There is evidence that the rab class of low molecular weight GTP-binding proteins is involved in vesicular transfer from endoplasmic reticulum to Golgi and between Golgi cisternae. To determine whether similar proteins play a role in regulated exocytosis, the effects of synthetic peptides derived from low molecular weight GTP-binding proteins on catecholamine secretion from digitonin-permeabilized chromaffin cells were investigated. The synthetic peptides represent the putative effector-binding domains of the rab, ras and ral classes of low molecular weight GTP-binding proteins and correspond to ras(33–48). Two rab peptides but neither a ras nor a ral peptide enhanced Ca2+-dependent secretion by approximately 30%. Maximal secretion in response to Ca2+ was increased. The enhancement was not blocked by the pseudosubstrate inhibitor of protein kinase C, PKC(19–31), thus indicating that activation of protein kinase C was not responsible for the enhancement of secretion. Similarly a rab peptide but neither a ras nor a ral peptide enhanced CppNHp-induced secretion 30–70%. The peptides had little or no effet in the absence of Ca2+ or GppNHp. The data are consistent with a protein of the rab class playing a role in regulated exocytosis.
