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Structure and Function
Most myosin molecules are composed of both a head and a tail domain.
Myosin I's function is unknown, but it is believed to be responsible for vesicle transport or the contraction vacuole of cells.
Myosin II is perhaps the best-studied example of these properties.
In muscle cells, it is myosin II that is responsible for producing the contractile force. Here, the long coiled-coil tails of the individual myosin molecules join together, forming the thick filaments of the sarcomere. The force-producing head domains stick out from the side of the thick filament, ready to walk along the adjacent actin-based thin filaments in response to the proper chemical signals.
Evolution and family tree
Myosin II, the most conspicuous of the myosin superfamily due to its abundance in muscle fibers, was the first to be discovered. However, beginning in the 1970s researchers began to discover new myosin variants, with one head (as opposed to myosin II's two) and largely divergent tail domains. These new superfamily members have been grouped according to their structural similarities, with each subfamily being assigned a Roman numeral. The now diverse array of myosins has evolved from an ancestral precursor (see picture).
Analysis of the amino acid sequences of different myosins shows great variability among the tail domains but almost perfect retention of the same head sequence. Presumably this is so the myosins may interact, via their tails, with a large number of different cargoes, while the goal in each case - to move along actin filaments - remains the same and therefore requires the same machinery in the motor. For example, the human genome contains over 40 different myosin genes.
These differences in shape also determine the speed at which myosins can move along actin filaments. The hydrolysis of ATP and the subsequent release of the phosphate group causes the "power stroke," in which the "lever arm" or "neck" region of the heavy chain is dragged forward. Since the power stroke always moves the lever arm by the same angle, the length of the lever arm determines how fast the cargo will move. A longer lever arm will cause the cargo to traverse a greater distance even though the lever arm undergoes the same angular displacement - just as a person with longer legs can move farther with each individual step. Myosin V, for example, has a much longer neck region than myosin II, and therefore moves 30-40 nanometers with each stroke as opposed to only 5-10.
Genes in humans
Note that not all of these genes are active.
costamere (dystrophin, α,β-dystrobrevin, syncoilin, synemin/desmuslin, dysbindin, sarcoglycan, dystroglycan, sarcospan), desminmyoblast, satellite cell, sarcoplasm, sarcolemma, sarcoplasmic reticulum, T-tubule
|cardiac muscle||myocardium, intercalated disc, nebulette|
|smooth muscle||calmodulin, vascular smooth muscle|