Which proteins form the core contractile machinery of the sarcomere, and which regulate their interaction?

• A. Thick filaments are actin; thin are myosin; regulated by troponin and tropomyosin.
• B. Thick filaments are myosin; thin are actin; regulated by troponin and tropomyosin (troponin C, I, T).
• C. Thick filaments are myosin; thin are actin; regulated by titin and nebulin.
• D. Thick filaments are myosin; thin are myosin; regulated by tropomodulin.

Answer: (B)

In muscle sarcomeres, the contractile engine is built from thick filaments of myosin and thin filaments of actin. The myosin heads form cross-bridges with actin to generate force through ATP-powered cycling. This interaction is tightly regulated by the troponin–tropomyosin complex on the actin filament. The troponin complex has subunits C, I, and T, which together sense calcium and modulate the position of tropomyosin on the actin surface. When calcium rises, troponin C binds Ca2+, causing a shift that moves tropomyosin away from the myosin-binding sites on actin, allowing myosin heads to attach and contractions to proceed. Other proteins like titin and nebulin are structural elements that help maintain sarcomere architecture, not directly regulating cross-bridge formation. Tropomodulin caps actin filament ends and does not regulate the interaction between actin and myosin during contraction.