Chapter 8

Spider Silk: a Mystery Starting to Unravel Mike Hinman, Zhcngyu Dong, Ming Xu, and Randolph V. Lewis 1

1 Historical Aspects Spiders are unique creatures, due to the presence of glands in their abdomen which produce silk. They are also unique in the use of this silk throughout their li fe span and the nearly total dependence on silk for their evolutionary success. Humans have viewed spiders with both dread and delight. The delight has been based on the beauty and precise construction of the classic orb web. There are few who would disagree about the beauty of a web glistening in the morning dew. Although spiders have undoubtedly been studied since earliest man, the first papers using a scientific approach to spiders webs and silk appeared in the 1800s. One of the earliest was by John Blackwell (1830), describing the construction of nets (webs) by spiders. The following decades resulted in studies of the biology of the spiders and their anatomy, but little information was published about the silk itself. Benton (1907) published one of the earliest studies describing properties of the silk itself. In that same year, Fischer (1907) demonstrated the protein nature of the silk by showing the presence of predominantly amino acids. There were periods of fairly intense study prior to World War II and in the late 1950s. However, progress, especially when compared to silkworm silk, was relatively meager. Beginning in the 1970s, the laboratories of Work, Gosline, and Tillinghast revived interest in spider silk with several papers describing physical, mechanical, and chemical properties of spider silks. The structure of the spider silk protein(s) remained unknown despite the efforts of these groups and others.

2 Biological Aspects of Spider Silk Production Spider webs are constructed from several different silks. Each of these silks is produced in a different gland. The different silks, the glands which produce them, and the use of each are listed in Table 1. The glands occur as bilaterally symmetrie paired sets. Although each of the glands has its own distinctive shape and size the functional organization of all of them is similar. The majority of the gland serves as a reservoir of soluble silk protein which is synthesized in specialized cells at the I

Molecular Biology Deparlment. Univ. of Wyoming, Box 3944 Laramie, WY 82071-3944, USA

Results and Problems in Cell Differentiation 19 Biopolymers Case. S. T. (Ed.) Springer- Verlag Beflin Heidelberg 1992


Mike Hinman el al.

Table I. Various spider silks, their uses, and glands of synlhcsis Silk gland



Major ampullate Minor Ampullate Piriform Aciniform Cylindrical (tubuliform) Aggregate Flagilliform

Dragline, frame threads Reinforces dragline Attachment disk Swathing silk Cocoon silk Sticky silk glue Thread for sticky silk

Anlerior Median Anlerior Median, posterior Median, posterior Posterior Posterior

distal end of the gland. The soluble silk is forced (pulIed) down a narrow duct during which the physical and chemical changes occur which produce the solid silk fiber. There is a muscular valve at the exit to the spinneret which can control the flow rate of the fiber and may control the fiber diameter to a small degree. The silk exits through the spinnerets, of which there are three pairs, anterior, median and posterior. The exit spinneret for each of the different silks is also listed in Table 1. Due to their size and ease of study, the major ampullate glands have recelved the most attention. Thus most of what is known about the synthesis of silk pro teins is based on the study of that gland. However, morphological and histochemical studies of the other glands support the ideas developed for the major ampullate gland. The synthesis of the silk protein(s) takes pi ace in specialized columnar epithelial cells (Bell and Peakall 1969) which appear to lack a Golgi apparatus. There appear to be at least two different types of cells producing pro tein (Kovoor 1972), which correlates with our data on the composition of the silk from these glands. The newly synthesized protein appears as droplets within the cell which are secreted into the lumen of the gland. The state ofthe protein in the lumen ofthe gland is unknown but it must be in a state which prevents fiber formation, as the fiber is not formed until passage down the duct. This is probably accomplished by a combination of protein structure and concentration which prevents aggregation in large pro tein arrays. It has been shown that the silk in the gland is not birefringent, whereas the silk becomes birefringent as it passes down the duct (Work 1977b). Thus the ordered array of pro tein seen in the final fiber is accomplished in the duct. This ordering appears to be due to the mechanical and frictional forces aligning the pro tein molecules and probably altering the secondary structure to the final fiber form. Iizuka (1983) has proposed a similar mechanism for silkworm silk formation. Experimental evidence for this has been the ability to draw silk fibers directly from the lumen ofthe major, minor and cylindrical glands (Hinman M, pers. commun.), implying that the physical forces of drawing the solution are sufficient for fiber formation.

3 Current Information About Spider Silk 3.1 Mechanical Properties One of the features which attracted attention to spider silk was its unique properties. The spider must be able to use the minimum amount of silk in its web to


Spider Silk: a Mystcry Starting to Unravel Table 2. Comparative data on dilTerent übers Material

Draglinc silk KEVLAR Ruhher Tendon


Energy to break

(Nm -2)

(lkg -1)

Ix 4x Ix Ix

IX 3X 8X 5X

10" 10 9 10 6 10 9

10 5 104 104 10 3

Data dcrived from Gosline et al. (1986)

catch prey in order to survive successfully. The web has to stop a rapidly ftying insect nearly instantly in a manner that allows it to become entangled and trapped. To do this, the web must absorb the energy of the insect without breaking and yet not act as a trampoline to send the insect back off the web. Gosline et al. (1986) have reviewed several aspects of this and concluded that spider silk and the web are nearly optimally designed for each other. As with any polymer, especially those made of pro tein, there are numerous factors which can affect the tensile strength and elasticity. These can include temperature, hydration state, and rate of extension. Even with all those caveats, it is clear that dragline silk is a unique biomaterial. As seen in Table 2, dragline silk will absorb more energy prior to breaking than nearly any commonly used material. Thus, although it is not as strong as several of the current synthetic fibers, it can outperform them in many applications.

3.2 Chemical Data The composition of spider silks has been known to be predominantly protein since the early studies of Fischer (1907). In fact, except for the sticky spiral thread, no significant amounts of anything but protein have been detected, including sugars, minerals, and lipids. The sticky spiral thread contains a number of watersoluble compounds such as potassium phosphate, potassium nitrate, gamma amino butyramide, and several other amine-containing compounds (Anderson and Tillinghast 1980). Although not proven, it is likely these compounds serve to absorb water to insure that the sticky spiral retains its adhesive catching properties. The various silks have significantly different amino acid compositions as do the same silks from different spiders. Table 3 lists the compositions of several different spider silks (da ta from Anderson 1970; Work and Young 1987; Lewis R V, unpubl.). In the major ampullate silks, the combination of Glu, Pro, Gly, and Ala comprises 80°;;, of the silk from cach species. However, the proportion of Pro is significantly different in each. As will be discussed below, these differences can be accounted for by different ratios of two proteins. The minor ampullate silks are more similar among species and differ from major ampullate silk in having significantly lower Pro values. Cylindrical (tubuliform) gland silk used for constructing egg cocoons is radically different from any of the other silks. The amount of Gly is reduced by nearly three-quarters and Ser, in particular, has increased to compensate as have

0.7 1.0 4.7 0.4 0.4


0.1 3.5 4.3 0.3 0.3


0.2 1.5 3.9 1.0 0.5



1.3 3.9 0.5 0.5


1.9 1.4 5.1 1.6 0.1 42.8 36.6 1.7

1.2 0.1 7.4 10.9 3.5 45.8 22.2 0.8

2.4 0.8 10.3 9.2 10.1 42.0 18.0 2.2

1.0 0.9 7.4 11.5 15.8 37.2 17.6 1.2

Minor Amp(Ad)

Major Amp(N)

Major Amp(Ag)

Major Amp(Ad)

0.4 2.2 8.4 0.9 0.5 2.9


2.8 0.3 10.9 4.5 0.6 42.6 20.0 2.5

1.2 0.3 5.2 8.0 0.5 51.6 24.1 1.2 0.4 1.5 7.9 1.0 0.5

Minor Amp(N)

Minor Amp(Ag)

Cylind (N) 3.6 5.2 23.4 9.3 0.4 7.3 30.0 1.8 1.9 6.9 2.1 5.2 1.0 3.2

Cylind (Ag) 5.9 3.6 28.1 10.0 0.4 9.3 23.7 4.9 1.8 6.6 1.5 3.2 2.0


Gland (species)

4.3 9.1 1.7 2.4 2.2 0.5 4.0

8.3 9.8 15.7 7.5 3.3 13.6 11.0 7.1

Aciniform (Ad)

9.2 7.6 6.8 9.8 10.8 14.5 6.2 5.8 4.7 5.5. 2.2 3.8 7.4 2.4 3.4

10.5 4.4 14.8 10.4 7.8 7.8 9.9 5.4 3.7 5.4 2.2 2.3 9.0 2.8 3.6

Pyriform Aggregate (Ad) (Ad)

Data derived from Anderson (1970). Work and Young (1987), and Lewis (unpub!.). Ad is A. diadematus, Ag is A. gemmoides, and N is N. claripes

Asp Thr Ser Glu Pro Gly Ala Val Cys Met He Leu Tyr Phe Lys His Arg

Amino acid

Table 3. Amino acid composition of different silks

1.0 1.4 2.6 l.l 1.4 0.7 1.1

2.7 2.5 3.1 2.9 20.5 44.2 8.3 6.7

Coronate (Ad)

Spider Silk: a Mystery Starting to Unravel


other amino acids to a much lesser extent. The coronate gland silk is also very different in having a very high proportion of Pro in relation to the other amino acids. The other three silks are more similar to each other in having no greatly predominant amino acids and having a more even distribution of the amounts of the various amino acids.

3.3 Biophysical Studies Virtually all ofthe data on spider silk have been obtained from major ampullate (dragline) silk. There are two major reasons for this. First, it can be easily obtained since the spiders trail it along behind them as they move. Thus the silk can be gathered or the spider can be forced to extrude it by causing the spider to fall and wrapping the silk onto something as it does. The second reason is the combination of mechanical properties of elasticity and high tensile strength, which will be discussed in detail below. Since it is not possible to have confidence in the extrapolation of the physical data from dragline silk to the other silks because the amino acid compositions are so different. The detailed biophysical data from the other silks will have to await further experimentation. There were several early studies of silk fibers using X-ray diffraction which provided some information, much of which was interpreted based on the structure of silkworm silk (reviewed in Fraser and MacRae 1973). These studies led to the classification of dragline silk as ß-sheet group 3, 4, or 5, depending on the species. These groups are distinguished by the intersheet distance between the ß-sheets. The higher the number, the larger that spacing. It was also clear that much of the structure was not ß-sheet and appeared to be random coi!. However, it is clear from the amino acid compositions of the different silks that large bulky groups are present and must be accommodated either in the ß-sheet or in the random coil regions. The amino acid sequences of the proteins from dragline silk put limits on what the structures can be and the X-ray data may need to be reinterpreted on that basis. Using Fourier transform infrared (FTIR) spectroscopy Dong et a!. (1991) have probed the structure of the dragline silk fiber in the relaxed and extended states. The data confirms the presence of significant ß-sheet-like structure which appears the same in both relaxed and extended forms (Fig. IA, B). Dragline silk, dissolved in 4.7 M LiCI0 4 , dialyzed against water, and dried to a film, also showed predominantly ß-sheet-like conformation, indicating this is a preferred secondary structure of the proteins in the solid state (Dong Z and Lewis RV, unpub!. data). However, in the extended state, the silk forms an :x-helical structure which returns to the original form when the tension is released (Fig. I A, B). This is seen by the formation of the peaks at 1557 and 1651 cm - 1. The parallel polarized spectra shows that the orientation is parallel to the fiber axis. These same spectral features were observed for both Nephila clavipes and Araneus yemmoides (Figs. 1 and 2). The :x-helical regions appear to be coming from the random or nonoriented regions. However, minor ampullate silk, which exhibits very low elasticity, showed no such :x-helix


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Spider silk: a mystery starting to unravel.

Chapter 8 Spider Silk: a Mystery Starting to Unravel Mike Hinman, Zhcngyu Dong, Ming Xu, and Randolph V. Lewis 1 1 Historical Aspects Spiders are un...
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