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A more recent version of this article appeared on February 1, 2006
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Submitted on May 3, 2005
Accepted on November 14, 2005
Department of Biochemistry, University of Wisconsin, Madison, WI 53706
Monitoring Editor: Keith Mostov
SNARE (soluble N-ethylmaleimide sensitive fusion protein attachment protein receptor) proteins mediate cellular membrane fusion events and provide a level of specificity to donor-acceptor membrane interactions. However, the trafficking pathways by which individual SNARE proteins are targeted to specific membrane compartments are not well understood. In neuroendocrine cells, SNAP25 (synaptosome-associated protein of 25 kDa) is localized to the plasma membrane where it functions in regulated secretory vesicle exocytosis, but it is also found on intracellular membranes. We identified a dynamic recycling pathway for SNAP25 in PC12 cells through which plasma membrane SNAP25 recycles in
3 h.
20% of the SNAP25 resides in a perinuclear recycling endosome-TGN compartment from which it recycles back to the plasma membrane. SNAP25 internalization occurs by constitutive, dynamin-independent endocytosis that is distinct from the dynamin-dependent endocytosis that retrieves secretory vesicle constituents following exocytosis. Endocytosis of SNAP25 is regulated by ARF6 (through PIP2 synthesis) and is dependent upon F-actin. SNAP25 endosomes, which exclude the plasma membrane SNARE syntaxin 1A, merge with those derived from clathrin-dependent endocytosis containing endosomal syntaxin 13. Our results characterize a robust ARF6-dependent internalization mechanism that maintains an intracellular pool of SNAP25, which is compatible with possible intracellular roles for SNAP25 in neuroendocrine cells.
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