In vitro biosynthesis of somatostatin. Evidence for two distinct preprosomatostatin molecules

mRNA isolated from angler fish islets of Langerhans was translated in the wheat germ cell-free protein-synthesizing system and the products identified by immunoprecipitation with specific antibodies to somatostatin followed by sodium dodecyl sulfate gel electrophoresis. As previously shown (Shields, d. (1980) Proc. Natl. Acad. Sci. U. S. A. 77, 4074), a major polypeptide of 18,000 dalton, designated preprosomatostatin, was immunoprecipitable. Here, evidence is presented for an additional somatostatin-immunoreactive polypeptide of apparent Mr = 19,000. The 19 kilodalton polypeptide was similar, but not identical with the 18 kilodalton preprosomatostatin, as determined by tryptic peptide analysis. Comparison of the tryptic peptides of the 19,000 dalton polypeptide with those of unlabeled somatostatin demonstrated that it contained the authentic somatostatin sequence. Like the 18,000 dalton precursor, the 19,000 dalton polypeptide had the mature somatostatin sequence located at its COOH terminus; it is proposed that this molecule is a minor species of preprosomatostatin.