Biosynthesis and secretion of rat salivary proteins by Xenopus laevis oocytes

Xenopus laevis oocytes injected with poly (A+) RNA isolated from rat parotid and submandibular glands synthesize and secrete salivary proteins. Amylase was identified in the media of cultured oocytes injected with rat parotid mRNA by size and immunoprecipitation with anti-human amylase serum. Secretion of the salivary proteins was detectable in the medium eight h following the parotid mRNA injection and continued in a time-dependent fashion for up to 96 h. In contrast to rat parotid slices in culture, which demonstrate a regulated pathway of secretion highly responsive to the secretagogue isoproterenol, secretion of salivary proteins by oocytes did not respond to the stimulation by isoproterenol. Though parotid mRNA is presumed to contain the templates encoding the regulated pathway of secretion, reconstitution of this pathway of secretion in oocytes was not observed in our experiments. Since Xenopus laevis oocytes secrete constitutively significant amounts of proteins when injected with salivary gland mRNA, they are a useful biological system for the analysis of secretion, processing, and function of salivary proteins.