Myosin IIs

Myosin II
Updated 21/5/02
The classic myosin first isolated from muscle, subsequently found in non-muscle cells and protists (Adelman & Taylor, 1969). It is now clear that there are likely to be no myosin II members expressed in plants as there is none in the complete genome of Arabidopsis thaliana (Reddy et al, 2001).
	Figure 1. The myosin II molecule is composed of two heavy chains and two sets of light chains. The light chains are the Essential Light Chains (ELC) and Regulatory Light Chains (RLC). Overall activity and polymerization can be modulated by phosphorylation on the light chains and the C-terminus of the heavy chains respectively.
The role of myosin II in muscle contraction. Myosin II provides the motive force behind the contraction of muscle cells, working in concert with the actin filaments. The sliding filament theory first suggested by Hugh Huxley (Huxley & Hanson, 1954) is now widely accepted. However the details are still hotly debated.
	Figure 2a. An electron microscope image of rat skeletal muscle showing the characteristic organisation of the sarcomere. With two prominent Z-discs, a "thick" filament composed of myosin and two sets of ""thin filaments composed largely of actin.
	2b. A diagram showing the main components of the sarcromere. Thick filaments in blue, thin filaments in red. Note that the thick filament is held between the thin filaments by "Titin" - an enormous spring-like molecule with many site for other muscle proteins including actin (See Titin at the Z disc for more info). The Z- disc to Z-disc distance is about 2.5mm depending of course on the state of contractility.

	Figure 3a. The Myosin Cross-bridge Cycle. A. ATP binding to a cleft at the “back” of the head causes a conformation which cannot bind actin. B. As the ATP is hydrolysed, the head swings back about 5nm to the “cocked” position the ADP and Pi remain bound. C+D. The force generating stages. When the Pi leaves the myosin, the head binds the actin and the “power stroke” is released as the head bind actin. ADP is released to continue the cycle. At this stage the head in bound to actin in the “rigor” or tightly bound state.
The role of myosin II in cell locomotion. Cell locomotion has been studied for more than a century. The earliest work involved the giant amoeba (Amoeba, Chaos) and two competing hypothesis, the frontal contraction hypothesis and the rear contraction hypothesis (Janson & Taylor, 1993). The discovery of myosin II in protists encouraged suggestions of its involvement in the production of contraction (either at the from or rear of the amoeba cortex), however, it was shown that the deletion of the single myosin II gene from Dictyostelium discoideum did not destroy its ability to crawl (although it did cause the amoeba to move in a less organised fashion)(Knecht & Loomis 1987). (See Cell Locomotion and Myosin & Cell locomotion for further details).
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