Amino acid periodicities and their structural implications for the evolutionarily conservative central domain of some silkmoth chorion proteins

The central domain is an evolutionarily conservative region that is invariant in length in the A and Hc-A families of silkmoth chorion proteins. This domain shows strong sixfold periodicities for various amino acid residues, such as glycine and large non-polar residues. The periodicities and their phase relationships, together with the documented prevalence of β-sheets and β-turns in the chorion, strongly support a secondary structure model in which short (4-residue) β-sheet strands alternate with β-turns, forming a compact antiparallel, probably twisted β-sheet. This structure should be important for the establishment of higher order structure in the chorion.