Submitted on October 18, 2007
Accepted on January 1, 2008
Non-redundant Function of Secretory Carrier Membrane Protein (SCAMP) Isoforms in Dense Core Vesicle Exocytosis
Haini Liao1, Jie Zhang1, Svetlana Shestopal1, Gabor Szabo2, Anna Castle1, and John David Castle1*
1 Cell Biology, University of Virginia, Charlottesville, Virginia, United States
2 Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, Virginia, United States
* To whom correspondence should be addressed. E-mail: jdc4r{at}virginia.edu.
Five secretory carrier membrane proteins (SCAMPs 1-5) have been characterized in mammalian cells. Previously, SCAMPs 1 and 2 have been implicated to function in exocytosis. RNAi-mediated deficiency of one or both of these SCAMPs interferes with dense core vesicle (DCV) exocytosis in neuroendocrine PC12 cells as detected by amperometry. Knockdowns of these SCAMPs each decreased the number and frequency of depolarization-induced exocytotic events. SCAMP2 but not SCAMP1 depletion also delayed the onset of exocytosis. Both knockdowns, however, altered fusion pore dynamics, increasing rapid pore closure and decreasing pore dilation. In contrast, knockdowns of SCAMPs 3 and 5 only interfered with the frequency of fusion pore opening and did not affect the dynamics of newly opened pores. None of the knockdowns noticeably affected upstream events including the distribution of DCVs near the plasma membrane and calcium signaling kinetics, although noradrenalin uptake/storage was moderately decreased by deficiency of SCAMPs 1 and 5. Thus SCAMPs 1 and 2 are most closely linked to the final events of exocytosis. Other SCAMPs collaborate in regulating fusion sites but the roles of individual isoforms appear at least partially distinct.
