Question

Actin binding sites are located on
A) Troponin
B) Tropomyosin
C) Meromyosin
D) Both tropomyosin and meromyosin
E) Both troponin and tropomyosin

Answer 1

Actin has myosin-binding sites on its filament. The meromyosin is the part of myosin which has active binding sites for ATP and proteins of muscle cells.

Complete answer: Actin and myosin are proteins present in muscle cells. Actin is a globular protein and is the microfilament protein in eukaryotic cells. It is an important contributor to the contractile function in muscles and other cells. Actin has troponin and tropomyosin, while myosin has meromyosin for regulation on filaments. These proteins help in the contraction and relaxation of the muscle. Troponin and tropomyosin are regulatory proteins which prevent contraction until there is the release of calcium ions. The ATP is also required for the myosin to move across the actin filament.
The meromyosin is a component of myosin protein. There are two types of meromyosin formed when it is proteolysed. The two subunits are heavy meromyosin (HMM) and light meromyosin (LMM). It has the actin-binding site. Heavy meromyosin has a globular portion or cross-bridge which binds to the actin filaments. There are two subunits in HMM: HMMS-1 and HMMS-2. The HMMS-1 has the major activity of myosin. The HMMS-1 has also actin-binding sites and ATP binding sites which function as myosin. It also is responsible for the rate of muscle contraction. The mechanism for contraction includes sliding of actin (thin filaments) and myosin filaments (thick filaments). The muscle contraction is signalled by acetylcholine which induces the release of calcium ions. Calcium binds to troponin on actin filaments and makes active binding sites for myosin. Myosin pulls actin towards the centre where the muscle is contracted but protein myosin is relaxed.
Hence, the correct answer is option C.

Note: Troponin and tropomyosin also bind to actin filaments which block the myosin-binding sites on actin. Tropomyosin blocks the cross-bridge formation and troponin makes sure that tropomyosin is binding to actin preventing muscle contraction. These support relaxation of muscles.