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Vol. 19, Issue 2, 722-734, February 2008

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Munc18-1 Is Critical for Plasma Membrane Localization of Syntaxin1 but Not of SNAP-25 in PC12 Cells

Lakshmanan Arunachalam^*,, Liping Han^*,, Nardos G. Tassew, Yu He^,, Li Wang^*, Li Xie^,, Yoshihito Fujita^*, Edwin Kwan^,, Bazbek Davletov^||, Philippe P. Monnier^,, Herbert Y. Gaisano^*,,, and Shuzo Sugita^*,

*Division of Fundamental Neurobiology and Division of Genetics and Development, Toronto Western Research Institute, University Health Network, Toronto, Ontario, M5T 2S8, Canada; Departments of Physiology and Medicine, Faculty of Medicine, University of Toronto, Ontario, M5S 1A8, Canada; and ^||Medical Research Council, Laboratory of Molecular Biology, Cambridge CB2 2QH, United Kingdom

Submitted July 13, 2007; Revised October 31, 2007; Accepted November 29, 2007
Monitoring Editor: Benjamin Glick

Although Munc18-1 was originally identified as a syntaxin1–interacting protein, the physiological significance of this interaction remains unclear. In fact, recent studies of Munc18-1 mutants have suggested that Munc18-1 plays a critical role for docking of secretory vesicles, independent of syntaxin1 regulation. Here we investigated the role of Munc18-1 in syntaxin1 localization by generating stable neuroendocrine cell lines in which Munc18-1 was strongly down-regulated. In these cells, the secretion capability, as well as the docking of dense-core vesicles, was significantly reduced. More importantly, not only was the expression level of syntaxin1 reduced, but the localization of syntaxin1 at the plasma membrane was also severely perturbed. The mislocalized syntaxin1 resided primarily in the perinuclear region of the cells, in which it was highly colocalized with Secretogranin II, a marker protein for dense-core vesicles. In contrast, the expression level and the plasma membrane localization of SNAP-25 were not affected. Furthermore, the syntaxin1 localization and the secretion capability were restored upon transfection-mediated reintroduction of Munc18-1. Our results indicate that endogenous Munc18-1 plays a critical role for the plasma membrane localization of syntaxin1 in neuroendocrine cells and therefore necessitates the interpretation of Munc18-1 mutant phenotypes to be in terms of mislocalized syntaxin1.

Address correspondence to: Shuzo Sugita (ssugita{at}uhnres.utoronto.ca)