The primary structure of a salivary calcium-binding proline-rich phosphoprotein (protein C), a possible precursor of a related salivary protein A.

The complete primary structure of a calcium-binding "proline-rich phosphoprotein" named salivary Protein C was determined from peptides obtained by enzymatic and chemical cleavages of the protein. The protein consists of a single polypeptide chain of 150 residues. It contains the entire primary structure of a previously isolated salivary Protein A in its NH2-terminal 106 residues. The COOH-terminal 44 residues consist mostly of glycine, glutamine, and proline, including a hexaproline sequence, but no polyproline structure could be detected by CD spectroscopy. There is extensive repetition of sequences in the protein, suggesting gene multiplication and recurrent folding. Comparison of the primary structure of salivary Proteins A and C with known protein sequences indicate that the salivary proteins constitute a new family. A mouse submaxillary protease will cleave salivary Protein C between residues 106 and 107 only, giving rise to salivary Protein A and a 44-residue COOH-terminal peptide. This cleavage and the sequence data suggest that salivary Protein C may be a precursor of salivary Protein A.