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J. Biol. Chem., Vol. 277, Issue 12, 9629-9632, March 22, 2002

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ACCELERATED PUBLICATION
Rabconnectin-3, a Novel Protein That Binds Both GDP/GTP Exchange Protein and GTPase-activating Protein for Rab3 Small G Protein Family^*

Fumiko Nagano^§, Hiroshi Kawabe^§, Hiroyuki Nakanishi, Masahiko Shinohara, Maki Deguchi-Tawarada^¶, Masakazu Takeuchi^¶, Takuya Sasaki, and Yoshimi Takai^**

From the  Department of Molecular Biology and Biochemistry, Osaka University Graduate School of Medicine/Faculty of Medicine, Suita 565-0871, the ^¶ KAN Research Institute Inc., 1 Chudoji-Awatacho, Shimogyo-ku, Kyoto 600-8815, and the  Department of Biochemistry, Tokushima University School of Medicine, Tokushima 770-8503, Japan

Rab3A, a member of the Rab3 small G protein family, regulates Ca²⁺-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.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) AF389880.

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