Chromobindin A. A Ca2+ and ATP regulated chromaffin granule binding protein

A variety of studies have shown that about 20 proteins can be isolated from bovine adrenal medullary cytosol by virtue of their ability to bind to chromaffin granule membranes in the presence of Ca2+. In the present study we have examined the properties of a group of seven of these proteins. This group of proteins binds to granule membranes in the presence of Ca2+, however, the proteins are not released from the membrane by the removal of Ca2+ unless ATP is present. The proteins range from 53 to 59 kDa and they form a multisubunit complex of about 800 kDa. This complex, which we have named chromobindin A, has 13 subunits joined together in a ring, 175 A in diameter and 115 A in height. The binding of chromobindin A to membranes is stimulated by Ca2+, Sr2+, and Ba2+. The release is stimulated by a variety of nucleotides, including the nonhydrolyzable nucleotide analog adenyl-5'-yl imidodiphosphate. At present the function of chromobindin A in vivo is not clear, although the observation that Ca2+ stimulates chromobindin A binding together with observations that imply that chromobindin A binds to a protease-sensitive receptor on the granule membrane suggest that the complex is involved in exocytosis and that it may be partially responsible for the ATP dependence of this process.