Practice question

Two distinct superfamilies of cytoskeletal motors evolved to walk along filaments using ATP hydrolysis. Myosin motors share N terminal head motor domain containing actin binding interface including helix turn helix, cardiomyopathy loop, P loop ATPase, Switch I II for gamma phosphate sensing and lever arm with IQ motifs binding calmodulin light chains. Upon interaction with F actin 7 nanometer helical filament, head undergoes conformational cycle releasing phosphate driving swing of lever arm producing force along filament. Diverse isoforms specialize: myosin II non processive forming bipolar filaments for contraction, myosin V processive dimer for cargo, myosin I single headed tension sensor. In contrast dynein heavy chain AAA plus ring and coiled coil stalk binds microtubule 25 nanometer tubule, kinesin motor domain with tubulin binding loops moves along protofilaments. Nexin is non motor linker between doublet microtubules in axoneme. Therefore actin filament based intracellular transport and contractility specifically require myosin family as dedicated actin associated motor providing directional movement.