We have used two actin-binding proteins of the intestinal brush border, TW 260/240 and villin, to examine the effects of filament cross-linking and filament length on myosin-actin interactions. TW 260/240 is a nonerythroid spectrin that is a potent cross-linker of actin filaments. In the presence of this cross-linker we observed a concentration-dependent enhancement of skeletal muscle actomyosin ATPase activity (150-560% of control; maximum enhancement at a 1:70-80 TW 260/240:actin molar ratio). TW 260/240 did not cause a similar enhancement of either acto-heavy meromyosin (HMM) ATPase or acto-myosin subfragment-one (S1) ATPase. Villin, a Ca2+-dependent filament capping and severing protein of the intestinal microvillus, was used to generate populations of actin filaments of various lengths from less than 20 nm to 2.0 microns; (villin:actin ratios of 1:2 to 1:4,000). The effect of filament length on actomyosin ATPase was biphasic. At villin:actin molar ratios of 1:2-25 actin-activated myosin ATPase activity was inhibited to 20-80% of control values, with maximum inhibition observed at the highest villin:actin ratio. The ATPase activities of acto-HMM and acto-S1 were also inhibited at these short filament lengths. At intermediate filament lengths generated at villin:actin ratios of 1:40-400 (average lengths 0.26-1.1 micron) an enhancement of actomyosin ATPase was observed (130-260% of controls), with a maximum enhancement at average filament lengths of 0.5 micron. The levels of actomyosin ATPase fell off to control values at low concentrations of villin where filament length distributions were almost those of controls. Unlike intact myosin, the actin-activated ATPase of neither HMM nor S1 showed an enhancement at these intermediate actin filament lengths.

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