Which proteins are contractile?

• A. Keratin and Collagen
• B. Tubulin and Dynein
• C. Actin and Myosin
• D. Fibrin and Albumin

Answer: (C)

Contractile proteins are the ones that generate muscle shortening. The classic pair is actin and myosin, whose interaction drives the sliding filament mechanism: myosin heads bind to actin, pull it inward using energy from ATP, and this action slides the thin filaments past the thick ones, shortening each sarcomere and producing force. That direct mechanism is what makes actin and myosin the contractile proteins.

Other proteins listed serve different roles. Keratin and collagen provide structural support in tissues, not active shortening. Tubulin and dynein are part of the cytoskeleton and intracellular transport systems, not the muscle’s contractile apparatus. Fibrin is involved in blood clotting, and albumin maintains osmotic balance in the blood; neither drives muscle contraction.