Kinesins microtubule binding sites

Kinesins (20) ATP Eleavy and light chains head domains with ATPase activity and microtubule-binding site Cytoplasm Transport of cargo vesicles and chromosomes during mitosis... 

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).

An impressive amount of kinetics data have also been collected for kinesin. It has been determined that ATP binds to kinesin with a rate constant of 4 pM s" [31] and the dissociation constant, K, is about 75 pM [31], (Therefore, ATP dissociates from its binding site with a rate constant of -150 s, ) In the absence of microtubule, the rate constant for ATP hydrolysis catalyzed by kinesin is 6s" [4] and ADP releases from a kinesin with a rate constant of -0,002 s [38], Both ATP hydrolysis [39] and ADP release [40] increase to 100-300 s" and -20 s", respectively, when kinesin binds microtubule. It was also found that the chemical processes at the two heads are cooperative the ADP release from one head is further accelerated (60-300 s ) when ATP is bound at the other head. However, it is believed that ADP release ranains the rate-limiting step [31,38], Regardless of the experimental conditions, P, releases at a rate of >100 s [31,41], The kinetic measurements also showed that an ADP occupied kinesin head only binds the microtubule weakly 10-20 pM [4]) compared to an empty head [31,42],... 

MAPs may be defoed as proteins with specific binding sites for tubulins. It is not known whether dynein, kinesin and the retrograde translocator (see IG-nesin) are MAPs by this definition, because their affinity to microtubules may be mediated by other MAPs. The classic MAPs are listed below. 

Even though dynein, kinesin, and myosin serve similar ATPase-dependent chemomechanical functions and have structural similarities, they do not appear to be related to each other in molecular terms. Their similarity lies in the overall shape of the molecule, which is composed of a pair of globular heads that bind microtubules and a fan-shaped tail piece (not present in myosin) that is suspected to carry the attachment site for membranous vesicles and other cytoplasmic components transported by MT. The cytoplasmic and axonemal dyneins are similar in structure (Hirokawa et al., 1989 Holzbaur and Vallee, 1994). Current studies on mutant phenotypes are likely to lead to a better understanding of the cellular roles of molecular motor proteins and their mechanisms of action (Endow and Titus, 1992). 

The spacing between identical subunits on microtubules is 8 nm. Thus, a kinesin with a step size that is not a multiple of 8 nm would have to be able to bind on more than one type of site on the microtubule surface. 

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