Difference Between Actin and Myosin

Actin vs Myosin
 

Some protein molecules can produce multi-subunit structures, which can be involved in motility of organisms. This motility may involve in cells, in part or whole organism. One of the most important examples of such motility system is the muscular system. The main contractile system of all muscular tissue is based on the interactions of two proteins called “Actin” and “Myosin”. The system of these proteins sometimes called “Actin-myosin contractile system”.

Actin

Actin can exist in two distinct forms within the cell. They are globular actin (G-actin) or filamentous actin (F-actin). G-actin is a ≈43kDa protein that can bind ATP and polymerize to form microfilaments known as F-actin filaments. The nucleation of F-actin filaments is polarized and has appositive (+) end and a negative (-) end. Both ends are highly dynamic but have different on/off rates; growth of filaments occurs primarily at the positive end as it has a much higher on rate. Actin filaments are highly cross-linked and bundled by proteins such as α-actinin, to increase their structural integrity. The cellular actin network has its highly dynamic nature to the actin-interacting proteins that facilitate its assembly, stabilization and disassembly.

Myosin

The myosins are a family of motor proteins associated with actin. Actin-myosin complexes generate the cellular forces used in cell contractility and migration. The majority of myosins are (+) end motors; means that they move along actin filaments towards the (+) end. There are several different types of myosins and each participates in specific cellular functions. Myosin “heavy chains” consist of one or more head, neck, and tail domains. Functionally, myosins also strengthen the actin network by cross-linking actin fibers. Myosin uses ATP to produce energy thus initiate the muscle contraction by forcing its head towards the actin fiber. One myosin molecule produces about 1.4 pN of force when it changes the confirmation.

What is the difference between Actin and Myosin?

• Actin filaments are thin filaments and myosin filaments are thick filaments.

• Actin filaments occur in two forms, the monomeric G-actin and the polymeric F-actin and myosin molecule has two components, a tail and a head. The tail is formed of heavy meromyosin (H-MM) while the head is formed of light meromyosin (L-MM).

• Actin forms both A and I bands, whereas myosin forms only A bands (A-band forms the dark anisotropic band of myofibril and I-band forms the light isotropic band of myofibril).

• ATP only binds to myosin ‘head’ and it does not bind to actin.

• Unlike actin, myosin produces the force by binding ATP in order to initiate muscle contractions.