Origin of evolutionary novelty in proteins: how a high-cysteine chorion protein has evolved.

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Abstract
The structure of unusual high-cysteine (Hc) proteins (.apprx. 30 mol%), which are characteristic of the chorion of the silkmoth Bombyx mori, was determined by determining the sequence of a corresponding c[complementary]DNA clone. The Hc protein sequence evolved from a family of more ordinary chorion genes, in large part through fixation of mutations leading to enhanced cysteine content. Mutations of different types are differentially distributed in different parts of the sequence. In 2 conservative parts, those encoding the amino-terminal signal peptide and the highly structured central region of the protein, only base substitutions were accepted. In 2 alternating parts, which encode varible arms flanking the central region, deletions and duplications of tandemly repetitive sequences are prominent. Both base substitutions and expansions or deletions of tandemly repetitive elements are important in the evolution of this type of protein; functional constraints of the various protein domains dictate which class of mutations can be accepted.