Vik1 Modulates Microtubule-Kar3 Interactions through a Motor Domain that Lacks an Active Site

Abstract

Conventional kinesin and class V and VI myosins coordinate the mechanochemical cycles of their motor domains for processive movement of cargo along microtubules or actin filaments. It is widely accepted that this coordination is achieved by allosteric communication or mechanical strain between the motor domains, which controls the nucleotide state and interaction with microtubules or actin. However, questions remain about the interplay between the strain and the nucleotide state. We present an analysis of Saccharomyces cerevisiae Kar3/Vik1, a heterodimeric C-terminal Kinesin-14 containing catalytic Kar3 and the nonmotor protein Vik1. The X-ray crystal structure of Vik1 exhibits a similar fold to the kinesin and myosin catalytic head, but lacks an ATP binding site. Vik1 binds more tightly to microtubules than Kar3 and facilitates cooperative microtubule decoration by Kar3/Vik1 heterodimers, and yet allows motility. These results demand communication between Vik1 and Kar3 via a mechanism that coordinates their interactions with microtubules.

Drosophila Ncd reveals an evolutionarily conserved powerstroke mechanism for homodimeric and heterodimeric kinesin-14s

“…Through our recent studies on S. cerevisiae Kar3Vik1 and Kar3Cik1, we discovered novel properties of these yeast kinesin14s that challenged the earlier models of how kinesin-14s generate force for their cellular functions (13,15,(26)(27)(28). The C-terminal globular domain of Vik1 exhibits the structure of a kinesin motor domain (MD), yet Vik1 as well as Cik1 lack a nucleotide-binding site (13,26).…”

“…The C-terminal globular domain of Vik1 exhibits the structure of a kinesin motor domain (MD), yet Vik1 as well as Cik1 lack a nucleotide-binding site (13,26). Because the C-terminal domain of Vik1 binds MTs independently of Kar3 and with high affinity (26), it was designated the Vik1 motor homology domain (MHD). A series of site-directed cross-links at the base of the coiled coil near the motor heads of Kar3Vik1 were introduced, and motility assays indicated that both the Kar3MD and Vik1MHD must interact with the MT lattice to generate sustained MT gliding, yet significant unwinding of the coiled coil was not required (>10 Å but <20 Å) (13).…”

“…The C-terminal globular domain of Vik1 exhibits the structure of a kinesin motor domain (MD), yet Vik1 as well as Cik1 lack a nucleotide-binding site (13,26). Because the C-terminal domain of Vik1 binds MTs independently of Kar3 and with high affinity (26), it was designated the Vik1 motor homology domain (MHD).…”

“…(ii) MT collision is followed by rapid ADP release (E0-E1) (24). (iii) By analogy to Kar3Vik1, the head that initially collides with the MT (E1) binds α-tubulin in a noncanonical MT binding configuration and is rotated relative to its partner head that binds β-tubulin (E4) (13,26). …”

“…S1). The following assumptions were made based on earlier studies (9,13,23,24,(26)(27)(28): (i) In solution, homodimeric Ncd exhibits an asymmetry in which one head holds ADP tightly bound while the partner head holds ADP weakly (E0) (24). (ii) MT collision is followed by rapid ADP release (E0-E1) (24).…”