Association of the GTP-binding protein Rab3A with bovine adrenal chromaffin granules

The Rab3A protein belongs to a large family of small GTP-binding proteins that are present in eukaryotic cells and that share amino acid identities with the Ras proteins (products of the ras protooncogenes). Rab3A, which is specifically located in nervous and endocrine tissues, is suspected to play a key role in secretion. Its localization was investigated in bovine adrenal gland by using a polyclonal antibody. Rab3A was detected in adrenal medulla but not in adrenal cortex. In cultured adrenal medulla cells. Rab3A was specifically expressed in the catecholamine-secreting chromaffin cells. Subcellular fractionation suggested that Rab3A is about 30% cytosolic and that particulate Rab3A is associated with the membrane of chromaffin granules (the catecholamine storage organelles) and with a second compartment likely to be the plasma membrane. The Rab3A localization on chromaffin granule membranes was confirmed by immunoadsorption with an antibody against dopamine beta-hydroxylase. Rab3A was not extracted from this membrane by NaCl or KBr but was partially extracted by urea and totally solubilized by Triton X-100, suggesting either an interaction with an intrinsic protein or a membrane association through fatty acid acylation. This study suggests that Rab3A, which may also be located on other secretory vesicles containing noncharacterized small GTP-binding proteins, is involved in their biogenesis or in the regulated secretion process.