Overview of Cell Biology/Actin and Myosin

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 * Study guide:
 * Wikipedia article: Myosin

Myosins
Myosins are mechanochemical enzymes and motor proteins that function through ATP hydrolysis.

Myosin Structure
Myosin is composed of heavy chains and light chains. The heavy chains have head, neck and tail domains. The head domain binds actin and has ATPase activity. The neck domain provides an attachment point for regulatory and essential lightchains such as calmodulin. The tail domain differs between myosins and determines the specific properties of each myosin.

Myosin II
Myosin II has a long α-helix tail which forms a dimer with another Myosin to create a coiled-coil dimer. The tails also mediate polymerization into bipolar thick filaments.

Myosin I
Myosin I has a short tail which does not assemble into filaments. Some Myosin I proteins have membrane binding sites and can move organelles into the cell.

Myosin Motility and ATP hydrolysis
Myosin motility is coupled to ATP hydrolysis. Most myosins move toward the (+) end of the filament. Myosin VI, however, moves towards the (-) end.

Myosin Cross-bridge Cycle
(1) Rigor: The ATP binding site is empty and myosin is tightly bound to actin

(2) ATP binding: The ATP binding cleft closes upon ATP binding and the actin binding cleft opens, which weakens the actin-myosin bond.

(3) ATP hydrolysis: The ATP molecule is hydrolyzed to ADP and Pi and the myosin head moves to a new position.

(4) Pi release: This step is also referred to as the power stroke. The actin fliament is moved relative to the myosin filament

(5) ADP release: myosin is restored to original rigor state.

Muscle Cell and Sarcomere Structure
Skeletal muscle have a regular internal structure and the muscle fivers of skeletal muscle (myofibers) are enormous cells that contain multiple nuclei. Muscle fibers are composed of individual contractile bundles called myofibrils. Each myofibril consists of small contractile units called sarcomeres.

Thin filaments are composed of actin filaments, CapZ, tropomodulin, tropomyosin, troponin, and nebulin. Thick filaments are composed of myosin II. Protease can cut myosin into the S1 and tail (motor) domains. The isolated tails can make filaments alone. Thick filaments are bipolar. They are 5 main parts to these CapZ and a-actinin mediate binding of the (+) end of actin to the Z disk.

The (-) ends of actin are capped by tropomodulin.

Nebulin wraps along the length of the filament.

Titin connects the ends of myosin thick filaments to the Z-disk and extends along the filaments to the M-line (½ the length of the sarcomere).

The Sliding Filament Model of Contraction
The sarcomere shortening caused by myosin filaments sliding past actin filaments with no change in the length of either filament. Because the myosin filament is bipolar, it pulls the thin filaments and Z-disk towards the center of the sarcomere, causing sarcomere shortening.