Figure 15.3: Actin Fibre

Myosin: Myosin is another major muscle protein. It is a hexameric protein. It consists of two heavy chains and four light chains. Molecular weight of the heavy chains and light chains are around 200 kda and 15 to 27 kda respectively. Heavy chain forms dimeric filaments by coiling α-helically each other except N terminal (Figure 16.4). Unwound N terminal of myosin is globular in nature and it serves the binding sites for other light chains as well as F-actin. N terminal of the myosin has ATPase activity. Carboxy terminal of myosin filament meet together at H zone whereas N terminals meet together at the margins of A band.

Figure 16.4:  Myosin fibre

Tropomyosin: Tropomyosin is a rod like fibrous molecule. Its molecular weight is around 66 kda. It made up of two different α and β peptides and both are α-helical in nature. Double stranded tropomyosin run parallel with thin filaments in grooves of the F-actin strands.

Troponin: Troponin is a globular protein and exit as three interconnected protein system. They are (a) TpT (tropomyosin binding protein), (b) TpC (calcium binding protein) and (c) TpI (actin and TpC binding protein). Troponin lie on thin filaments with an interval of 38.5 nm, but the rest attached with F-actin and tropomyosin.
Polymerization of desmin results intermediate filaments which is predominantly present in Z line. Α-α-actinin is a homodimeric protein and often anchors the ends of f-actin molecule to the Z line.