| GRAB : a physiologic guanine nucleotide exchange factor for Rab3A , which interacts with inositol hexakisphosphate kinase . ^^^ We now report the identification , cloning , and characterization of a novel protein , GRAB ( guanine nucleotide exchange factor for Rab3A ) , which interacts with both InsP6K1 and Rab3A , a Ras like GTPase that regulates synaptic vesicle exocytosis . ^^^ GRAB is a physiologic GEF ( guanine nucleotide exchange factor ) for Rab3A . ^^^ Consistent with a role of Rab3A in synaptic vesicle exocytosis , GRAB regulates depolarization induced release of dopamine from PC 12 cells and nicotinic agonist induced hGH release from bovine adrenal chromaffin cells . ^^^ |