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J Biol Chem. 2002 Mar 22;277(12):9629-32. Epub 2002 Jan 24.

Rabconnectin-3, a novel protein that binds both GDP/GTP exchange protein and GTPase-activating protein for Rab3 small G protein family.

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Department of Molecular Biology and Biochemistry, Osaka University Graduate School of Medicine/Faculty of Medicine, Suita 565-0871, Japan.


Rab3A, a member of the Rab3 small G protein family, regulates Ca(2+)-dependent exocytosis of neurotransmitter. The cyclical activation and inactivation of Rab3A are essential for the Rab3A action in exocytosis. GDP-Rab3A is activated to GTP-Rab3A by Rab3 GDP/GTP exchange protein (Rab3 GEP), and GTP-Rab3A is inactivated to GDP-Rab3A by Rab3 GTPase-activating protein (Rab3 GAP). It remains unknown how or in which step of the multiple exocytosis steps these regulators are activated and inactivated. We isolated here a novel protein that was co-immunoprecipitated with Rab3 GEP and GAP by their respective antibodies from the crude synaptic vesicle fraction of rat brain. The protein, named rabconnectin-3, bound both Rab3 GEP and GAP. The cDNA of rabconnectin-3 was cloned from a human cDNA library and its primary structure was determined. Human rabconnectin-3 consisted of 3,036 amino acids and showed a calculated M(r) of 339,753. It had 12 WD domains. Tissue and subcellular distribution analyses in rat indicated that rabconnectin-3 was abundantly expressed in the brain where it was enriched in the synaptic vesicle fraction. Immunofluorescence and immunoelectron microscopy revealed that rabconnectin-3 was concentrated on synaptic vesicles at synapses. These results indicate that rabconnectin-3 serves as a scaffold molecule for both Rab3 GEP and GAP on synaptic vesicles.

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