Rabphilin-3A binds to a M_r 115,000 polypeptide in a phosphatidylserine- and Ca²⁺-dependent manner

Rabphilin-3A is a putative target protein for Rab3A/Smg 25A, which is a member of the Ras-related small GTP-binding protein and implicated in neurotransmitter release from the synapse. Rabphilin-3A is composed of two functionally different domains: the N-terminal Rab3A-binding and the C-terminal phosphatidylserine- and Ca²⁺-binding domains. The C-terminal domain has two copies of an internal repeat that are homologous to the C2 domains of protein kinase C, synaptotagmin, and phospholipase A₂ and C-γl, which are known to bind phosphatidylserine and Ca²⁺. In this study, we attempted to identify the Rabphilin-3A-interacting molecule in bovine brain by use of an overlay assay technique. The ³²P-labeled C-terminal fragment of Rabphilin-3A (281-704 amino acids) bound to a protein molecule with a M_r of about 115 kDa which was immobilized on a nitrocellulose sheet. This protein was highly purified and characterized. The binding of the ³²P-labeled C-terminal fragment to this protein was dependent on both phosphatidylserine and Ca²⁺, and inhibited by an excess amount of the C-terminal fragment and the C2 domain fragment (396-704 amino acids) but not by the N-terminal fragment (1-280 amino acids). These results indicate that Rabphilin-3A binds to a protein molecule with a M_r of 115 kDa through the C2 domain in the presence of phosphatidylserine and Ca²⁺.