The biosynthesis of a component
SGM 110, specifically localized to the membrane of
insulin secretory granules, was studied in rat
insulinoma cells and in normal islets of Langerhans. Cells or islets were labelled with [35S]
methionine or [3H]
mannose and
SGM 110 was immunoprecipitated by using a
monoclonal antibody. Pulse-chase experiments demonstrated that the nascent
polypeptide was cotranslationally glycosylated to form a 97,000 Da
peptide which in turn was processed to the mature 110,000 Da form. A 50,000 Da form detected by immunoblotting with the same antibody was not conspicuously labelled even after a 20 h chase incubation, suggesting that it represented late processing of
SGM 110 in lysosomes. With
insulinoma cells, an increase in medium
glucose concentration from 3 mM to 20 mM was without effect on the secretion of
insulin or on the biosynthesis of (pro)
insulin or
SGM 110. In normal islets, however, 20 mM-
glucose produced a 17-fold increase in (pro)
insulin biosynthesis and a 13-fold increase in
SGM 110 biosynthesis, compared with only a 2-fold increase in total
protein synthesis, as judged by incorporation of [35S]
methionine during a 1 h incubation. The effect of
glucose on both (pro)
insulin and
SGM 110 biosynthesis was blocked by the addition of
mannoheptulose, but not by the removal of extracellular
calcium, both of which conditions inhibit insulin secretion. In contrast
tolbutamide, an agent which stimulates insulin secretion, did not enhance the biosynthesis of (pro)
insulin or
SGM 110. It is concluded that at least one
protein component of the
insulin secretory granule membrane is synthesized co-ordinately with
proinsulin and is subject to similar regulatory mechanisms. Factors which acutely control insulin secretion may also control granule biogenesis, although the two processes are not coupled in an obligatory fashion.