Silkmoth chorion proteins. Their diversity, amino acid composition, and the NH-terminal sequence of one component

Silkmoth eggshell (chorion) proteins have been characterized by electrophoresis on sodium dodecyl sulfate and isoelectric focusing polyacrylamide gels; up to 33 and 41 components, respectively, were detected from a single chorion. Some of these components are polymorphic, being absent from chorions of certain animals. A system of nomenclature for all chorion proteins is presented, based on their separation on sodium dodecyl sulfate and isoelectric focusing gels. The chorion is enriched in glycine, alanine, cysteine, and tyrosine and poor in methionine and histidine. The proteins were fractionated into four partially overlapping groups; all four are enriched in the above amino acids, although significant differences exist. Further fractionation by isoelectric focusing of one of the above groups, s/s, yielded seven components, two of which are homogeneous both on sodium dodecyl sulfate and isoelectric focusing gels. The amino acid compositions, molecular weights, and solubility properties of the components share certain features which distinguish s/s as a group from the other three groups. The sequence of the first 67 NH2-terminal residues of a homogeneous protein purified from s/s has been determined. The protein contains a cysteine-rich tail (3 cysteines in the first 18 residues) followed by a 49-residue segment which contains only a single cysteine residue. This latter segment also contains two different tetrapeptide sequences which are each repeated, one twice and the other four times.