In smooth muscle, contraction is initiated by Ca2+ binding to calmodulin (CaM) rather than troponin C. The downstream phosphorylation target is:

• A Myosin light chain (MLC) via Ca2+-CaM-dependent myosin light chain kinase (MLCK) ✓
• B Troponin I — reducing inhibition of actin-myosin interaction
• C Actin directly phosphorylated by Ca2+-CaM complex
• D Rho kinase (ROCK) independently of Ca2+

Explanation

In smooth muscle, intracellular Ca2+ binds to calmodulin (4 Ca2+ per CaM). The Ca2+-CaM complex activates myosin light chain kinase (MLCK), which phosphorylates the 20 kDa myosin light chain (MLC-20) at serine 19. Phosphorylated MLC then activates the Mg2+-ATPase of myosin heads, enabling cross-bridge cycling and contraction. Relaxation occurs when MLCP (myosin light chain phosphatase) dephosphorylates MLC. ROCK (Rho-kinase) enhances contraction by inhibiting MLCP, increasing sensitivity to Ca2+ (Ca2+ sensitisation).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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