Spider flagelliform silk: lessons in protein design, gene structure, and molecular evolution.

Spiders spin multiple types of silks that are renowned for their superb mechanical properties. Flagelliform silk, used in the capture spiral of an orb-web, is one of the few silks characterized by both cDNA and genomic DNA data. This fibroin is composed of repeating ensembles of three types of amino acid sequence motifs. The predominant subrepeat, GPGGX, likely forms a beta-turn, and tandem arrays of these turns are thought to create beta-spirals. These spring-like helices may be critical for the exceptional ability of capture silk to stretch and recoil. Each ensemble of motifs was found to correspond to a different exon within the flagelliform gene. The pattern of sequence similarity among exons indicates intragenic concerted evolution. Surprisingly, the introns between the iterated exons are also homogenized with each other. This unusual molecular architecture in the flagelliform silk gene has implications for the evolution and maintenance of spider silk proteins.