Kinesins are molecular motors that transport various cargoes along microtubule tracks using energy derived from ATP hydrolysis. Although the motor domains of kinesins are structurally similar, the family contains members that move on microtubules in opposite directions. Recent biochemical and biophysical studies of several kinesins make it possible to identify structural elements responsible for the different directionality, suggesting that reversal of the motor movement can be achieved through small, local changes in the protein structure. Molecular motors use energy derived from ATP hydrolysis to move unidirectionally along cytoskeletal ‘tracks’; myosins use actin filaments, kinesins and dyneins — microtubules. Kinesin motors constitute a large superfamily of motor proteins which participate in numerous biological processes such as transport of vesicles and organelles, organization of spindle microtubules, and chromosome segregation (reviewed by Hirokawa, 1998; Woehlke & Schliwa, 2000). Kinesins share a common force-generating element, called ‘motor domain ’ that hydrolyzes ATP and binds to microtubules. Structural analyses of kinesins have revealed that the topography of their nucleotide-binding pockets is similar to those of myosins and G-proteins. This raises the possibility that all these protein structures evolved from a common ancestor (Kull et al., 1998). Besides the well conserved motor domain kinesins possess a superhelical segment (‘stalk’) that differs for each of th
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