25.16 : Overview of Myosin Structure and Function

Myosins are a family of molecular motor proteins, first identified in the skeletal muscles, where they are responsible for muscle contraction. Along with their role in muscle contraction, these proteins also play a role in the intracellular transport of molecules and vesicles. There are twenty-four classes of myosins based on their domain sequence and organization. Of the twenty-four, six classes (Myosin I, Myosin II, Myosin V, Myosin VI, Myosin VII, and Myosin X) have been well characterized. The other roles of these molecular motor proteins include forming contractile rings during cytokinesis, organelle transport across polar actin filaments, aiding cell polarization, and signal transduction.

Myosin I is a monomeric protein with a globular head and a short tail. Unlike other myosin proteins, the tail domain can bind with a lipid membrane. The globular head with the actin-binding domain attaches to the F-actin. This myosin protein allows the intracellular transport of molecules and vesicles. Along with actin filaments, they are also present in intestinal cells as a component of the small cellular projections in microvilli.

Myosin II is the most widely studied member of this class of proteins, and its role in high-speed motility for muscle contraction has been well established. The general structure of myosin II is a dimer composed of two helically arranged polypeptide chains, each having a globular head, a narrow neck, and an alpha-helical tail. Each globular head has an actin-binding and ATP-binding domain where ATP hydrolysis occurs. The neck portion comprises a shorter light chain domain that acts as a lever for the neck movement, and the tail region has a long heavy chain domain that extends into the tail. The two tail polypeptide chains form a coiled-coil structure.