The two major families of eukaryotic cellular motor ATPases, kinesin and myosin, constitute the myosin-kinesin superfamily within the TRAFAC class of GTPases. The ATPase domain of myosins along with that of the kinesin family of microtubule based motors is believed to have evolved from the core GTPase domain through deletion of strands 6 and 7 and addition of two N-terminal strands [1,2]. Kinesin [3,4,5] is a microtubule-associated force-producing protein that may play a role in organelle transport. Kinesin is an oligomeric complex composed of two heavy chains and two light chains. The kinesin motor activity is directed toward the microtubule's plus end.

The heavy chain is composed of three structural domains: a large globular N-terminal domain which is responsible for the motor activity of kinesin (it is known to hydrolyze ATP, to bind and move on microtubules), a central α-helical coiled coil domain that mediates the heavy chain dimerization; and a small globular C-terminal domain which interacts with other proteins (such as the kinesin light chains), vesicles and membranous organelles.

The kinesin motor domain comprises five motifs, namely N1 (P-loop), N2 (Switch I), N3 (Switch II), N4 and L2 (KVD finger) [7]. It has a mixed eight stranded β-sheet core with flanking solvent exposed α-helices and a small three-stranded antiparallel β-sheet in the N-terminal region [8].

A number of proteins have been recently found that contain a domain similar to that of the kinesin 'motor' domain [3,9]:

• Drosophila claret segregational protein (ncd). Ncd is required for normal chromosomal segregation in meiosis, in females, and in early mitotic divisions of the embryo. The ncd motor activity is directed toward the microtubule's minus end.
• Drosophila kinesin-like protein (nod). Nod is required for the distributive chromosome segregation of nonexchange chromosomes during meiosis.
• Human CENP-E [4]. CENP-E is a protein that associates with kinetochores during chromosome congression, relocates to the spindle midzone at anaphase, and is quantitatively discarded at the end of the cell division. CENP-E is probably an important motor molecule in chromosome movement and/ or spindle elongation.
• Human mitotic kinesin-like protein-1 (MKLP-1), a motor protein whose activity is directed toward the microtubule's plus end.
• Yeast KAR3 protein, which is essential for yeast nuclear fusion during mating. KAR3 may mediate microtubule sliding during nuclear fusion and possibly mitosis.
• Yeast CIN8 and KIP1 proteins which are required for the assembly of the mitotic spindle. Both proteins seem to interact with spindle microtubules to produce an outwardly directed force acting upon the poles.
• Fission yeast cut7 protein, which is essential for spindle body duplication during mitotic division.
• Emericella nidulans bimC, which plays an important role in nuclear division.
• Emericella nidulans klpA.
• Caenorhabditis elegans unc-104, which may be required for the transport of substances needed for neuronal cell differentiation.
• Caenorhabditis elegans osm-3.
• Xenopus Eg5, which may be involved in mitosis.
• Arabidopsis thaliana KatA, KatB and katC.
• Chlamydomonas reinhardtii FLA10/KHP1 and KLP1. Both proteins seem to play a role in the rotation or twisting of the microtubules of the flagella.
• Caenorhabditis elegans hypothetical protein T09A5.2.

The kinesin motor domain is located in the N-terminal part of most of the above proteins, with the exception of KAR3, klpA, and ncd where it is located in the C-terminal section.

The kinesin motor domain contains about 340 amino acids. An ATP-binding motif of type A is found near position 80 to 90, the C-terminal half of the domain is involved in microtubule-binding. The signature pattern for that domain is derived from a conserved decapeptide inside the microtubule-binding part.