Isolation and characterization of six proteins from rabbit parotid saliva belonging to a unique family of proline-rich proteins

Abstract

Proline-rich proteins are major components of salivary secretion from humans, nondashhuman primates, rats, hamsters and rabbits. They are also synthesized in mice in response to chronic stimulation by β agonists. This study seeks to provide an understanding of the structural and genetic relationships within these families of proteins and to determine the possible function of the proline-rich proteins. Rabbit parotid saliva was collected and proline-rich proteins were affinity purified using goat antibodies to human proline-rich proteins. Further purification was achieved by repeated cation exchange chromatography on a Mono S column using a Fast Protein Liquid Chromatography system. Six basic proline-rich proteins were purified. The apparent molecular weights were between 75,000 and 125,000, based on their mobilities in sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Glycine, glutamine (and glutamate) and proline accounted for 79–87% of total amino acids in all proteins, but proline was present in smaller amounts (17–21%) than in proline-rich proteins from other species. All proteins were glycosylated but not phosphorylated. Circular dichroism of two proline-rich proteins, MS7A and MS5B, indicated the absence of secondary structure. The ndashterminal sequences of three proteins electro-eluted after preparative gel electrophoresis were determined. A high degree of similarity was found in various regions of mouse, rat, monkey and human proline-rich proteins. Rabbits thus synthesize constitutively a family of proteins that are immunologically and structurally related to proline-rich proteins from other species.