Abstract
Kar3Vik1 is a heterodimeric kinesin with one catalytic subunit (Kar3) and one noncatalytic subunit (Vik1). Vik1 experiences conformational changes in regions analogous to the force-producing elements in catalytic kinesins. A molecular mechanism by which Kar3 could trigger Vik1's release from microtubules was revealed. These findings will serve as the prototype for understanding the motile mechanism of kinesin-14 motors in general. It is widely accepted that movement of kinesin motor proteins is accomplished by coupling ATP binding, hydrolysis, and product release to conformational changes in the microtubule-binding and force-generating elements of their motor domain. Therefore, understanding how the Saccharomyces cerevisiae proteins Cik1 and Vik1 are able to function as direct participants in movement of Kar3Cik1 and Kar3Vik1 kinesin complexes presents an interesting challenge given that their motor homology domain (MHD) cannot bind ATP. Our crystal structures of the Vik1 ortholog from Candida glabrata may provide insight into this mechanism by showing that its neck and neck mimic-like element can adopt several different conformations reminiscent of those observed in catalytic kinesins. We found that when the neck is α-helical and interacting with the MHD core, the C terminus of CgVik1 docks onto the central β-sheet similarly to the ATP-bound form of Ncd. Alternatively, when neck-core interactions are broken, the C terminus is disordered. Mutations designed to impair neck rotation, or some of the neck-MHD interactions, decreased microtubule gliding velocity and steady state ATPase rate of CgKar3Vik1 complexes significantly. These results strongly suggest that neck rotation and neck mimic docking in Vik1 and Cik1 may be a structural mechanism for communication with Kar3.
Highlights
Kar3Vik1 is a heterodimeric kinesin with one catalytic subunit (Kar3) and one noncatalytic subunit (Vik1)
Based on this new information, we propose that the neck of Vik1 must change positions during the motile cycle of Kar3Vik1, and interactions between the neck and C terminus of Vik1 may provide the critical link for coupling release of Vik1 and Cik1 from microtubules to microtubule binding and subsequent neck rotation in Kar3 during nucleotide exchange
In both processive and nonprocessive forms of these kinesins, the neck region is observed to be either docked onto or undocked from the motor core in crystal structures, and these states of the motor are generally determined by the nucleotide state of the motor domain
Summary
Kar3Vik is a heterodimeric kinesin with one catalytic subunit (Kar3) and one noncatalytic subunit (Vik). As seen in a separate CgVik structure, this docking of the C terminus seems to be completely abolished when the neck loses its ␣-helical structure and is stabilized in a state where it is undocked from the MHD Based on this new information, we propose that the neck of Vik must change positions during the motile cycle of Kar3Vik, and interactions between the neck and C terminus of Vik may provide the critical link for coupling release of Vik and Cik from microtubules to microtubule binding and subsequent neck rotation in Kar during nucleotide exchange
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