Gel electrophoresis of native gelsolin and gelsolin-actin complexes.

BHK gelsolin migrated on non-denaturing 8-25% polyacrylamide gels with an apparent molecular mass of 80 kDa. In the absence of Ca2+ no complex formation occurred between BHK gelsolin and actin. In the presence of Ca2+ two complex species were found: a ternary complex, GA2, of apparent molecular mass of 210 kDa at gelsolin:actin ratio of 1:2, and a novel quaternary complex, GA3, of apparent molecular mass of 247 kDa when actin was in excess. Both cytoplasmic and plasma gelsolin species form GA3 with skeletal muscle actin. No complexes larger than GA3 were observed. The formation of GA3 involves the binding of a third actin to the gelsolin molecule at the site previously assumed to be masked, rather than to the actin molecules already present in GA2. In preference to GA3, GA2 was incorporated into actin filaments stabilized with phalloidin. On chelation of free Ca2+, both GA2 and GA3 dissociated to form the EGTA stable binary complex (GA) with an apparent molecular mass of 140 kDa and free actin.