Localization of Rabphilin-3A, a Putative Target Protein for Rab3A, at the Sites of Ca²⁺-Dependent Exocytosis in PC12 Cells

Rab3A/Smg 25A, a small GTP-binding protein, is highly concentrated in presynapse of neurons and implicated in neurotransmitter release. We have recently identified a putative target protein for Rab3A, isolated its cDNA, and designated it as Rabphilin-3A. To examine whether Rabphilin-3A as well as Rab3A is localized at the sites of Ca²⁺-dependent exocytosis, we investigated here localization of Rabphilin-3A and Rab3A in comparison with the sites of exocytosis ira the differentiated PC12 cells. Rabphilin-3A as well as Rab3A was highly concentrated at the tips of the neurites where Ca²⁺-dependent exocytosis took place. Inversely, neither Rabphilin-3A nor Rab3A was concentrated at the tips of the neurites where Ca²⁺-dependent exocytosis did not take place. These results suggest that Rabphilin-3A as well as Rab3A constitutes a part of the machinery necessary for neurotransmitter release.