Clathrin light chains and secretory vesicle binding proteins are distinct

Abstract

Recently, several groups^1–4 have initiated studies on cytosolic proteins that bind to isolated secretory vesicle membranes in the presence of Ca²⁺ in order to identify proteins that may regulate exocytosis. Two major chromaffin granule binding proteins, of molecular weights 32,000 (32K) and 34,000 (34K), were reported to have the same mobilityon one-dimensional SDS gels as clathrin-associated light chains from the adrenal medulla, and the 34K granule binding protein the same one-dimensional peptide map as the 34K clathrin light chain⁵. These observations support the hypothesis that Ca²⁺ -dependent recruitment of soluble light chains to the vesicle membrane may nucleate the assembly of a clathrin coat and initiate endocytosis. Here we report that two-dimensional peptide maps of the clathrin light chains and of all chromaffin granule membrane binding proteins in the 30K range are distinct, and therefore fail to support this hypothesis. It has also been suggested that some or all of the vesicle binding proteins require calmodulin for their interaction with the membrane⁵. However, we find that antagonism of calmodulin by trifluoperazine does notprevent the association of the other cytosolic proteins with the chromaffin granule membrane.