Visualization of Rab3A dissociation during exocytosis: a study by total internal reflection microscopy.

Rab3A is a small G protein in the Rab3 subfamily, and is thought to act at late stage of exocytosis. However, the detailed mechanism of its action is not completely understood. To study the role of Rab3A in exocytosis, we used a total internal reflection fluorescence microscope to examine the fluorescence changes of EGFP-Rab3A-labeled and NPY-EGFP-labeled vesicles in PC12 cells upon stimulation. The fluorescence of EGFP-Rab3A-labeled and NPY-EGFP-labeled vesicles decreased while showing different patterns. The NPY-EGFP-labeled vesicles that exocytosed showed a transient fluorescence increase before NPY-EGFP fluorescence disappearance, which represents fusion and NPY release. This transient increase was diminished in cells that co-expressed the GDP-bound Rab3A mutant. The fluorescence of EGFP-Rab3A-labeled vesicles dispersed before disappearance, which represents the dissociation of Rab3A from the vesicles. The dispersion was not found in GTP-bound Rab3A mutant-labeled vesicles. Interestingly, EGFP-Rab3A F59S, a mutant unable to bind rabphilin, dissociates slower from the vesicles than wild type Rab3A and caused a slower release of NPY-EGFP. The results provide direct evidence to support the hypothesis that GTP hydrolysis and rabphilin are involved in Rab3A dissociation from the vesicles and the occurrence of exocytosis.