Proprotein-Processing Endopeptidases of the Insulin Secretory Granule

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Abstract
Enzymological studies have implicated two Ca^2+ dependent endopeptidases in the conversion of proinsulin to insulin: a type 1 activity and a type 2 activity which cleave on the C-terminal side of R^31R^32 and K^64R^65 in proinsulin, respectively. These activities were further characterized and their relationship to the mammalian family of subtilisin-like proteases was investigated. PC2 was expressed in neuroendocrine tissues and in insulinoma secretory granule fractions predominantly as a 65kDa protein. On anion-exchange chromatography of solubilized granules, PC 1/3 immunoreactivity comigrated with a peak of type 1 activity whereas PC2 immunoreactivity coeluted with the peak of type 2 endopeptidase activity. PC2 antiserum gave a specific immunoprécipitation of type 2 activity from insulin granule extracts. It was concluded that the PC2 gene-product has type 2 endopeptidase activity.

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