Exocytotic stimulation promotes association of the ADP-ribosylation factor with PC12 cell membranes.
ADP-ribosylation factors (ARFs) are a family of small molecular, monomeric GTP-binding (G) proteins, initially identified by their ability to enhance cholera toxin (CTX) ADP-ribosyltransferase activity. ARFs have been implicated in protein transport and vesicle and endosome fusion. Although several reports show that synthetic peptides of the N-terminus of ARF inhibited Ca(2+)-dependent exocytosis in permeabilized adrenal chromaffin cells, the role of ARFs in exocytosis has not been established. In this study, we investigated the translocation of ARFs to the membrane fraction from the cytosol fraction in PC12 cells after exocytotic stimulation by measuring the immunoreactivity of ARFs (with anti-ARF anti-serum and with anti-ARF3 antibodies) and enzymatic ARF activity, which enhances the CTX effect. Both the immunoreactivity and the enzymatic activity of ARF in the membrane fraction increased about twofold, significantly, after exocytotic stimulation with ATP and KCl. The translocation of ARF and noradrenaline release was observed in the presence of extracellular CaCl2, but not in the absence of CaCl2. The ARF translocated to the membrane fraction after stimulation in intact cells seemed to be an inactive, perhaps is the GDP form, because ARF did not activate CTX in the absence of guanosine 5'-O-(thiotriphosphate) (GTP gamma S). As previously reported, ARF in the active, GTP gamma S-bound state bound to the membrane fractions. Thus ARF may have been active during translocation and inactivated later. The immunoreactivity of Gs alpha, one of the trimeric G proteins, was not changed before or after stimulation. These findings suggest that ARFs translocate to membranes from the cytosolic fraction after exocytotic stimulation in PC12 cells, and raise the possibility that ARFs regulate exocytosis.