Kinesins protofilaments

With regard to microtubular ultrastructure, micro filaments (5-7 run in diameter) are composed of filamentous actin. The tubule-like structures are formed by a, P-tubulin heterodimers. The wall is composed of 13 parallel protofilaments. Various microtubule-associated proteins and motor proteins (kinesin and dynein) are bound to the wall. The microtubule is a polar structure, i.e., plus and minus ends. 

Fig. 19-4).212b However, single kinesin heads, which lack the coiled-coil neck region, have a duty ratio of <0.45. The movement is nonpro-cessive.213 The Ned motor is also nonprocessive.214-216 As mentioned previously, the Ned and kinesin motor domains are at opposite ends of the peptide chain, and the motors move in opposite directions along microtubules.217 218 The critical difference between the two motor molecules was found in the neck domains, which gave rise to differing symmetries in the two heads.219 The latter are shown in Fig. 19-20, in which they have been docked onto the tubulin protofilament structure.

Figure 19-20 Model showing the ned and kinesin dimer structures docked onto a tubulin protofilament. The bound ned and kinesin heads are positioned similarly. Because of the distinct architectures of the kinesin and ned necks, the unbound kinesin head points toward the plus end, whereas the unbound ned head is tilted toward the minus end of the protofilament. From Sablin et al.219 Courtesy of Ronald Vale.

Fig. 5. 3D EM shows how kinesin and tau bind to microtubules. (A) Reconstruction of a microtubule decorated with kinesin heads (ochre). One kinesin head binds per afi-tubulin heterodimer (grey) and, due to its asymmetric form, can be used to distinguish between the subunits. (B) Inside view of a microtubule that was coassembled with gold-labeled tau and decorated with kinesin heads. The kinesin heads can be seen on the outside through the holes between the protofilaments. The labeled repeat motif of tau binds to the inside face of microtubule. The averaged nanogold density (yellow), which is attached to a repeat motif of tau through a linker, can only be seen near the Taxol binding site of -tubulin, but not on the a subunit (Kar et al, 2003a). The ribbon diagram of the refined zinc-sheet structure is also shown for reference (see Figure 3).

Ogawa, T., Nitta, R., Okada, Y., and Hirokawa, N. (2004). A common mechanism for microtubule destabilizers-M type kinesins stabilize curling of the protofilament using the class-specific neck and loops. CeU, 116, 591-602. 

This is illustrated in Fig. 4, which shows schematically how a kinesin dimer could walk along a microtubule protofilament. 

Fig. 4. Dimeric kinesin moving along a microtubule protofilament. Four snapshots taken from an animated cartoon that illustrates how a kinesin dimer could walk along a microtubule. The pictures shown here are all from one half-cycle one head (green/blue) is fixed while the other one (yellow/red) orbits around the common neck and stalk (stalk not shown). During the second half-cycle, the motor domains change their roles. (The animated cartoon is available from http //www. mpasmb-hamburg.mpg.de/.)...

FIGURE 1.1 (See color insert following page 172.) Kinesin and microtubule. (A) Conventional kinesins are homodimer, each of the monomer is made of head, neck-linker, and neck-helix domain. The neck-linkers of two heads are colored in green and yellow, respectively. The neck-helices from the two monomers associate the two subunits. (B) A microtubule with 13 protofilaments, each of which is made of an 8nm periodic head-to-taU alignment of the tubulin dimer subunits. A single protofilament that is used as a track for kinesin is colored in red. Kinesins take steps hand-over-hand along the protofilament. 

Berhner E., Young E.C., Anderson K., Mahtani H.K., and GeUes J. 1995. Failure of a single-headed kinesin to track parallel to microtubule protofilaments. Nature 373 718-721. 

---