Central strand well-developed

Insects and arachnoids produce well-known amyloids. Silk and spider webs, like P-keratin, also differ from amyloids in being fibrous P-sheet proteins composed of peptide strands that are parallel, rather than perpendicular, to the direction of the fibril axis. For the process of silk formation by spiders, it has been proposed that fibrils in the silk gland have an initial cross-P structure (Kenney et al. 2002 Table 3) that, when stretched, assume parallel P-structures. However, X-ray diffraction for a peptide derived from the central domain of the A class of chorion proteins, derived frovaAntheraea polyphemus eggshells, displayed P-sheets perpendicular to the fibril axis, the same cross-P structure that occurs in amyloid proteins (Iconomidou et al. 2000 Table 3). The stability and strength of the amyloid fibres provides mechanical and biological protection for the oocyte and developing embryo from a variety of environmental and predatory hazards. 

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