Short Speech on ‘Contractile Proteins’

Myosin:

This is the protein of thick filaments of the myofibril. It has a molecular weight of about 500,000 which is about 10 times the molecular weight of actin.

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Its molecule is much more compli­cated than the actin and one of its most important properties is that it is an ATPase, i.e., it .will enzymatically hydrolyse ATP to form ADP and inorganic phosphate. The reaction is activated by calcium ions but inhibited by magnesium ions.

Myosin in its structural constitution probably consists of two major subunits and two-three minor ones.

The larger subunit has a molecular weight of about 200,000, whereas the smaller one a mole­cular weight of about 20,000.

Even if the molecule has three of the smaller subunits, the total molecular weight does not exceed the value of 500,000 obtained for whole molecule.

Whatever the resolu­tion of this particular discrepancy, it is obviously true that the mole­cule has a highly elongated form.

The long straight tail section of the molecule has the two a-helical subunits intertwined to make a larger helix, and has a width of only 15 to 20A.

At one end each of the main subunits appears to coil up to form a globular head region, which includes one of the small subunits as well.

This gives the molecule two head pieces approximately 200A long and 50A in diameter. It is this part of the molecule which serves as an ATPase and which has an affinity for actin.

Tip information on the mole­cular structure has come from the fact that a brief digestion of myosin by trypsin splits it into two different types of particles.

These derivatives are called light and heavy meromyosin. The light meromyosin aggregates to form filaments and has no enzyme activity: The heavy meromyosin, on the other hand, is water soluble and will split ATP.

Thus, the light meromyosin, seems to be part of the double-stranded tail region of the myosin, whereas the heavy mero­myosin represents the rest of the molecule (including both head piece, a portion of double-stranded tail, and the small subunits).

The thick filaments in the myofibrils appear to consist almost entirely of myosin molecule which has their tail regions organized in a parallel array.

The cross bridges which stick out from the thick filaments at regular intervals are now thought to contain the globular head pieces of the myosin molecules.

This part of the molecule also contains a region with an affinity for actin, and this is probably concentrated at the tips of the cross bridges where they approach the thin filaments.

Actin:

This is the protein of thin filaments of myofibril. It has molecular weight about 4,000 and N-acetylaspartic acid as its N- terminal residue.

It contains one molecule of 3-methyI-histidin per mole of protein and is a single polypeptide chain.

It exists in a globular form, G-actin in the absence of salts, which has a high affi­nity for calcium and can form dimers.

In the presence of salts there is a transformation of G-actin to a fibrous state F-actin which is highly polymerized. Each form of actin has a high affinity for ATP.

According to Selby and Bear, 1956 the thin filaments of all muscles consist principally of the fibrous form of actin and they also pointed out that the actin monomeres are arranged either in a net­like structure or in helics.

Recently Hanson, 1963 and Lowy, 1963 has reached the conclusion that F-actin consists of two chains of monomeres connected together in a double helical form.

The most interesting feature of this protein is that it undergoes polymerisation in solution in the presence of ATP, Mg²⁺, and KCL.

n G-actin + n ATP—–à (F-actin+ADP)n + phosphate

The ADP produced in this reaction remains firmly bound to each unit of the actin polymer, and no further hydrolysis of ATP takes place until the ADP is displaced.

The thin filaments can also bind Ca²⁺ ions, and in this form they will activate the ATPase of myosin, although, as mentioned earlier, it now appears that this binding is due to tropomyosin.

Tropomyosin:

It is the most recently studied protein compo­nent of the myofibrils which is present together with actin in thin filaments and forms a specific complex with F-actin in vitro. Its molecule is rod-like, 400A long and 20A diameter with a molecular weight 70,000.

It has two subunits in the form of a-helices in an extended coiled—coil conformation. When added to actomyosin solution tropomyosin inhibits its calcium activated ATPase activity but not its magnesium activated ATPase activity.

Troponin:

This protein has globular molecules and is found in the thin filaments together with actin and tropomyosin. It promotes the aggregation of purified tropomyosin. Its biological function is not clear.

In addition to these above proteins there are some other pro­teins found in the muscles, these are a-actinin and paramyosin α-actinin protein has a molecular weight of about 160,000 and interacts strongly with actin, causing cross-linkage of the F-actin filaments and gel formation. It has been established that a-actinin and tropomyosin are contained in the Z-lines.

Paramyosin is the contractile protein of muscle myofibrils of annelids and molluscs. It has a molecular weight of about 151,000.