Evidence fore biosynthesis and differential post-translational proteolytic processing of different (pre)prosomatostatins in pancreatic islets.

In anglerfish (AF) pancreatic islets, somatostatin-14 (88-14) is synthesized via a precursor-product pathway. Sequence analyses of cDNAs prepared from AF islet mRNA have demonstrated the presence of mRNAs coding for three different AF preprosomatostatins. From these sequences it was predicted that two of the three precursors contain SS-14 at their COOH terminus whereas the third has an analog form, [Tyr7,Gly"]SS14, as its predicted COOH terminus. Results from our studies on AF SS biosynthesis have demonstrated that one major form of SS synthesized is SS-14 and that [Tyr7,Gly'O]SS-14 is not found in readily detectable amounts in AF islets. However, the possibility remained that [Tyr7,Gly"]SS-14 is expressed as part of a larger polypeptide. To examine this alternative, reverse-phase high pressure liquid chromatography was used to perform peptide mapping on the S-carboxymethylated (CM) tryptic products of Mr = 8,000-15,000, M, = 2,500-8,000, and M, = 1,000-2,000 peptides previously labeled in vitro with C3H]tryptophan and [3sS] cysteine. Tryptic peptides generated from the M, = 8,000-15,000 and M, = 2,500-8,000 polypeptides and labeled with 3H or 36S were eluted under different chromatographic conditions. Several of these peptides had retention times which did not deviate significantly from those of the CM tryptic products from both synthetic SS-14 and [Tyr7,Gly'o]SS-14. The M, = 1,000-2,000 peptides yielded only tryptic fragments identical with those generated from SS-14. The identities of the peptides that behaved in reverse-phase high pressure liquid chromatography like the COOH-terminal tryptic fragments of SS-14 and [Tyr7,Gly"]SS-14 were confirmed by amino acid and sequence analyses. These results demonstrate that the gene coding for the [Tyr7,Gly'o]SS-14-containing precursor is expressed and that the product of proteolytic processing of this precursor is significantly larger than SS-14. This indicates that the precursors which contain SS-14 and uyr7,Gly'o]SS-14 are apparently subjected to differential post-translational proteolytic processing.