Structural models of the evolutionarily conservative central domain of silk-moth chorion proteins

Silk-moth chorion proteins belong to a small number of families: A, B, C, Hc-A and Hc-B. The central domain is an evolutionarily conservative region in each family, of variable length and composition between families. This domain shows clear 6-fold periodicities for various amino acid residues, e.g. glycine. The periodicities, together with the well-documented prevalence of beta-sheet and beta-turn secondary structure of chorion proteins, strongly support a structural model in which four-residue beta-strands alternate with beta-turns, forming a compact antiparallel, probably twisted beta-sheet. Conformational analysis, aided by interactive graphics refinement and recent experimental findings, further suggest that this structure consists of beta-strands, alternating with I' and II' beta-turns, and apparently forms the basis for the molecular and supramolecular assembly of chorion.