The complete primary structure of a proline-rich phosphoprotein from human saliva

The complete amino acid sequence of a calcium-binding "proline-rich phosphoprotein," named Protein A, from human saliva was determined by automated and manual Edman degradation of peptides obtained by enzymatic and chemical cleavage of the intact protein. The NH2-terminal pyrrolidone carboxylic acid was identified by means of NMR. The protein consists of 106 amino acids, including 24 residues of proline. The NH2-terminal 32 residues contain 13 of the 15 negatively charged residues including 2 phosphoserines, but only 1 proline. In spite of a high concentration of proline in the COOH-terminal part of the molecule, the longest oligoproline sequence is tetraproline. The protein contains a number of repeated sequences and there are also several sequences of 3 or 4 residues identical with known sequences of collagen, but the characteristic occurrence of glycine in every third position in collagen is not found in salivary Protein A.