Practice question

Non muscle contractility underpins stress fiber maintenance, adhesion maturation, morphogenesis and wound closure, distinct from skeletal muscle sarcomeres employing troponin regulation. Myosin II family in non muscle cells includes isoforms IIA, IIB and IIC sharing hexameric composition two heavy chains plus essential and regulatory light chains, forming short bipolar filaments that transiently associate with actin rather than stable thick filaments. Activation occurs via phosphorylation of 20 kilodalton regulatory light chain RLC at Ser19 by calcium calmodulin dependent MLCK and Rho associated kinase ROCK, converting autoinhibited 10S folded conformation to extended 6S assembly competent state enhancing actin activated ATPase. Coordinated pulling of antiparallel actin bundles generates isometric tension, retrograde flow and cortex stiffness. Isoform IIA drives rapid adhesion turnover at leading edge, IIB sustains prolonged tension at rear. Myosin I single headed tension sensor, myosin III adaptation in photoreceptors, myosin VI minus end directed for endocytosis, not primary contractile generator. Therefore myosin II remains central for non muscle contractility.