A novel rabconnectin-3-binding protein that directly binds a GDP/GTP exchange protein for Rab3A small G protein implicated in Ca2+-dependent exocytosis of neurotransmitter
✍ Scribed by Hiroshi Kawabe; Toshiaki Sakisaka; Masato Yasumi; Tatsushi Shingai; Genkichi Izumi; Fumiko Nagano; Maki Deguchi-Tawarada; Masakazu Takeuchi; Hiroyuki Nakanishi; Yoshimi Takai
- Publisher
- John Wiley and Sons
- Year
- 2003
- Tongue
- English
- Weight
- 284 KB
- Volume
- 8
- Category
- Article
- ISSN
- 1356-9597
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✦ Synopsis
Abstract
Background: 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). We have recently found a novel protein, named rabconnectin‐3, which is co‐immunoprecipitated with Rab3 GEP or GAP from the extract of the crude synaptic vesicle (CSV) fraction of rat brain. Rabconnectin‐3 is abundantly expressed in the brain where it is associated with synaptic vesicles. We have found that two more proteins are co‐immunoprecipitated with Rab3 GEP from the CSV fraction of rat brain. We attempted here to isolate and characterize one of them.
Results: We determined its partial amino acid sequence, cloned its cDNA from a human cDNA library, and determined its primary structure. The protein consisted of 1490 amino acids (aa) and showed a calculated molecular weight of 163 808. The protein had 7 WD domains. The protein was abundantly expressed in the brain where it co‐localized with rabconnectin‐3 on synaptic vesicles. The protein formed a stable complex with rabconnectin‐3. We named this protein rabconnectin‐3β and renamed rabconnectin‐3 rabconnectin‐3α. Rabconnectin‐3β, but not rabconnectin‐3α, directly bound Rab3 GEP. Neither rabconnectin‐3α nor ‐3β directly bound Rab3 GAP.
Conclusion: These results indicate that rabconnectin‐3 consists of the α and β subunits and binds directly Rab3 GEP through the β subunit and indirectly Rab3 GAP through an unidentified molecule(s).