Myosin interacts with actin during its enzymatic cycle, and actin stimulates myosin's ATPase activity. There are extensive interaction surfaces on both actin and myosin. Several surface loops of myosin play different roles in actomyosin interaction. However, the functional role of loop 4 in actin binding is still ambiguous. We explored the role of loop 4 by either mutating its conserved acidic group, Glu-365, to Gln (E365Q), or by replacing the entire loop with three glycines (ΔAL) in a Dictyostelium discoideum myosin II motor domain (MD) containing a single tryptophan residue. This native tryptophan (Trp-501) is located in the relay loop and is sensitive to nucleotide binding and lever-arm movement. Fluorescence and fast kinetic measurements showed that the mutations in loop 4 do not alter the enzymatic steps of the ATPase cycle in the absence of actin. By contrast, actin binding was significantly weakened in the absence and presence of ADP and ATP in both mutants. Because the strength of actin-myosin interaction increases in the order of rigor, ADP, and ATP complex, we conclude that loop 4 is a functional actin-binding region that stabilizes actomyosin complex, particularly in weak actin-binding states.
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