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Myosin refers to a group of motor proteins that are essential for muscle contraction. These proteins may also have ATPase activity, catalyzing the hydrolysis of ATP, leaving ADP and an inorganic phosphate. This reaction releases energy which is then used to drive motion e.g. muscle movement.

Myosin is common in all eukaryotes. The most common form is myosin II, used in muscle contraction, but there are 40 different types in the human genome alone.
Myosin II is formed from heavy chains and light chains. The heavy chain consists of three domains: the head region, the tail region and the neck region. The action of myosin also relies on actin. The myosin binds to the actin filaments at the head domain to allow movement along the filament, generally towards the plus end of the actin filament. ATP also binds at this region to provide energy for the movement. The tail region can bind to the molecule the myosin is moving or can connect to other myosin molecules. The neck region provides the site of interaction between the heavy and light chains. It can also act as a lever to allow flexibility of the tail.

Myosin II is made up of two heavy chains which form the bulk the protein and two light chains. The light chains have structural and regulatory roles. They have a similar structure to the protein calmodulin, found in smooth muscle cells[1].


  1. Alberts B. Molecular biology of the cell. 6th ed. New York, NY [u.a.]: Garland Science Taylor and Francis; 2015

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