Proc. Nati. Acad. Sci. USA Vol. 88, pp. 5041-5045, June 1991

Biochemistry

Similarity of the three-dimensional structures of actin and the ATPase fragment of a 70-kDa heat shock cognate protein KEVIN M. FLAHERTY*, DAVID B. MCKAY*, WOLFGANG KABSCHt, AND KENNETH C. HOLMESt *Beckman Laboratories for Structural Biology, Department of Cell Biology, Stanford University School of Medicine, Stanford, CA 94305-5400; and tMax-Planck-Institut fur Medizinische Forschung, Abteilung Biophysik, Jahnstrasse 29, 6900 Heidelberg, Federal Republic of Germany Communicated by Lubert Stryer, March 8, 1991 (received for review January 14, 1991)

Actin is an ubiquitous cytoplasmic protein involved in the formation of filaments that are major components of the cytoskeleton (for reviews, see refs. 1-3). Actin filaments interact cyclically with myosin filaments to produce a mutual sliding, which is the basis of muscular contraction and many other aspects of cell motility, including cytokinesis. The actin protomer binds one ATP or ADP molecule and has a single high-affinity and several low-affinity sites for divalent cations (Mg2+ or Ca2+). The molecule has been highly conserved throughout evolution. The protein consists of a single polypeptide chain of typically 375 amino acid residues. Acetylation of the N-terminal aspartate and methylation ofHis-73 are posttranslational modifications. Actin remains in monomeric form at low salt. On raising the salt concentration, filaments form rapidly with consequent hydrolysis of ATP. Actin from rabbit skeletal muscle and from many other sources forms a tight 1:1 complex with bovine pancreatic deoxyribonuclease I (DNase I), although the biological significance of the complex is still obscure. Upon complex formation, actin inhibits DNase I activity (4) and also loses its capability to polymerize (5). This complex can be crystallized, and its structure has been solved (refs. 6 and 7; Protein Data Bank, reference 1ATN). When cells are subjected to an external stress, such as a sudden increase in temperature, they begin to synthesize a class of cellular proteins, commonly referred to as chaperones, which are thought to facilitate the refolding of denatured proteins, to aid the correct assembly or disassembly of

some oligomeric protein complexes, and to participate in the transmembrane targeting of proteins to cellular organelles (8). The ATP-dependent chaperones fall into two classes. The first is represented by the groEL protein in Escherichia coli, a 14-mer of 60-kDa protometers (9). The second class includes the 70-kDa heat shock proteins (HSP70) as well as constitutive representatives (HSC70) present in the cell cytoplasm under normal conditions (10). There is evidence that the HSC70 proteins participate in the ATP-driven disassembly of clathrin cages (11). Proteolytic digestion of the bovine HSC70 protein yields a 44-kDa N-terminal fragment that retains ATPase activity but cannot bind clathrin (12). This ATPase fragment has been crystallized (13), and its threedimensional structure has been reported (ref. 14; Protein Data Bank, ref. 1HSC). Unexpectedly, the atomic structures ofboth rabbit skeletal muscle actin (6, 7) and the 44-kDa ATPase fragment of the bovine HSC70 protein (14) are strikingly similar. In addition, the nucleotide-binding domains ofeach of these proteins have been recognized to closely resemble the substrate-binding domains of hexokinase (15). Here, we compare the structures of actin and the HSC70 ATPase fragment in detail. Structure Alignment. The optimal superposition between actin and the HSC70 ATPase fragment was determined by an iterative process. Because the two proteins have a very similar folding topology, it was straightforward to initially superimpose the structures manually and to identify equivalent a-carbon (Ca) atoms in the ,-sheets of both structures. An improved transformation was then computed that gave a rotation and translation that minimized the distances between the Ca-atom positions of equivalent residues (16). The superimposed molecules were inspected by eye to guide the classifications of Ca positions into equivalent, such as corresponding atoms in aligned a-helices and 8-strands, versus not equivalent, such as those in loops that traverse different routes or are of different lengths. Additional stretches of equivalent polypeptide chains were then identified and included in the calculation of a different optimal superposition between the two molecules. This procedure was terminated when all equivalent residues had been identified. As a consequence of the similarity in folding topology of the two proteins, when segments of the Ca backbone are superimposed, it is generally clear from visual inspection at which residue the backbones diverge. Hence, an empirical identification of equivalent residues was used in preference to computational methods that rely primarily on distance criteria. The result is shown in Fig. 1; amino acid residues are specified by one-letter code. The equivalent amino acid residues in the molecules are aligned above each other and indicated by uppercase letters. Lowercase letters identify residues in the sequence that are not considered equivalent.

The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Abbreviations: HSP70, 70-kDa heat shock protein; HSC70, 70-kDa heat shock constitutive representatives present in cell cytoplasm under normal conditions; Ca, a carbon; Wat 546, water molecule 546.

Although there is very little sequence identity ABSTRACT between the two proteins, the structures of rabbit skeletal muscle actin (375-amino acid residues) and the 44-kDa ATPase fragment of the bovine 70-kDa heat shock cognate protein (HSC70; 386 residues) are very similar. The a-carbon positions of 241 pairs of amino acid residues that are structurally equivalent within the two proteins can be superimposed with a root-mean-square difference in distance of 2.3 A; of these, 39 residues are identical, and 56 are conservative substitutions. In addition, the conformations of ADP are very similar in both proteins. A local sequence "fingerprint," which may be diagnostic of the adenine nucleotide f8-phosphate-binding pocket, has been derived. The fingerprint identifies members of the glycerol kinase family as candidates likely to have a similar structure in their nucleotide-binding domains. The structural differences between the two molecules mainly occur in loop regions of actin known to be involved in interactions with other monomers in the actin filament or in the binding of myosin; the corresponding regions in heat shock proteins may have functions that are as yet undetermined. Placing the Ca2+ ATP of actin on the ATPase fragment structure suggests Asp-206 (corresponding to His-161 of actin) as a candidate proton acceptor for the ATPase reaction.

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Biochemistry: Flaherty et al. RCTIN HSCO

Proc. Natl. Acad Sci. USA 88 (1991)

1 dedetTRLUCINGSGLUKAGFa gddapraUFPSIUGRPrhq gvumgmgqKDSYUGDEaQSKRGIL TLKYPlemskg PRUGIDLGTTYSCUGUfqhgkvei iandqgnrTTPSYURFTd TERLIGD^RfKMQURtlNptIITUFDHIr

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ACTIN 74 giit MIDDfEK WHHTFYne, ru)pee HPTLLTEAPLIP HSC70 73 igrrfddavvqsdmkhwpfmvvndagrpkvqveykgetksfyPEEUSShlULTKMlKEiceay gktuTMAUUTUPRYFt t ~~~~~~~~C..-..

MMOE_>(..

ACT IN 113 KAMREKhTQIflFetfnvpaHYURIQAULSLYASGR tt GIULDSGDGUTHNUPIYE GYALohAR1l1LD HSC70 152 DSQRQATKDAGTiogInv LRIII1EPTARI1IYGLdkk9gaernVLIFFDLGGTFDUSILTIedg!FEVK STRGDTr

y fvt ACTIN 180 LAGRELTDYLI1KILTERG tREREIURDI KEKLcyvoIdfenema taoas333IEKSYELp ogc s3StASIEIc9iy e HSC70 228 LGGEDFDNRt1UNHFIREFkrkhkkdi3enkravrRLRTACERRKRTL

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ACTIN 247 UITIGn ERFRCPETLFqpsfigmesaGIHETTYNSIfKCd;dir;kd yAIYURN SGGTTnYPGi!DRnQKEITGI HSC70 291 idfYTS ITraRFEELHADLF rGTLDPUEKALRDfl kldksq! HDIULVGGSTRIPK IQKLLQDFF1

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ACTIN 321 apstmkIIKIIAPPERKYSUU1GGSILASLstfqqmwitkqeydeagpsivhr HSC?0 356 g I Sdk KELNKS I1PDERVRYGRRUQAAI

FIG. 1. Structure alignment between actin and HSC70. The structurally equivalent amino acid residues of actin are aligned above those of HSC70 and are indicated by uppercase letters. Thirty-nine equivalent amino acid residues of identical type are marked by solid bars. Lowercase letters identify residues in the sequence that are not considered structurally equivalent. Secondary structure (17) is summarized above and below the two sequences: arrows represent ,B-strand residues, open cylinders represent a-helices, and cylinders with 3 inside represent 310-helices. Loops that interrupt 8-strands are shown as continuous lines. Amino acid residues are specified by one-letter code.

Secondary structure (17) is summarized above and below the two sequences; arrows indicate a,-sheet residue, and the open cylinder and cylinder containing 3s describe residues in an a-helix or a 310-helix, respectively. The root-mean-square (rms) distance between the Ca positions of the 241 pairs of equivalent residues is 2.3 A. Of these, 39 residue pairs are identical, and another 56 pairs are conservative substitutions. Comparison of Structures. The structures of actin and the HSC70 ATPase fragment are very similar. Fig. 2 shows a Ca-stereo plot of the two optimally aligned structures. The view was chosen identical to Fig. la of Kabsch et al. (6). Subdomains 1-4 of actin correspond to HSC70 domains IA, IB, IIA, and IIB, except for the difference of assignment of the helices that traverse the base of the cleft. As described in

ref. 14, Flaherty et al. chose a convention that simplified the discussion of folding topology of the protein, whereas the convention of Kabsch et al. (6) is consistent with a more globular appearance in the subdomains. The similarity of the two structures also extends to the bound ADP. Here, the rms distance is 0.66A between corresponding atoms of the nucleotide using the rotation and translation determined previously at the best superposition of the 241 equivalent Ca coordinates. When only corresponding atoms from ADP are used to determine the best superposition, the rms distance is 0.41A. Fig. 3 shows the environment of ADP in both actin and the HSC70 ATPase fragment. In both structures the nucleotide is bound in a cleft between domains, with the adenine base in the anti conformation and

FIG. 2. Superimposed Ca traces of actin and HSC70. For actin, residues considered structurally equivalent in the comparison are green, and the remaining (not equivalent) residues are blue. For the HSC70 ATPase fragment, equivalent residues are red, and residues not equivalent are magenta. Backbone appears yellow where the equivalent residues of actin (green) and the ATPase fragment (red) superimpose. A few amino acid residue sequence numbers are specified.

Biochemistry: Flaherty et al.

Proc. Natl. Acad. Sci. USA 88 (1991)

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FIG. 3. Stereoplot of the atoms in the environment of ADP. (Upper) The ADP environment in actin. The nucleotide is green; residues that identical in the two structures are red; other residues are magenta. (Lower) The equivalent amino acids of the HSC70 ATPase fragment surrounding the ADP, shown with the same color code.

are

the ribose in a 2'-endo pucker. In actin, the adenine base fits into a hydrophobic pocket in the large domain formed by a portion (Arg-210-Glu-214) of an a-helix, a 310-helix (Gly-302Tyr-306), and by Lys-336. On this a-helix, Glu-214 forms a hydrogen bond to the 02' atom of the ribose and also a salt bridge across one turn to Arg-210, whereas Lys-213 is involved in a hydrogen bond to the carbonyl group of Gly-182. The hydrophobic parts of Glu-214 and Lys-213 form the top of the pocket. The floor of the pocket consists of the 310-helix (residues 302-305), which connects the fourth P-strand with the following a-helix in the right-handed PaB motif in subdomain 3. Tyr-306 forms the back of the pocket. In HSC70, the adenine-binding pocket is formed in analogy with actin by one turn (Glu-268-Arg-272) of a longer a-helix, a 310-helix (Gly-339-Ile-343), and by Asp-366. On this a-helix, Arg-272 forms a salt bridge across one turn to Glu-268, which is hydrogen bonded to the 02' atom of the ribose, whereas Lys-271 is involved in a hydrogen bond to the carbonyl group of Gly-230. Note that, compared with actin, the order of Glu-268 and Arg-272 in the salt bridge is exchanged, but their roles are retained. At the back of the pocket one finds Ile-343 in place of Tyr-306. The 310-helix (Gly-339-Arg-342) has the same length as the equivalent one found in actin. Conservation of glycine at position 302 in actin and at the equivalent position 339 in HSC70 is essential because this residue lies so close to the a phosphate that no side chain could be accommodated. Moreover, it is involved in hydrogen bonding to the 01 oxygen of the a phosphate. The neighboring Ser-340 is also important because it makes a hydrogen bond with the carbonyl of Gly-201 in the phosphate-binding loop. Ser-340 corresponds to Thr-303 in actin that hydrogen bonds to the carbonyl of Gly-156 in a completely equivalent way. The phosphate forms hydrogen bonds with the amides of Ser-14, Gly-15, and Leu-16 in actin

and with the equivalent loop residues Thr-13, Thr-14, and Tyr-15 in HSC70. The main structural differences between the two proteins are summarized in Table 1. They usually comprise loop regions that, in actin, are involved in interactions with other monomers in the actin filament (7) or in binding the myosin heavy chain subfragment 1 (Si) in the rigor state (6). The actin segment His-40-Gln-49, which has no counterpart in HSC70, is included in the main contact with DNase I in the crystal Table 1. Structural differences between HSC70 and actin Actin HSC70 Actin function

Met-1-Gly-4 Phe-21-Arg-36

Asp-1-Thr-5 Ala-22-Ala-29 His40-Gln-49

Lys-71-Tyr-115 Ile-130-Val-139 Ile-164-Val-169 Asp-186-Asn-194 Glu-213-Ile-216

Glu-72-Thr-77 Asn-92-Glu-100 Glu-125-Ala-131 Thr-148-Thr-149

S1 binding S1 binding Actin-actin contact, DNase I binding Methylated His-73 S1 binding

Pro-172-His-173 His-227 Lys-246-Arg-261 Ser-275-Thr-278 Asp-285-Phe-293 Arg-299-Ala-300

Tyr-198-Thr-203 Cys-217-Leu-236

Arg-311

Gln-263-Ala-272

Lys-325-Ile-331

Pro-243-Gln-246

Actin-actin contact Tropomyosin binding? Actin-actin contact

Asn-252

Hypothetical trimer contact

Actin-actin contact Asp-286-Tyr-294 Actin-actin contact Ala-319-Lys-326 Actin-actin contact Leu-380-Lys-384 Ser-350-Arg-372 S1, myosin heavy chain subfragment 1.

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Proc. Natl. Acad Sci. USA 88 (1991)

structure (6) and is believed to interact with another monoalong the long-pitch two-start actin helix (7). The unconserved (white) part of domain IB comprising residues mer

Lys-71-Tyr-115 in HSC70 is replaced in actin by a d loop containing the methylated His-73, a posttranslational modification found in almost all actins studied to date. The function of this modification is unknown, but His-73 is involved in a network of salt bridges connecting subdomains 2 and 4. The region Cys-217-Leu-236 in actin contains an a-helix that does not appear in HSC70. This helix is located at the outside of the filament (7), and inspection of figure 4b of Milligan et al. (18) suggests that this region might be involved in interactions with tropomyosin. This suggestion is in agreement with the observation that the sequence of Tetrahymena actin (19, 20), which does not bind muscle tropomyosin (21), differs in 8 amino acids in region 222-233 from the rabbit muscle sequence. The exposed hydrophobic loop region Gln-263-Ala-272 in actin is thought to participate in binding to two monomers from the opposite strand to hold the filament together across the axis. In HSC70, the corresponding loop is absent. Although no specific sites of protein-protein interaction have been delineated within the HSP70-related proteins, it is intriguing to question whether some of the equivalent regions of HSC70 may mediate specific functions. Despite the lack of significant global sequence similarity between actins, HSP70-related proteins, and the hexokinases, there is a local sequence "fingerprint" characteristic ofthe nucleotide-binding structural motif (the lower domains) common to these proteins. In particular, the two N-terminal ,1-strands, which interact with the 8 and 'y phosphates of the nucleotide, show a sequence similarity (see Table 2). In this motif, the aspartic acid chelates the metal ion of the nucleotide-metal complex; the invariant glycine abuts the y phosphate in a manner that would be sterically precluded by a residue with a Cp atom; hydroxyls from the next two residues hydrogen bond to the phosphate oxygens; and the positively charged lysine or arginine residue (which is not present in the HSP70 protein family) forms a salt bridge to the nucleotide phosphates. This sequence fingerprint suggests that some other kinases-specifically glycerol kinase and glucokinase-may have the same nucleotide-binding motif. Crystallographic work in progress on glycerol kinase should soon reveal whether this is the case (27). Inferences. We believe that the structural similarity between actin and HSC70 is not accidental: 241 pairs of equivalent residues from the two proteins can be superimposed with an rms discrepancy of 2.3A. It suggests the existence of a common ancestral molecule along the evolutionary pathway. Hexokinase, too, probably evolved for this precursor. The dnaK protein of E. coli, which is a HSP70related protein, shares typically -48% sequence identity with its eukaryotic counterparts. Additionally, the recent finding of actin-like sequences in E. coli and Salmonella typhimurium (28) is well in agreement with the existence of a common prokaryotic ancestor. The branching into actin and the heat Table 2. Alignment of nucleotide HSC70 Actin Hexokinase Glucokinase Glycerol kinase

shock proteins must have occurred a very long time ago because there is little sequence similarity between them. Only 39 of 241 structural equivalent amino acid residues are identical, and another 56 are similar. The strongest sequence identity found consists of 7 matches of 24 residues in the actin region Ala-295-Thr-318, which is part of the adenine-binding pocket. The structural differences occur mainly in regions of the actin molecule known to be involved in rather specific functions. At this time, much less is known about the functions of HSC70. However, looking at Fig. 2 and Table 1, we may suggest that regions Phe-21-Arg-36, Lys-71-Tyr-115, Asp-186-Asn-194, Glu-213-Ile-216, Lys-246-Arg-261, and Asp-285-Phe-293 are candidate sites that might have specific functions in HSC70. Because the environment and geometry of the nucleotide in the two proteins is rather similar, one can use information from both structures to formulate a hypothesis for the ATPase mechanism. In HSC70 the coordinates of the ion and the y phosphate in ATP are not yet clear, whereas the ATP form of actin:DNase I is well determined in this region. On the other hand, the HSC70 structure is determined to higher resolution, which permits placement of some water molecules. When we introduce the ATP and Ca2+ coordinates from the actin into HSC70, the following picture emerges: The Ca2+ ion probably lies between the Asp-10 and Asp-199 (equivalent to Asp-11 and Asp-154 in actin) and is flanked by Glu-175 (Gln-137 in actin). A particular water molecule, denoted Wat 546 in HSC70, is then situated with its oxygen atom at a distance 3.5 A from the Py atom of the ATP molecule; further, the 03'-Py bond of the ATP is approximately in line with a line joining the Py atom to the oxygen ofWat 546. Therefore, Wat 546 is a strong candidate for an in-line attack on the y phosphate. Moreover, Wat 546 is hydrogen bonded to Thr-204 (Val-159 in actin) and is polarized by Asp-206. This fact suggests that Asp-206 may function as a proton acceptor (general base) in the ATPase reaction. Interestingly, a comparison with hexokinase (29) shows that the 60H of the glucose analogue N-o-toluolylglucosamine comes close to the position of Wat 546. The proximity ofThr-204 to Wat 546 suggests that a possible side reaction of HSC70 could be autophosphorylation of Thr-204. In actin, His-161 is the residue that aligns with Asp-206 of HSC70. By itself, the histidine imidazole would be capable of acting as a proton acceptor at physiological pH. However, its PKa could be shifted, and it could act as a more effective proton acceptor if the carboxyl of an acidic residue were brought into close proximity of the imidazole, thereby forming a charge-relay system. If such a charge-relay system were formed upon polymerization of actin, it would rationalize how the actin molecule can "switch" from having a relatively slow ATPase activity in the monomeric (G actin) state to having a much more rapid ATPase activity in the polymerized (F actin) state. The striking structural similarity of actin and the HSC70 ATPase domain suggests there may be mechanistic similarities in their functions. The specific nature of these similar-

(3-phosphate-binding sequences ... AVGIDLGTTYS£V18 ... ALVCDNGSGLV.KAlq ... YLAIDLGGTNL.RVc4 ... FLSLDLGGTNFRV86, ...

IVALDQGTTSSRA18

... (I/L/V)X(I/L/V/C)DXG(T/S/G) (T/S/G)XX(R/K/C) Consensus Alignment of the ATP P-phosphate-binding sequences of bovine HSC70 (22), rabbit skeletal muscle actin (23), yeast hexokinase B (24), rat liver glucokinase (25), and E. coli glycerol kinase (26). X = any amino acid. The consensus sequence was derived from the aligned sequences as shown, in addition to inspection of the actin and ATPase fragment structures to see what other side chains might be allowed. Initial and final residues of the consensus region are underlined; strictly conserved residues are shown in boldface type.

Biochemistry: Flaherty et al. ities is obscure, however, because few biochemical activities have been well-characterized for the HSP70-related proteins. The role of ATP in actin appears to be to regulate the monomer-polymer transition: ATP-actin polymerizes; polymerization induces ATP hydrolysis; release of products (ADP + Pi) destabilizes the filament (30). The HSP70-related proteins are schematically more complex because their activities are mediated through the C-terminal substrate recognition domain and because they interact with a diversity of target protein substrates. However, in the specific example of clathrin uncoating by HSC70, nucleotide hydrolysis and product release result in disassembly of clathrin (a polymeric substrate) into individual triskelions. For other activities in which HSC70 is thought to participate, such as protein synthesis (31) and transmembrane targeting (32, 33), its activity is suggested to involve disassembly of, or at least preventing stable formation of, nonspecific protein aggregates; we may presume that this activity is mediated through nucleotide hydrolysis and product release as well. Hence, we may suggest that actin and the HSP70-related proteins use a similar mechanism, mediated through ATP hydrolysis and release of ADP + Pi, to regulate quite different oligomeric assembly-disassembly processes in proteins. This work was supported, in part, by Grant GM-39928 from the National Institutes of Health to D.B.M.

1. 2. 3. 4.

Pollard, T. D. (1990) Curr. Opinion Cell Biol. 2, 33-40. Vandekerckhove, J. (1990) Curr. Opinion Cell Biol. 2, 41-50. Carlier, M.-F. (1989) let. Rev. Cytol. 115, 139-170. Lazarides, E. & Lindberg, U. (1974) Proc. Natl. Acad. Sci. USA 71, 4742-4746. 5. Hitchcock, S. E. (1980) J. Biol. Chem. 255, 5668-5673. 6. Kabsch, W., Mannherz, H. G., Suck, D., Pai, E. F. & Holmes,

K. C. (1990) Nature (London) 347, 37-44. 7. Holmes, K. C., Popp, D., Gebhard, W. & Kabsch, W. (1990) Nature (London) 347, 44-49. 8. Ellis, R. J., van der Vies, S. M. & Hemmingsen, S. M. (1989) Biochem. Soc. Symp. 55, 145-153. 9. Hohn, T., Hohn, B., Engel, A. & Wurtz, M. (1979) J. Mol. Biol. 129, 359-373. 10. Pelham, H. R. B. (1986) Cell 46, 959-%1.

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11. Schlossman, D. M., Schmid, S. L., Braell, W. A. & Rothman, J. E. (1984) J. Cell Biol. 99, 723-733. 12. Chappell, T. G., Konforti, B. B., Schmid, S. L. & Rothman, J. E. (1987) J. Biol. Chem. 262, 746-751. 13. DeLuca-Flaherty, C., Flaherty, K. M., McIntosh, L. J., Bahrami, B. & McKay, D. B. (1988) J. Mol. Biol. 200, 749-750. 14. Flaherty, K. M., DeLuca-Flaherty, C. & McKay, D. B. (1990) Nature (London) 346, 623-628. 15. Steitz, T. A., Fletterick, R. J., Anderson, W. A. & Anderson, C. M. (1976) J. Mol. Biol. 104, 197-222. 16. Kabsch, W. (1978) Acta Crystallogr. Sect. A 34, 827-828. 17. Kabsch, W. & Sander, C. (1983) Biopolymers 22, 2577-2637. 18. Milligan, R. A., Whittaker, M. & Safer, D. (1990) Nature (London) 348, 217-221. 19. Hirono, M., Endoh, H., Okada, N., Numata, 0. & Watanabe, Y. (1987) J. Mol. Biol. 194, 181-192. 20. Cupples, C. G. & Pearlman, R. E. (1986) Proc. Natl. Acad. Sci. USA 83, 5160-5164. 21. Hirono, M., Tanaka, R. & Watanabe, Y. (1990) J. Biochem. (Tokyo) 107, 32-36. 22. DeLuca-Flaherty, C. & McKay, D. B. (1990) Nucleic Acids Res. 18, 5569. 23. Elzinga, M., Collins, J. H., Kuehl, W. M. & Adelstein, R. S. (1973) Proc. Natl. Acad. Sci. USA 70, 2687-2691. 24. Stachelek, C., Stachelek, J., Swan, J., Botstein, D. & Konigsberg, W. (1986) Nucleic Acids Res. 14, 945-963. 25. Andreone, T. L., Printz, R. L., Pilkis, S. J., Magnuson, M. A. & Granner, D. K. (1989) J. Biol. Chem. 264, 363-369. 26. Pettigrew, D. W., Ma, D.-P., Conrad, C. A. & Johnson, J. R. (1988) J. Biol. Chem. 263, 135-139. 27. Faber, H. R., Pettigrew, D. W. & Remington, S. J. (1989) J. Mol. Biol. 207, 637-639. 28. Garcia-Cuellar, C., Tenorio, V., Cisneros, B., Montanez, C., Hernandez, J. M., Alvarez, J. & De la Garza, M. (1990) J. Cell Biol. Abstr. 111, 31a. 29. Anderson, C. M., McDonald, R. C. & Steitz, T. A. (1978) J. Mol. Biol. 123, 1-13. 30. Korn, E. D., Carlier, M.-F. & Pantaloni, D. (1987) Science 238, 638-644. 31. Beckmann, R. P., Mizzen, L. A. & Welch, W. J. (1990) Science 248, 850-854. 32. Deshaies, T. J., Koch, B. D., Werner-Washburne, M., Craig, E. A. & Schekman, R. (1988) Nature (London) 332, 800-805. 33. Chirico, W. J., Waters, M. G. & Blobel, G. (1988) Nature (London) 332, 805-810.

Similarity of the three-dimensional structures of actin and the ATPase fragment of a 70-kDa heat shock cognate protein.

Although there is very little sequence identity between the two proteins, the structures of rabbit skeletal muscle actin (375-amino acid residues) and...
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