Rab3D, a small GTPase, is localized on mast cell secretory granules and translocates to the plasma membrane upon exocytosis

Although mast cell secretion has been intensively studied because of its pi votal role in allergic reactions and its advantages as a physiologic model: the molecular composition of the secretory machine is virtually unknown. I n view of the guanine-nucleotide dependency of mast cell exocytosis and the participation of Rab3 proteins in synaptic vesicle release, we hypothesize d that a Rab3 isoform regulates mast cell secretion. Fragments of Rab3A, 3B , and 3D were cloned from RBL-2H3 mast cells by reverse transcription-polym erase chain reaction (RT-PCR). Northern blot analysis revealed Rab3D transc ripts to be relatively abundant, Rab3B substantially less so, and Rab3A and 3C undetectable. By ribonuclease (RNase) protection assay, Rab3D transcrip ts were at least 10-fold more abundant than those of other isoforms, and by immunoblot analysis, Rab3D protein was at least 60-fold more abundant than that of Rab3B. Rab3D was more abundant in RBL cells than in brain, but the total mass of Rab3 proteins in RBL cells was 10-fold less than in brain. R ab3D only partly colocalized with secretory granules in RBL cells, but full y colocalized in mature peritoneal mast cells. There was a descending conce ntration gradient of Rab3D from peripheral to central granules, and no cyto plasmic pool was detectable in resting mast cells. Following exocytotic deg ranulation, Rab3D translocated to the plasma membrane and remained there fo r at least 15 min. These studies suggest that Rab3D is a component of the r egulated exocytotic machine of mast cells, and identify differences between mast cells and neurons in Rab3 expression and trafficking.