Biosynthesis and Processing of Silk Proteins
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Biosynthesis and Processing of Silk Proteins David L. Kaplan, Stephen Fossey, and Charlene M. Mello with Steven Arcidiacono, Kris Senecal, Wayne Muller, Scott Stockwell, Richard Beckwitt, Christopher Viney, and Keven Kerkam Introduction Silks produced by silkworms (e.g., Bombyx mori) and orb-web weaving spiders (e.g., Nephila clavipes)] are essentially pure protein, that is, complexes of amino acid polymers. They are the most common fibers spun by biological systems. There has been a long-standing interest in the use of these and similar fibers in textiles, cables, fiber reinforcement in composites, in addition, for example, to cross hairs in optical instruments, and fishing nets. Both nylon, a homo-polymer of the amino acid glycine, and Kevlar, a polymer of a nonnatural aromatic amino acid, can be considered modified, synthetic versions of silk and are used for some of the applications mentioned above. The potential for genetic manipulation, through recombinant DNA technology, of the natural biosynthetic process for these natural proteins (see the article by Cappello in this issue) has renewed interest in the production of new silklike proteins.2 The natural silks are characterized by a /3-sheet secondary structure which is stabilized by interchain hydrogen bonds and intersheet hydrophobic interactions (Figure 1). Silks can be considered block copolymers, with crystalline domains consisting of short side chain monomers (the amino acids glycine, alanine, and serine) interspersed in amorphous domains consisting of bulkier side chain amino acids.3'4 This family of fibers is naturally tailored to perform functions such as catching MRS BULLETIN/OCTOBER 1992
prey (orb web) or serving as a barrier against environmental challenges (cocoon). The domestic silkworm (B. mori) produces only one type of silk, cocoon silk, at only one stage in its lifecyle, during the fifth
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- - -H-N
C=O N-H---
--O=C N-H
O=C
C=O
H-N
--H-N
c=o---
--O=C
N-HN-H
c=o
H-N
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I Figure 1. Molecular structure of silk protein. Hydrogen bonds are shown as dotted lines connecting protein strands whose N-terminal and C-terminal chain ends run counter to each other along neighboring chains (anti-parallel /3-sheet). R-groups on the alpha carbons are not shown.
larval instar just before molt to the pupa. The silk is produced in modified salivary glands and spun from the mouth. The cocoons are harvested and processed commercially in a labor-intensive process for the production of a fine continuous filament silk fiber. Spiders produce many different types of silk such as dragline and orb frame (from the major ampullate gland), capture (from the f lagelliform gland), glue (from the aggregate gland), support fibers for the orb frame (from the minor ampullate gland), cocoon (from the cylindrical gland), wrapping prey (from the aciniform gland), and attachment silk for environmental substrates (from the piriform gland). Some of these silks are produced throughout the spider's lifecycle. Each type of silk generally has a unique amino acid sequence, an
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