Myosins (/ˈməsɪn, --/[1][2]) are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-based motility.

Part of the myosin II structure. Atoms in the heavy chain are colored pink (on the left-hand side); atoms in the light chains are colored faded-orange and faded-yellow (also on the left-hand side).

The first myosin (M2) to be discovered was in 1864 by Wilhelm Kühne. Kühne had extracted a viscous protein from skeletal muscle that he held responsible for keeping the tension state in muscle. He called this protein myosin.[3][4] The term has been extended to include a group of similar ATPases found in the cells of both striated muscle tissue and smooth muscle tissue.

Following the discovery in 1973 of enzymes with myosin-like function in Acanthamoeba castellanii, a global range of divergent myosin genes have been discovered throughout the realm of eukaryotes.[5]

Although myosin was originally thought to be restricted to muscle cells (hence myo-(s) + -in), there is no single "myosin"; rather it is a very large superfamily of genes whose protein products share the basic properties of actin binding, ATP hydrolysis (ATPase enzyme activity), and force transduction. Virtually all eukaryotic cells contain myosin isoforms. Some isoforms have specialized functions in certain cell types (such as muscle), while other isoforms are ubiquitous. The structure and function of myosin is globally conserved across species, to the extent that rabbit muscle myosin II will bind to actin from an amoeba.[6]

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