Characterization of the chromobindins. Soluble proteins that bind to the chromaffin granule membrane in the presence of Ca super(2+)

A group of proteins that bind to the chromaffin granule membrane in the presence of Ca super(2+) has been isolated by affinity chromatography of bovine adrenal medullary cytosol on granule membranes coupled to Sepharose 4B. Twenty-two of these proteins were resolved into classes depending upon the Ca super(2+) concentration at which they were eluted from the affinity column, upon their affinities for native granule membranes or for liposomes prepared from extracted granule lipids, and upon the requirement of seven of the proteins for ATP in the cytosol freaction and column buffers to promote binding. The molecular weights and isoelectric points of these proteins were determined by two-dimensional electrophoresis. Two of the granule-binding proteins were identified: synexin and calmodulin. Two major granule-binding proteins were found to present a pattern in two-dimensional electrophoresis that was very similar to but shifted slightly toward the basic end of the gel from the pattern generated by light chains associated with clathrin in adrenal medullary coated vesicles. In the chromaffin cell, these proteins, by associating with the granule membrane in the presence of an increased cytosolic Ca super(2+) concentration, might play a variety of roles in the process of exocytosis.