Spider silk ampullate glands

The primary structure of the silk fibroin from S.c. ricini has recently been determined by Yukihiro et al. (personal communication). The silk mainly consists of about 100 repeated similar sequences where there are alternative appearances of a polyalanine region and a glycine-rich region as in spider (major ampullate) silk. As a result, the most striking conformational characteristic of these silk fibroins in the silk gland or in aqueous solution is the presence of a-helical domains consisting of Ala residues. 

The first successful expression of cDNA clones of silks were with partial cDNA clones from N. clavipes major ampullate silk gland cDNA [76]. The 1.7-kb fragment was from the 3 -terminus of the major ampullate silk gene and a 43 kDa recombinant silk protein was expressed and characterized. Most recently, partial cDNA clones of the 3 end of spider silks cDNAs from Araneus diadematus were cloned and expressed in mammalian cells as part of an effort to develop transgenic animals that express silk proteins [77]. Transgenic expression of silks in plants (tobacco and potato) and mammalian epithelial cells has been reported [71,77] and may point the way toward more substantive production of these proteins in the future. 

Cunniff, P.M., Fossey, S.A.. Auerbach, M.A. and Song, J.W. (1994) Mechanical properties of major ampullate gland silk fibers extracted from Nephila clavipes spiders. In Silk Polymers Materials Science and Biotechnology, pp. 234-251, D. Kaplan, W.W. Adams, B. Farmer and C. Viney (Eds.). American Chemical Society. Washington, DC. 

In addition to silk moths/worms, silks are produced by many other species of insects and spiders (Kaplan et al., 1992, 1993, 1998). Unlike silk moth-derived silk, spider silks are not widely used in the textile industry because of their limited availability. Spiders naturally produce less silk than a silkworm cocoon ( 137 m of fiber can be obtained from the ampullate gland of a spider while one silkworm cocoon yields 600-900 m of fiber) (Lewis, 1996) and, spiders being solitary and predatory in nature, cannot be raised in large numbers. However, it was documented that spider silks are just as suitable for textile production as their insect counterparts (Kaplan et al., 1993). Consequentially, for biomaterial development, silk moths/worms and spiders are the main silk sources. 

Silk fibers formed by the secretion of the major ampullate glands of the spider Nephila clavipes also involve the linear supramolecular aggregation of a globular protein (fibroin). The formation of rodlike aggregates is consistent with the observation of a precursor liquid crystalline phase before fiber solidification [135,136]. Details of this supramolecular polymerization are still unclear. 

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