Cotranslational and Posttranslational Proteolytic Processing of Preprosomatostatin-I in Intact Islet Tissue

• Published in: The Journal of cell biology 1986-10, Vol.103 (4), p.1205-1211
• Main Authors: Noe, Bryan D., Andrews, Phillip C., Dixon, Jack E., Spiess, Joachim
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Amino acids; Analytical, structural and metabolic biochemistry; Animals; Biochemistry; Biological and medical sciences; Biosynthesis; Body tissues; Cell biology; Elution; Fishes - metabolism; Fundamental and applied biological sciences. Psychology; Gels; Islets of Langerhans; Islets of Langerhans - metabolism; Protein hormones. Growth factors. Cytokines; Protein Precursors - metabolism; Protein Processing, Post-Translational; Protein Sorting Signals - metabolism; Proteins; Radioactive decay; Radioimmunoassay; Somatostatin - biosynthesis; Somatostatin - metabolism
• ISSN: 0021-9525; 1540-8140
• DOI: 10.1083/jcb.103.4.1205

Preprosomatostatin-I (PPSS-I) is processed in anglerfish islets to release a 14-residue somatostatin (SS-14). However, very little is known regarding other processing events that affect PPSS-I. This is the first study to identify and quantify the levels of nonsomatostatin products generated as a result of processing of this somatostatin precursor in living islet tissue. The products of PPSS-I processing in anglerfish islet tissue were identified in radiolabeling studies using a number of criteria. These criteria included immunoreactivity, specific radiolabeling by selected amino acids, radiolabel sequencing, and chromatographic comparison to isolated, structurally characterized fragments of anglerfish PPSS-I using reverse-phase high performance liquid chromatography. Intact prosomatostatin-I (aPSS-I) was isolated from tissue incubated with [3 H]tryptophan and [14 C]leucine. Significant 14 C radioactivity was observed in the products of 11 of the first 44 sequencer cycles in positions consistent with the generation of a 96-residue prosomatostatin. These results indicate that signal cleavage occurs after the cysteine located 25 residues from the initiator Met of PPSS-I, resulting in a signal peptide 25 amino acids in length. Nonsomatostatin-containing fragments of the precursor were also found in tissue incubated with a mixture of 3 H-amino acids. Only a small quantity of the dodecapeptide representing residues 69-80 in the prohormone was found (10 nmol/g tissue). Two other fragments of aPSS-I, also observed to be present in low abundance, were found to correspond to residues 1-27 (16 nmol/g tissue) and to residues 1-67 (7 nmol/g tissue) of aPSS-I. No evidence for the presence of the fragment corresponding to residues 29-67 was found. However, large quantities of SS-14 were observed (287 nmol/g tissue), indicating that the major site of aPSS-I cleavage is at the basic dipeptide immediately preceding SS-14. Recovery of much lower levels of the nonsomatostatin fragments of aPSS-I suggests that prohormone processing at the secondary sites identified in this study occurs at a low rate relative to release of SS-14 from aPSS-I.