The generation of force and the active transport of material over long distances are processes that are important in a number of complex functions. Both force generation and transport are driven by molecular motors that utilize the energy derived from ATP hydrolysis to move along polar filaments of the cytoskeleton. This movement generates the forces needed for synthesis, packaging, and transport of cellular components, and enables the cell to do work and maintain tension in tissues. The best-understood motor proteins are myosin and kinesin, which move on actin filaments and microtubules, respectively. Other motor proteins that move on DNA, on RNA, and on the protein backbones have roles in synthesis, packaging, and degradation processes. In a broad view, molecular motors couple the Brownian movements of proteins (vibrational, rotational, or translational) to the dissipation of energy (from ATP or GTP hydrolysis, or from coupling to a proton gradient). The three major types of motors, myosin, kinesin, and AAA superfamily of ATPases, are significantly different in their designs and functions. Although single-molecule studies have provided many insights into the mechanisms of motor protein function, the rules that control their motility are not well understood. The mechanisms that regulate motor protein activity are complex yet highly important because the level of force and/or displacement that is generated must be tightly controlled in order for the complex functions to be carried out properly (for more background on motors, see Howard, 2000).

Motor protein complexes constitute a major class of proteins that are needed for many functions, as outlined in the textbox above. Analyses of the genetics of motor proteins have been somewhat frustrating because knocking out individual motor proteins often produces ‘unremarkable’ phenotypes. For example, individual myosin I motor proteins, which represent a diverse group of single-headed motor proteins, can be knocked out in the amoeboid cells of Dictyostelium discoideum, with little effect. Only when several myosin I proteins are knocked out do cells show significant changes in function. In the early days of protein knockouts, this result was viewed by many as an indication that the myosin I proteins were unimportant and young faculty were discouraged from working on them because it would be doubly hard to get tenure.