Recent studies have suggested that two small GTPases, Rab3A and Rab27A, play a key role in the late steps of dense-core vesicle exocytosis in endocrine cells; however, neither the precise mechanisms by which these two GTPases regulate dense-core vesicle exocytosis nor the functional relationship between them is clear. In this study, we expressed a number of different Rab proteins, from Rab1 to Rab41 in PC12 cells and systematically screened them for those that are specifically localized on dense-core vesicles. We found that four Rabs (Rab3A, Rab27A, Rab33A, Rab37) are predominantly targeted to dense-core vesicles in PC12 cells, and that three of them (Rab3A, Rab27A, Rab33A) are endogenously expressed on dense-core vesicles. We further investigated the effect of silencing each Rab with specific small interfering RNA on vesicle dynamics by total internal reflection fluorescence microscopy in a single PC12 cell. Silencing either Rab3A or Rab27A in PC12 cells significantly decreased the number of dense-core vesicles docked to the plasma membrane without altering the kinetics of individual exocytotic events, whereas silencing of Rab33A had no effect at all. Simultaneous silencing of Rab3A and Rab27A caused a significantly greater decrease in number of vesicles docked to the plasma membrane. Our findings indicate that Rab3A and Rab27A cooperatively regulate docking step(s) of dense-core vesicles to the plasma membrane.
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