A comparison of the composition of silk proteins produced by spiders and insects

Proteins that are highly expressed and composed of amino acids that are cos tly to synthesize are likely to place a greater drain on an organism's ener gy resources than proteins that are composed of ingested amino acids or one s that are metabolically simple to produce. Silks are highly expressed prot eins produced by all spiders and many insects. We compared the metabolic co sts of silks spun by arthropods by calculating the amount of ATP required t o produce their component amino acids. Although a definitive conclusion req uires detailed information on the dietary pools of amino acids available to arthropods, on the basis of the central metabolic pathways, silks spun by herbivorous, Lepidoptera larvae require significantly less ATP to synthesiz e than the dragline silks spun by predatory spiders. While not enough data are available to draw a statistically based conclusion, comparison of homol ogous silks across ancestral and derived taxa of the Araneoidea seems to su ggest an evolutionary trend towards reduced silk costs. However, comparison of the synthetic costs of dragline silks across all araneomorph spiders su ggests a complicated evolutionary pattern that cannot be attributed to phyl ogenetic position alone. We propose that the diverse silk-producing systems of the araneoid spiders (including three types of protein glues and three types of silk fibroin), evolved through intra-organ competition and that ta xon-specific differences in the composition of silks drawn from homologous glands may reflect limited or fluctuating amino acid availability. The diff erent functional properties of spider silks may be a secondary result of se lection acting on different polypeptide templates. (C) 1999 Elsevier Scienc e B.V. All rights reserved.