ42 [23]. Consequently, membrane disruption by β-strand-rich oligomers formed selectively on prokaryotic membranes has been suggested [23]. Our comparison shows that some other disease-associated proteins may also harbor both amphipathic and aggregation-prone segments. Membrane-bound α-synuclein contains both an amphipathic helix and an aggregation-prone helix with high β-sheet propensity [29,62]. A possible antimicrobial activity has not yet been investigated, but α-synuclein toxicity is known to be associated with refolding and oligomerization [2]. Huntingtin-1 also contains an N-terminal amphipathic helix. While it is spatially separated from the aggregation-prone polyglutamine segment, its molecular interactions are crucial in the control of aggregation [36,37,63]. While the aggregating and amphipathic segments are separated in hIAPP, α-synuclein, and huntingtin-1, computational analysis of the antimicrobial protein SAA suggests that its amphipathic N-terminal helix itself may trigger aggregation by refolding into a β-hairpin [47]. In all these cases, the presence of pro-helical segments not only does convey the ability to bind to membranes but also exacerbates the effects of their most aggregation-prone regions. As shown for huntingtin-1 (see above), unfolded AMYs may oligomerize and act as folding partners for each other's pro-helical segments [36]. The aggregation-prone sequences are aligned in the resulting helical homomolecular complexes and can nucleate amyloid formation [37]. A similar mechanism has been suggested for IAPP [58], and short-lived helical intermediates have been observed during Aβ aggregation despite its low helical propensity in solution [64].
membrane binding ability that governs their target selectivity and is safely contained by unambiguous structural preferences. In contrast, these features are distorted in the uncontrolled membrane interactions of AMYs and give rise to toxicity. In the same way, any modulation of peptide charge, structure, length, hydrophobicity, or self-association propensity can dramatically alter cytotoxicity of AMPs [25]. However, since most naturally occurring helical AMPs are not amyloidogenic, the targeted induction of AMP expression to battle antimicrobial infections does not convey a risk for protein aggregation disease. Some amyloids, however, may include antimicrobial properties as part of their biological function, as suggested for hIAPP and dermaseptin S9 [21,23]. In these cases, it is tempting to speculate that amyloid formation and cell membrane damage can represent aberrant activities of a special class of AMPs. This implies that the separation between biological function and disease depends on chaperoning mechanisms to keep the amyloidogenic peptides in a nontoxic state or careful dosing to keep the concentration below the toxicity threshold. Different chaperoning mechanisms for hIAPP have been described [65], and functional amyloids are part of the host defense system [25]. hIAPP is likely antimicrobial in the helical state [21], but the antimicrobial activity of Aβ42 from brain homogenates correlates with Alzheimer's disease pathology from toxic Aβ oligomers [18]. In the light of this, it appears possible that the antimicrobial activity of a few AMYs is not their primary biological function but may in some cases be the manifestation of an exceptionally broad toxicity. However, select properties of AMYs could be harnessed to generate functional amyloid-like antimicrobial agents that would, for example, selectively assemble on the surface of specific pathogens. In conclusion, we find that amyloid formation and antimicrobial activity generally have distinct determinants but may co-exist in special cases with mixed properties, often from separate regions, and sometimes with functional consequences.
AMYs are Not Generally AMPs by Design
Appendix A. Supplementary data
The impact of peptide folding on toxicity depends on the ability of a given structure to induce membrane strain. Helical AMPs possess a carefully balanced
Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.jmb.2014.03. 005.
Acknowledgements This work as supported by the Swedish Research Council.
Molecular determinants in antimicrobial and amyloid-forming peptides
Received 3 December 2013; Received in revised form 17 February 2014; Accepted 6 March 2014 Available online 17 March 2014 Keywords: antimicrobial peptides; amyloid formation; peptide folding; membrane binding; discordant helices Abbreviations used: AMP, antimicrobial peptide; AMY, amyloidogenic peptide; SAA, serum amyloid A; hIAPP, human islet amyloid polypeptide; Aβ42, amyloid β 1–42 peptide; MD, molecular dynamics.
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