In smooth muscle, cross-bridge formation is regulated not by troponin but by which calcium-binding regulatory protein that activates myosin light-chain kinase?

1. Smooth muscle fibers contain actin and myosin but lack troponin, unlike skeletal and cardiac muscle. 2. In their place, cross-bridge formation is regulated by the protein calmodulin. 3. Calcium ions bind calmodulin, and the resulting Ca2+-calmodulin complex activates myosin light-chain kinase, which phosphorylates the myosin heads to allow attachment. 4. Troponin is the calcium-binding regulator used in skeletal and cardiac muscle instead, not in smooth muscle. 5. Tropomyosin works alongside troponin in skeletal and cardiac muscle to block binding sites, again not the smooth-muscle regulator. 6. Dystrophin has a structural anchoring role linking filaments to the sarcolemma, unrelated to calcium-triggered regulation. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 10 "Muscle Tissue", section 10.8, p. 434_