Definition of an N-terminal Actin-binding Domain and a C-terminal Ca2+ Regulatory Domain in Human Brevin

Brevin is a Ca2+-modulated actin-associated protein that will sever F-actin and cap barbed filament ends. Limited proteolysis with chymotrypsin or subtilisin cleaves the molecule approximately in half. Cleavage is approximately 10-fold more rapid in Ca2+ than in EGTA. The two fragments are readily separated from each other and from undigested brevin by high pressure liquid chromatography on a DEAE resin. A 40,000-mol-wt fragment from the N-terminal is not retained by DEAE, while a 45,000-mol-wt C-terminal fragment binds more tightly than brevin. The N-terminal fragment retains approximately 10% of the nucleation activity, caps barbed ends, and retains 50% of the total severing activity defined by dilution induced depolymerization of pyrenyl actin, but, in contrast to brevin, none of these functions are affected by Ca2+. Fluorescent actin binding studies and gel-filtration demonstrate that the 40,000-mol-wt fragment binds two actin monomers. The 45,000-mol-wt C-terminal fragment has no severing, nucleating, or capping activity. Cross-reaction with two monoclonal antibodies against two specific Ca2+-induced conformations of human platelet gelsolin suggest that both Ca2+ binding sites are located on the carboxyl half of the brevin molecule. One epitope, defined as the rapidly exchanging Ca2+ binding site in the gelsolin-actin complex, is lost when a 20,000-mol-wt fragment is cleaved from the carboxyl terminal. The second epitope, related to the poorly exchanging Ca2+ binding site in the complex, is nearer the middle of the brevin molecule.