What steps are involved in the myosin powerstroke?2018-02-06T10:17:13+00:00

What steps are involved in the myosin powerstroke?

Each myosin motor protein possesses ATPase activity and functions in a cyclical manner that couples ATP binding and hydrolysis to a conformational change in the protein. This process is known as the ‘powerstroke cycle’ (reviewed in [1][2][3]) and is outlined in the steps below using myosin II as an example. T

myosin-powerstroke

The “power stroke” mechanism for myosin movement along actin filaments:

The direction in which the actin filament will be moved is dictated by the structural orientation of myosin in relation to the filament. A complete round of ATP hydrolysis produces a single ‘step’ or movement of myosin along the actin filament. This process is regulated by changes in the concentration of intracellular free calcium (reviewed in [4]). The steps involved are detailed below:

Step 1: At the end of the previous round of movement and the start of the next cycle, the myosin head lacks a bound ATP and it is attached to the actin filament in a very short-lived conformation known as the ‘rigor conformation’.

Step 2: ATP binding to the myosin head domain induces a small conformational shift in the actin-binding site that reduces its affinity for actin and causes the myosin head to release the actin filament.

Step 3: ATP binding also causes a large conformational shift in the ‘lever arm’ of myosin that bends the myosin head into a position further along the filament. ATP is then hydrolysed, leaving the inorganic phosphate and ADP bound to myosin.

Step 4: The myosin head makes weak contact with the actin filament and a slight conformational change occurs on myosin that promotes the release of the inorganic phosphate.

Step 5: The release of inorganic phosphate reinforces the binding interaction between myosin and actin and subsequently triggers the ‘power stroke’. The power stroke is the key force-generating step used by myosin motor proteins. Forces are generated on the actin filament as the myosin protein reverts back to its original conformation.

Step 6: As myosin regains its original conformation, the ADP is released, but the myosin head remains tightly bound to the filament at a new position from where it started, thereby bringing the cycle back to the beginning.

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References

  1. Vale RD, and Milligan RA. The way things move: looking under the hood of molecular motor proteins. Science 2000; 288(5463):88-95. [PMID: 10753125]
  2. Volkmann N, and Hanein D. Actomyosin: law and order in motility. Curr. Opin. Cell Biol. 2000; 12(1):26-34. [PMID: 10679363]
  3. Hwang W, and Lang MJ. Mechanical design of translocating motor proteins. Cell Biochem. Biophys. 2009; 54(1-3):11-22. [PMID: 19452133]
  4. Ebashi S, and Endo M. Calcium ion and muscle contraction. Prog. Biophys. Mol. Biol. 1968; 18:123-83. [PMID: 4894870]