Molecular motors

Molecular motors are responsible for almost all biologically interesting motion. They support efficient, sustained, directional motility of cellular components within cells, of entire cells over surfaces and of entire organisms. Motors allow cells to set up complex structure, and then continuously to maintain and adjust it, by directing packets of molecular components to localised, and sometimes distant, reaction sites. Without motorised transport, cellular components, and cells themselves, would need to diffuse to their destinations, and diffusion is inefficient over distances of more than a few microns. There are several families of molecular motors. In eukaryotic cells, networks of actin filaments and microtubules ramify through the cytoplasm, and cytoskeletal motors engage both in forcefully sliding the actin filaments and microtubules into place, and in trafficking cargo along them. The classical cytoskeletal motor is myosin. In muscle fibres, polymers of myosin pull on arrays of actin filaments, driving muscle contraction. Members of the myosin family are also involved in cell motility, in endocytosis and vesicle transport, in cytokinesis and in the gastrulation stage of embryonic development. Two other sorts of cytoskeletal motor, the kinesins and the dyneins, move along microtubules to actuate the directional motility of membranous vesicles, organelles, chromosomes, protein rafts and RNA. Certain dyneins power the beating of cilia and flagella. Cytoskeletal motors move linearly. Cells also contain rotary motors and track-laying motors. The F1 ATPase is a rotary machine that sits and spins in mitochondrial membranes, is powered by a proton gradient and ordinarily generates ATP. But its rotor can be driven backwards if ATP is supplied, whereupon it becomes a highly efficient rotary motor. Bacterial rotary motors drive the spinning of flagella, and so allow bacterial chemotaxis. Several ribosomal elongation factors and DNA and RNA manipulating enzymes are motors, in that they move directionally along the track that they synthesise. What do molecular motors look like? Most, but not all, have a heads-on-a-stalk configuration. The heads contain the ATPase and track-binding functions and the stalk recognises and binds to other motors, or to adaptor proteins, or directly to cargo. In some cases the tail may also fold up and bind reversibly to the head, thereby turning it on and off. Kinesin and myosin are structurally related to each other and to the G-proteins, and it is possible they diverged from a common, G-protein-like ancestor by the incorporation of different track-binding insertions.