Actin‐binding proteins regulate the work performed by myosin II motors on single actin filaments

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Abstract
Regulation of actin/myosin II force generation by calcium [Kamm and Stull, Annu. Rev. Physiol. 51:299–313, 1989] and phosphorylation of myosin II light chains [Sellers and Adelstein, “The Enzymes,” Vol. 18, Orlando, FL: Academic Press, 1987, pp. 381–418] is well established. However, additional regulation of actin/myosin II force generation/contraction may result from actin‐binding proteins [Stossel et al., Ann. Rev. Cell Biol. 1:353–402, 1985; Pollard and Cooper, Ann. Rev. Biochem. 55:987–1035, 1986] as they affect the gel state of the actin cytomatrix [reviewed in Taylor and Condeelis, Int. Rev. Cytol., 56:57–143, 1979]. Regulation of the gel state of actin may determine whether an isotonic or isometric contraction results from the interaction between myosin and actin. We have extended the single actin filament motility assay of Kron and Spudich [Proc. Natl. Acad. Sci. U.S.A. 83:6272–6276, 1986] by including filamin or α‐actinin on the substrate with myosin II to examine how actin‐crosslinking proteins regulate the movements of single actin filaments. Increasing amounts of actin‐crosslinking proteins inhibit filament velocity and decrease the number of filaments moving. Reversal of crosslinking yields increased velocities and numbers of moving filaments. These results support the solation‐contraction coupling hypothesis [see Taylor and Fechheimer, Phil. Trans. Soc. London B 299:185–197, 1982] which proposes that increased crosslinking of actin inhibits myosin‐based contraction. This study also illustrates the potentially varied roles of different actin‐crosslinking proteins and offers a novel method to examine actin‐binding protein activity and their regulation of motility at the single molecule level.