A camel milk whey protein rich in half-cystine. Primary structure, assessment of variations, internal repeat patterns, and relationships with neurophysin and other active polypeptides.

The amino acid sequence of a recently isolated camel milk protein rich in half-cystine has been determined by peptide analyses. The 117-residue protein has 16 half-cystine residues, concluded to correspond to disulfide bridges and suggesting a tight conformation of the molecule. Comparisons of the structure with those of other proteins reveal several interesting relationships. The camel protein is clearly homologous with a previously reported rat whey phosphoprotein of possible importance for mammary gland growth regulation, and with a mouse protein of probable relationship to neurophysins. The camel, rat and mouse proteins may represent species variants from a rapidly evolving gene. Residue identities in pairwise comparisons are 40% for the camel/rat proteins and 33% for the camel/mouse proteins, with 38 positions conserved in all three forms. The camel protein also reveals an internal repeat pattern similar to that for the other two proteins. The homology between the three milk whey proteins has wide implications for further relationships. Thus, previously noticed similarities, involving either of the milk proteins, include limited similarities to casein phosphorylation sites for the camel protein, to neurophysins in repeat and half-cystine patterns for the mouse and rat proteins, and to an antiprotease for the rat protein. These similarities are reinforced by the camel protein structure and the recognition of the three whey proteins as related. Finally a few superficial similarities with the insulin family of peptides and with some other peptides of biological importance are noticed. Combined, the results relate the camel protein in a family of whey proteins, and extend suggestions of relationships with some binding proteins.