Myosin
Myosin is a big protein (520 kDa) which consisting of 6 polypeptide chains: two heavy chains (220 kDa each), and two pairs of light chains (20 kDa each). This big protein has three biological activities:
- Spontaneously Myosin molecules assemble into filaments in solutions of physiological pH and ionic strength;
- Myosin is an ATPase, hydrolyzing ATP to ADP and Pi;
- Myosin binds the polymerized which is form of actin.
Myosin consists of a double-headed globular region joined to a long rod. The rod is a two-stranded α-helical coiled-coil formed by the 2 heavy chains, whereas the globular heads are also part of each heavy chain with the light chains attached. Restricted proteolysis of myosin with trypsin results in its dissection into two fragments: and heavy meromyosin (HMM) and light meromyosin (LMM). Functional studies of these two fragments reveal that LMM still can form filaments but lacks ATPase activity, where HMM does not form filaments but possesses ATPase activity and may bind to actin. HMM may be further split into 2 identical globular subfragments (S1) and one rod-shaped subfragment (S2) by another protease, papain. The S1 subfragment, whose structure has been determined by X-ray crystallography that contains an ATPase site, an actin-binding site and two light chain-binding sites. At flexible hinge regions the proteolytic cleavage of myosin occurs within the protein that separate the globular S1 domains from the rod-like S2 and LMM domains. These hinges play a crucial role in the contraction of muscle.