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

The structure of unusual high-cysteine (Hc) proteins (ca. 30 mol %), which are characteristic of the chorion of the silkmoth Bombyx mori, has been determined by determining the sequence of a corresponding cDNA clone. The Hc protein sequence has 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 two conservative parts, those encoding the amino-terminal signal peptide and the highly structured central region of the protein, only base substitutions have been accepted. By contrast, in two alternating parts, which encode variable 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.