A Non-Periodic Lattice Model for Crystals in Nephila clavipes Major Ampullate Silk
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into a microstructure. The monomeric units in a protein have the form: NH3-CJHR-COOH where the α-carbon is bonded to a functional "R" group that distinguishes the monomer as being one of the 20 commonly occurring natural α-amino acids. To help v i s u a l i z e the complex microstructure of MAS, it is expedient to consider briefly a simpler system: degummed cocoon fiber from the s i l k w o r m Bombyx mori. The single structural protein (fibroin) is dominated by a [Gly-Ala-Gly-Ala-Gly-Ser]n motif 5 7 (for which the corresponding sequence of f u n c t i o n a l groups is H-CH 3 -H-CH 3 -H-CH 2 OH). When the molecules are placed into an extended conformation, one side of the molecule is populated exclusively by hydrogen R-groups, whereas the opposite side has only methyl and methanolic groups (Figure la). Carboxyl and amide groups on adjacent molecules can hydrogen-bond the extended chains together to form (pleated) /3-sheets (Figure lb). These sheets subsequently stack to form threedimensional /3-sheet crystals (Figure lc). The spacing of these sheets is determined by the interdigitation of the R8 groups on juxtaposed faces. Hence, juxtaposed methyl-methanolic faces exhibit a much greater separation (5.7 A) than do glycyl faces (3.5 A). The simple repeat motif of B. mori silk allows the formation of extensive crystalline
regions in the silk fibers. At points where the alignment or register of the molecules becomes disrupted, the ordered crystals give way to an "amorphous" matrix (i.e., there is no longer spatial correlation between molecules). Consequently, a classic composite is formed wherein stiff crystals are dis910 persed in a tough matrix. Silk fibers from the major ampullate glands of Nephila clavipes spiders (i.e., golden orb weavers) have a more complicated microstructure, commensurate with their more sophisticated mechani11 14 cal properties. ' It has not yet been determined whether MAS consists of one or two types of protein molecules, but it is clear that the structural motifs are considerably more complex than that of B. mori silk. Since two very different polypeptide sequences have been identified (from analysis of cDNA clones of the messenger RNA in major ampullate glands), it is convenient to refer to them as two distinct molecules, Spidroins 1 and 2—although we recognize the evidence that both sequences may instead occur within a single MAS fibroin molecule. Our microstructural model does not depend on whether or not the two spidroin sequences are covalently linked. Spidroin 1 is characterized by short runs of polyalanine (5-7 residues) interspersed by approximately five repeats of a loosely conserved Gly-Gly-X motif where X is predominantly Gin, Tyr, and Leu. 15 Spidroin 2 comprises slightly longer runs of polyalanine (6-10 residues), as well as several proline-containing pentapeptides, including Gly-Tyr-Gly-Pro-Gly, Gly-Pro-Gly-Gly-Tyr, and Gly-Pro-GlyGln-Gln.16 Two Views of Supramolecular Order The assembly of these motifs into a supramolecular structure has been investigated by expectedly compl
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