Membrane localization and biological activity of SNAP-25 cysteine mutants in insulin-secreting cells.

The tSNARE SNAP-25 is expressed in pancreatic (beta)-cells and is involved in the regulated release of insulin. It has been shown previously that SNAP-25 associates with the plasma membrane consequent to palmitoylation of one or more cysteines in the central region of the molecule. The importance of palmitolyation in the biological function of SNAP-25 in exocytosis was not addressed. Furthermore, studies on both SNAP-25 and its non-palmitoylated homologues SNAP-29 and sec9, have suggested an alternative or complementary mechanism for membrane association involving interaction with syntaxin. To address these issues, we have now studied the behavior and biological activity of cysteine mutant SNAP-25 in insulin-secreting (HIT) cells. While 91% of native SNAP-25 was associated with the membrane, this value decreased to 56% for the single cysteine mutant C85/A and to 10% for the double (C85,88/A) and quadruple (C85,88,90,92/A) mutants. The mutant SNAP-25 forms were all found to bind syntaxin 1A with equal efficacy. Over-expression of syntaxin 1A in HIT cells allowed for partial relocalization of both the double and quadruple SNAP-25 cys mutants to the membrane. By introducing a further mutation to the SNAP-25 molecules to render them resistant to botulinum neurotoxin E, it was possible to study their ability to reconstitute regulated insulin secretion in toxin-treated HIT cells. Native SNAP-25 was able to fully reconstitute secretory activity in such cells. Despite the fact that the single cysteine mutant was significantly displaced to the cytosol, it still displayed 82% activity in the secretion reconstitution assay, and a similar discrepancy was seen for the double mutant. Even the quadruple mutant with no remaining cysteines was able to support a minimal level of secretion. It is concluded that both palmitoylation and binding to syntaxin are implicated in membrane association of SNAP-25. This as well as the discrepancy between membrane localization and biological activity of the cysteine mutants, suggests a complex, multi-component process for association of SNAP-25 with the membrane and its recruitment to a biologically productive state.