Low molecular weight GTP binding proteins in the secretory machinery of adrenal chromaffin cells

A group of low molecular weight ras-like GTP-binding proteins have been described on the cytoplasmic surface of mammalian secretory vesicles. Adrenal chromaffin cell catecholamine storage granules contain at least 6 low molecular weight GTP-binding proteins. Investigations were undertaken to determine the molecular identities of these proteins and whether they function in secretion.;Studies on GTP binding to purified chromaffin granule membranes indicates that a substantial number of sites exist on each granule. The binding sites are specific for guanine nucleotides, GTP;Subcellular fractionation studies show chromaffin granules contain rab3a, a low molecular weight GTP-binding protein also thought to be attached to brain synaptic vesicles. Immunocytochemical localization of dopamine ;A water soluble peptide, rab3AL(33-48) which corresponds to the effector binding region of rab3a was shown to enhance calcium-dependent and guanine nucleotide-dependent norepinephrine secretion in permeabilized chromaffin cells. The peptide may either bind an effector protein that enhances secretion or inactivate an inhibitor of secretion. Thus, a rab protein on chromaffin granules may modulate exocytosis.