Rab proteins are Ras-like GTPases that regulate traffic along the secretory or endocytic pathways. Within the Rab family, Rab3 proteins are expressed at high levels in neurons and endocrine cells where they regulate release of dense core granules and synaptic vesicles. Immunoelectron microscopy shows that Rab3A and Rab3D can coexist on the same granule before and after docking. Using electron microscopy of transfected PC12 cells, we report that expression of wild-type Rab3A (or Rab3D) increases the total number of granules and the percentage that is docked at the plasma membrane. Mutated Rab3A N135I (or Rab3D N135I) decreases the total granule number and the fraction of granules docked to the plasma membrane. These data show that at least one of the functions of Rab3A and Rab3D proteins is to control the number of granules docked at the plasma membrane.
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