Complete covalent structure of a proline-rich phosphoprotein, PRP-2, an inhibitor of calcium phosphate crystal growth from human parotid saliva

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Abstract
Human salivary secretions contain many proteins in which proline forms an unusually large fraction of the amino-acid residues present, typically from 20% to over 40% (1,2). These proteins are also unusually rich in glycine and glutamine, generally account for over half the total protein in saliva, and include acidic (3,4), basic (5-7) and glycosylated (8) molecules. The functions of most of these are not clearly defined. One group, however, the acidic proline-rich phosphoproteins (PRP), have been shown to be potent inhibitors of secondary precipitation (crystal growth) of calcium phosphate salts (9-11). Acting together with a salivary protein inhibitor of primary precipitation of calcium phosphates, statherin (12), the PRP stabilize saliva which is supersaturated with respect to the calcium phosphate salts which form dental enamel (13). These inhibitory activities act to provide a protective, reparative, but stable environment for dental enamel, which is important for maintaining the health of the teeth (14). The PRP are a complex group of phosphoproteins which include four major (3) and at least eight minor members (15). The primary structures of three of the major proteins have been determined. These are PRP-1 also designated Protein-C (16), PRP-3, also designated Protein-A (17), and PRP-4 (18). The designations PRP-1,-2,-3 and -4 (3) will be used here. The purpose of this paper is to report the complete primary structure of PRP-2 as a further step towards establishing the structural basis of the biological activity of the PRP, and clarifying the genetic and biosynthetic relationships of these closely related proteins.