Calcium-sensitive modulation of the actomyosin ATPase by fodrin.

Abstract

Fodrin, a spectrin-like protein isolated from brain, is a long flexible molecule which binds calmodulin and cross-links F-actin. The effects of fodrin on the actin-activated ATPase of myosin have been examined. When added after ATP, fodrin inhibited the actomyosin ATPase. Two to three times as much fodrin was required for inhibition in the presence of Ca2+ as in its absence. Complete inhibition in the absence of Ca2+ occurred at about one fodrin to 200 actins. Inhibition does not appear to result from fodrin cross-linking F-actin, and, thereby, preventing the myosin filaments from reaching the actin filaments; but cross-linking may promote inhibition by trapping the myosin filaments within the cross-linked F-actin. When added before ATP, fodrin stimulated the actomyosin ATPase almost 3-fold in the presence of Ca2+ and by less than 50% in the absence of Ca2+. Stimulation is thought to result from fodrin cross-linking F-actin. After several minutes the stimulations in Ca2+ were greatly reduced, and in the absence of Ca2+ the actomyosin ATPases were substantially inhibited. Whether added before or after ATP, fodrin inhibited the actin-activated ATPase of myosin subfragment 1. This inhibition was also slightly Ca2+ sensitive.