Nonconverted, amino acid analog-modified proinsulin stays in a Golgi-derived clathrin-coated membrane compartment.

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Abstract
The secretion of insulin by the [rat] pancreatic B-cell involves a passage of the newly synthetized (pro)insulin polypeptides across the Golgi apparatus, at the trans pole of which secretory proteins are released as a population of secretory granules characterized by a clathrinlike coat on segments of their limiting membrane. When the conversion of radiolabeled proinsulin to insulin was inhibited by replacing Arg and Lys with the aminoacid analogs, canavainine and thialysine, the nonconverted radioactive material remained associated with Golgi-derived, coated secretory granules. The coat was characterized as clathrin-containing by immunocytochemistry. Under analog treatment, the noncoated, storage secretory granules did not become markedly during the pulse-chase experiment. These data are compatible with the hypothesis that in normal conditions, the maturation of the coated compartment into noncoated granules is linked to the effective conversion of the prohormone.