Motor Proteins (Molecular Biology)

A motor protein hydrolyzes ATP to generate movement or force along the cytoskeleton in eukaryotes. The actin filaments and microtubular components of the cytoskeleton have distinct plus and minus ends, which extend to different regions of the cell. Cells use motor proteins that read the filament polarity to direct specialized cargoes to specific locations. Two motor protein families move along microtubules. The dyneins move toward the minus ends of the microtubules, and the kinesins move toward the plus ends of microtubules. Flagellar dyneins generate the sliding force between axonemal microtubules that is the motive force for eukaryotic cilia and flagella bending. Cytoplasmic dyneins are responsible for moving membrane-bounded organelles and, kinetochores and for assembling the spindle. Kinesin family members are also responsible for intracellular transport of membranous organelles moving chromosomes, and assembling the spindle. Myosins, the third motor protein family, interact with actin filaments and move toward the plus end of actin filaments. Myosin family members are responsible for a variety of functions in cells: contraction in muscle, cytokinesis in dividing cells, crawling locomotion, some forms of membrane-bounded organelle transport, and localization of determinants. The three motor proteins are multisubunit complexes that have specific heavy and light chains. All of the members of the three motor protein families are defined by a conserved, force-generating "head" domain that is part of the heavy chain. The heads bind the appropriate cytoskeletal polymer and hydrolyze ATP to move along the polymer. The divergent domains of the heavy chains, the "tails," and the light chains target the motors to various cargoes or locations and in some cases specify oligomerization of the molecules.