Practice question

Among eighteen myosin classes class II myosins dominate muscle mechanics due to capacity to assemble into higher order structures essential for force scaling. Each myosin II molecule comprises two heavy chains with N terminal motor domains hydrolyzing ATP IQ motifs binding essential and regulatory light chains forming lever and C terminal extended coiled coil driving dimerization plus further self association into filaments. In skeletal muscle dimers polymerize antiparallel into 1.6 micron long thick filaments containing about 300 molecules bare zone in middle heads projecting helically outward 14.3 nm periodicity. These interdigitate with thin actin filaments decorated with troponin tropomyosin facilitating sliding filament mechanism with ordered cross bridge cycling. Non muscle myosin II isoforms form smaller filaments for cortical tension and adhesion. Myosin V and VI transport cargo on actin toward plus and minus ends myosin I tethers membranes myosin X bundles filopodia none form stable contractile thick filaments in sarcomeres. Therefore skeletal and cardiac contractility fundamentally depends on myosin II filament assembly and ATP driven power stroke scaled by light chain phosphorylation.