Basic proline-rich proteins from human parotid saliva: relationships of the covalent structures of ten proteins from a single individual.

Eleven basic proline-rich proteins were purified from the parotid saliva of a single individual. The complete amino acid sequences of six of these were determined by conventional protein sequence methodology, bringing to nine the number of known primary structures of nonglycosylated basic proline-rich proteins from the same individual. The partial sequence of one additional protein is also reported. All of the basic proline-rich proteins studied contain segments with identical or very similar sequences, but with two possible exceptions, none of the proteins is derived from another secreted proline-rich protein. The amino acid sequences of nine nonglycosylated basic proline-rich proteins were compared with primary structures deduced from published nucleotide sequences of DNA coding for human parotid proline-rich proteins. The sequences align well, in general, but differences also exist pointing to the complexity of the genetics of these proteins. Seven secretory basic proline-rich proteins appear to be formed from three larger precursors by selective posttranslational proteolyses of arginyl bonds. One of the basic proline-rich proteins appears to derive from human acidic proline-rich proteins. The remaining two proteins studied do not conform to any DNA structure as yet reported. Two of the basic proline-rich proteins studied are phosphoproteins and exhibit abilities to inhibit hydroxyapatite formation in vitro.