Primary structure of a novel human salivary acidic proline-rich protein

Human salivary acidic proline-rich proteins (PRPs) form a significant fraction of the total salivary protein and fulfill several biologically important roles in the oral cavity. Five commonly occurring PRP polymorphisms, Db, Pa, PIF, Pr2 and Pr1, have been identified, their structures determined, and several uncommon polymorphisms (frequencies < 1:100) have been reported. Most PRPs occur as protein pairs, because of an unusual, limited but well-controlled post-translational cleavage. We now describe an additional uncommon polymorphism, found in the saliva of one of 127 individuals examined in a recent study, identified by high performance anion-exchange liquid chromatography. By analogy with previous terminology, we designate this protein pair as PRP-5, for the primary 150-residue polypeptide gene product, and PRP-6, for the secondary 106-residue cleavage product. Amino acid analysis of intact PRP-6 and sequence determination of PRP-6 chymotryptic peptides, residues 15-24 and 26-35, show a single difference in PRP-6, compared to the most similar, characterized PRP, PRP-4, in that residue 30 is histidine in PRP-6, rather than arginine as in PRP-4 and in all the other sequenced PRPs. This substitution may have implications for the resistance of this polymorphic variant to degradation by trypsin-like enzymes originating from the oral microflora.