Proteolysis of Synaptobrevin, Syntaxin, and Snap-25 in Alveolar Epithelial Type II Cells

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Abstract
Synaptobrevin‐2, syntaxin‐1, and SNAP‐25 were identified in rat alveolar epithelial type II cells by Western blot analysis. Synaptobrevin‐2 was localized in the lamellar bodies, and syntaxin‐1 and SNAP‐25 were found in 0.4% Nonidet P40‐soluble and ‐insoluble fractions, respectively, of the type II cells. When the isolated type II cells were stimulated for secretion with calcium ionophore A23187 or with phorbol 12‐myristate 13‐acetate, these proteins were found to have been proteolyzed. Preincubation of cells with calpain inhibitor II (N‐acetylleucylleucylmethionine), however, prevented the proteolysis. Treatment of the cell lysate with exogenous calpain resulted in a time‐dependent decrease of these proteins. The data suggest that synaptobrevin, syntaxin, and SNAP‐25 are subject to proteolytic modification by activated calpain in intact type II cells stimulated for secretion.

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