Kinesin Kar3Cik1 ATPase pathway for microtubule cross-linking

Modeling Study of the Dynamics of Kinesin-14 Molecular Motors

Kinesin-14s constitute a subfamily of the large superfamily of adenosine triphosphate-dependent microtubule-based motor proteins. Kinesin-14s have the motor domain at the C-terminal end of the peptide, playing key roles during spindle assembly and maintenance. Some of them are nonprocessive motors, whereas others can move processively on microtubules. Here, we take budding yeast Cik1-Kar3 and human HSET as examples to study theoretically the dynamics of the processive kinesin-14 motor moving on the single microtubule under load, the dynamics of the motor coupled with an Ndc80 protein moving on the single microtubule, the dynamics of the motor moving in microtubule arrays, and so on. The dynamics of the nonprocessive Drosophila Ncd motor is also discussed. The studies explain well the available experimental data and, moreover, provide predicted results. We show that the processive kinesin-14 motors can move efficiently in microtubule arrays toward the minus ends, and after reaching the minus ends, they can stay there stably, thus performing the function of organizing the microtubules in the bipolar spindle into polar arrays at the spindle poles.

Modeling processive motion of kinesin-13 MCAK and kinesin-14 Cik1-Kar3 molecular motors

Kinesin-13 MCAK, which is composed of two identical motor domains, can undergo unbiased one-dimensional diffusion on microtubules. Kinesin-14 Cik1-Kar3, which is composed of a Kar3 motor domain and a Cik1 motor homology domain with no ATPase activity, can move processively toward the minus end of microtubules. Here, we present a model for the diffusion of MCAK homodimer and a model for the processive motion of Cik1-Kar3 heterodimer. Although the two dimeric motors show different domain composition, in the models it is proposed that the two motors use the similar physical mechanism to move processively. With the models, the dynamics of the two dimers is studied analytically. The theoretical results for MCAK reproduce quantitatively the available experimental data about diffusion constant and lifetime of the motor bound to microtubule in different nucleotide states. The theoretical results for Cik1-Kar3 reproduce quantitatively the available experimental data about load dependence of velocity and explain consistently the available experimental data about effects of the exchange and mutation of the motor homology domain on the velocity of the heterodimer. Moreover, predicted results for other aspects of the dynamics of the two dimers are provided.

Heterodimerization of Kinesin-2 KIF3AB Modulates Entry into the Processive Run

Mammalian KIF3AB is an N-terminal processive kinesin-2 that is best known for its roles in intracellular transport. There has been significant interest in KIF3AB to define the key principles that underlie its processivity but also to define the mechanistic basis of its sensitivity to force. In this study, the kinetics for entry into the processive run were quantified. The results show for KIF3AB that the kinetics of microtubule association at 7 μm^-1 s^-1 is less than the rates observed for KIF3AA at 13 μm^-1 s^-1 or KIF3BB at 11.9 μm^-1 s^-1 ADP release after microtubule association for KIF3AB is 33 s^-1 and is significantly slower than ADP release from homodimeric KIF3AA and KIF3BB, which reach 80-90 s^-1 To explore the interhead communication implied by the rate differences at these first steps, we compared the kinetics of KIF3AB microtubule association followed by ADP release with the kinetics for mixtures of KIF3AA plus KIF3BB. Surprisingly, the kinetics of KIF3AB are not equivalent to any of the mixtures of KIF3AA + KIF3BB. In fact, the transients for each of the mixtures overlay the transients for KIF3AA and KIF3BB. These results reveal that intermolecular communication within the KIF3AB heterodimer modulates entry into the processive run, and the results suggest that it is the high rate of microtubule association that drives rebinding to the microtubule after force-dependent motor detachment.

Candida albicans Kinesin Kar3 Depends on a Cik1-Like Regulatory Partner Protein for Its Roles in Mating, Cell Morphogenesis, and Bipolar Spindle Formation

Candida albicans is a major fungal pathogen whose virulence is associated with its ability to transition from a budding yeast form to invasive hyphal filaments. The kinesin-14 family member CaKar3 is required for transition between these morphological states, as well as for mitotic progression and karyogamy. While kinesin-14 proteins are ubiquitous, CaKar3 homologs in hemiascomycete fungi are unique because they form heterodimers with noncatalytic kinesin-like proteins. Thus, CaKar3-based motors may represent a novel antifungal drug target. We have identified and examined the roles of a kinesin-like regulator of CaKar3. We show that orf19.306 (dubbed CaCIK1) encodes a protein that forms a heterodimer with CaKar3, localizes CaKar3 to spindle pole bodies, and can bind microtubules and influence CaKar3 mechanochemistry despite lacking an ATPase activity of its own. Similar to CaKar3 depletion, loss of CaCik1 results in cell cycle arrest, filamentation defects, and an inability to undergo karyogamy. Furthermore, an examination of the spindle structure in cells lacking either of these proteins shows that a large proportion have a monopolar spindle or two dissociated half-spindles, a phenotype unique to the C. albicans kinesin-14 homolog. These findings provide new insights into mitotic spindle structure and kinesin motor function in C. albicans and identify a potentially vulnerable target for antifungal drug development.

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

Drosophila melanogaster kinesin-14 Ncd cross-links parallel microtubules at the spindle poles and antiparallel microtubules within the spindle midzone to play roles in bipolar spindle assembly and proper chromosome distribution. As observed for Saccharomyces cerevisiae kinesin-14 Kar3Vik1 and Kar3Cik1, Ncd binds adjacent microtubule protofilaments in a novel microtubule binding configuration and uses an ATP-promoted powerstroke mechanism. The hypothesis tested here is that Kar3Vik1 and Kar3Cik1, as well as Ncd, use a common ATPase mechanism for force generation even though the microtubule interactions for both Ncd heads are modulated by nucleotide state. The presteady-state kinetics and computational modeling establish an ATPase mechanism for a powerstroke model of Ncd that is very similar to those determined for Kar3Vik1 and Kar3Cik1, although these heterodimers have one Kar3 catalytic motor domain and a Vik1/Cik1 partner motor homology domain whose interactions with microtubules are not modulated by nucleotide state but by strain. The results indicate that both Ncd motor heads bind the microtubule lattice; two ATP binding and hydrolysis events are required for each powerstroke; and a slow step occurs after microtubule collision and before the ATP-promoted powerstroke. Note that unlike conventional myosin-II or other processive molecular motors, Ncd requires two ATP turnovers rather than one for a single powerstroke-driven displacement or step. These results are significant because all metazoan kinesin-14s are homodimers, and the results presented show that despite their structural and functional differences, the heterodimeric and homodimeric kinesin-14s share a common evolutionary structural and mechanochemical mechanism for force generation.

Kinesin Motor Enzymology: Chemistry, Structure, and Physics of Nanoscale Molecular Machines

Molecular motors are enzymes that convert chemical potential energy into controlled kinetic energy for mechanical work inside cells. Understanding the biophysics of these motors is essential for appreciating life as well as apprehending diseases that arise from motor malfunction. This review focuses on kinesin motor enzymology with special emphasis on the literature that reports the chemistry, structure and physics of several different kinesin superfamily members.

Non-catalytic motor domains enable processive movement and functional diversification of the kinesin-14 Kar3

Motor proteins of the conserved kinesin-14 family have important roles in mitotic spindle organization and chromosome segregation. Previous studies have indicated that kinesin-14 motors are non-processive enzymes, working in the context of multi-motor ensembles that collectively organize microtubule networks. In this study, we show that the yeast kinesin-14 Kar3 generates processive movement as a heterodimer with the non-motor proteins Cik1 or Vik1. By analyzing the single-molecule properties of engineered motors, we demonstrate that the non-catalytic domain has a key role in the motility mechanism by acting as a 'foothold' that allows Kar3 to bias translocation towards the minus end. This mechanism rivals the speed and run length of conventional motors, can support transport of the Ndc80 complex in vitro and is critical for Kar3 function in vivo. Our findings provide an example for a non-conventional translocation mechanism and can explain how Kar3 substitutes for key functions of Dynein in the yeast nucleus.

Kinesin-2 KIF3AB exhibits novel ATPase characteristics

KIF3AB is an N-terminal processive kinesin-2 family member best known for its role in intraflagellar transport. There has been significant interest in KIF3AB in defining the key principles that underlie the processivity of KIF3AB in comparison with homodimeric processive kinesins. To define the ATPase mechanism and coordination of KIF3A and KIF3B stepping, a presteady-state kinetic analysis was pursued. For these studies, a truncated murine KIF3AB was generated. The results presented show that microtubule association was fast at 5.7 μm(-1) s(-1), followed by rate-limiting ADP release at 12.8 s(-1). ATP binding at 7.5 μm(-1) s(-1) was followed by an ATP-promoted isomerization at 84 s(-1) to form the intermediate poised for ATP hydrolysis, which then occurred at 33 s(-1). ATP hydrolysis was required for dissociation of the microtubule·KIF3AB complex, which was observed at 22 s(-1). The dissociation step showed an apparent affinity for ATP that was very weak (K½,ATP at 133 μm). Moreover, the linear fit of the initial ATP concentration dependence of the dissociation kinetics revealed an apparent second-order rate constant at 0.09 μm(-1) s(-1), which is inconsistent with fast ATP binding at 7.5 μm(-1) s(-1) and a Kd ,ATP at 6.1 μm. These results suggest that ATP binding per se cannot account for the apparent weak K½,ATP at 133 μm. The steady-state ATPase Km ,ATP, as well as the dissociation kinetics, reveal an unusual property of KIF3AB that is not yet well understood and also suggests that the mechanochemistry of KIF3AB is tuned somewhat differently from homodimeric processive kinesins.

Common mechanistic themes for the powerstroke of kinesin-14 motors

Kar3Cik1 is a heterodimeric kinesin-14 from Saccharomyces cerevisiae involved in spindle formation during mitosis and karyogamy in mating cells. Kar3 represents a canonical kinesin motor domain that interacts with microtubules under the control of ATP-hydrolysis. In vivo, the localization and function of Kar3 is differentially regulated by its interacting stoichiometrically with either Cik1 or Vik1, two closely related motor homology domains that lack the nucleotide-binding site. Indeed, Vik1 structurally resembles the core of a kinesin head. Despite being closely related, Kar3Cik1 and Kar3Vik1 are each responsible for a distinct set of functions in vivo and also display different biochemical behavior in vitro. To determine a structural basis for their distinct functional abilities, we used cryo-electron microscopy and helical reconstruction to investigate the 3-D structure of Kar3Cik1 complexed to microtubules in various nucleotide states and compared our 3-D data of Kar3Cik1 with that of Kar3Vik1 and the homodimeric kinesin-14 Ncd from Drosophila melanogaster. Due to the lack of an X-ray crystal structure of the Cik1 motor homology domain, we predicted the structure of this Cik1 domain based on sequence similarity to its relatives Vik1, Kar3 and Ncd. By molecular docking into our 3-D maps, we produced a detailed near-atomic model of Kar3Cik1 complexed to microtubules in two distinct nucleotide states, a nucleotide-free state and an ATP-bound state. Our data show that despite their functional differences, heterodimeric Kar3Cik1 and Kar3Vik1 and homodimeric Ncd, all share striking structural similarities at distinct nucleotide states indicating a common mechanistic theme within the kinesin-14 family.

Spindle pole body-anchored Kar3 drives the nucleus along microtubules from another nucleus in preparation for nuclear fusion during yeast karyogamy

Nuclear migration during yeast karyogamy, termed nuclear congression, is required to initiate nuclear fusion. Congression involves a specific regulation of the microtubule minus end-directed kinesin-14 motor Kar3 and a rearrangement of the cytoplasmic microtubule attachment sites at the spindle pole bodies (SPBs). However, how these elements interact to produce the forces necessary for nuclear migration is less clear. We used electron tomography, molecular genetics, quantitative imaging, and first principles modeling to investigate how cytoplasmic microtubules are organized during nuclear congression. We found that Kar3, with the help of its light chain, Cik1, is anchored during mating to the SPB component Spc72 that also serves as a nucleator and anchor for microtubules via their minus ends. Moreover, we show that no direct microtubule-microtubule interactions are required for nuclear migration. Instead, SPB-anchored Kar3 exerts the necessary pulling forces laterally on microtubules emanating from the SPB of the mating partner nucleus. Therefore, a twofold symmetrical application of the core principle that drives nuclear migration in higher cells is used in yeast to drive nuclei toward each other before nuclear fusion.

Functional asymmetry in kinesin and dynein dimers

Active transport along the microtubule lattice is a complex process that involves both the Kinesin and Dynein superfamily of motors. Transportation requires sophisticated regulation much of which occurs through the motor's tail domain. However, a significant portion of this regulation also occurs through structural changes that arise in the motor and the microtubule upon binding. The most obvious structural change being the manifestation of asymmetry. To a first approximation in solution, kinesin dimers exhibit twofold symmetry, and microtubules exhibit helical symmetry. The higher symmetries of both the kinesin dimers and microtubule lattice are lost on formation of the kinesin-microtubule complex. Loss of symmetry has functional consequences such as an asymmetric hand-over-hand mechanism in plus-end-directed kinesins, asymmetric microtubule binding in the Kinesin-14 family, spatially biased stepping in dynein and cooperative binding of additional motors to the microtubule. This review focusses on how the consequences of asymmetry affect regulation of motor heads within a dimer, dimers within an ensemble of motors, and suggests how these asymmetries may affect regulation of active transport within the cell.

Neck rotation and neck mimic docking in the noncatalytic Kar3-associated protein Vik1

BACKGROUND

Kar3Vik1 is a heterodimeric kinesin with one catalytic subunit (Kar3) and one noncatalytic subunit (Vik1).

RESULTS

Vik1 experiences conformational changes in regions analogous to the force-producing elements in catalytic kinesins.

CONCLUSION

A molecular mechanism by which Kar3 could trigger Vik1's release from microtubules was revealed.

SIGNIFICANCE

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.

The ATPase pathway that drives the kinesin-14 Kar3Vik1 powerstroke

Kar3, a Saccharomyces cerevisiae microtubule minus-end-directed kinesin-14, dimerizes with either Vik1 or Cik1. The C-terminal globular domain of Vik1 exhibits the structure of a kinesin motor domain and binds microtubules independently of Kar3 but lacks a nucleotide binding site. The only known function of Kar3Vik1 is to cross-link parallel microtubules at the spindle poles during mitosis. In contrast, Kar3Cik1 depolymerizes microtubules during mating but cross-links antiparallel microtubules in the spindle overlap zone during mitosis. A recent study showed that Kar3Vik1 binds across adjacent microtubule protofilaments and uses a minus-end-directed powerstroke to drive ATP-dependent motility. The presteady-state experiments presented here extend this study and establish an ATPase model for the powerstroke mechanism. The results incorporated into the model indicate that Kar3Vik1 collides with the microtubule at 2.4 μm(-1) s(-1) through Vik1, promoting microtubule binding by Kar3 followed by ADP release at 14 s(-1). The tight binding of Kar3 to the microtubule destabilizes the Vik1 interaction with the microtubule, positioning Kar3Vik1 for the start of the powerstroke. Rapid ATP binding to Kar3 is associated with rotation of the coiled-coil stalk, and the postpowerstroke ATP hydrolysis at 26 s(-1) is independent of Vik1, providing further evidence that Vik1 rotates with the coiled coil during the powerstroke. Detachment of Kar3Vik1 from the microtubule at 6 s(-1) completes the cycle and allows the motor to return to its initial conformation. The results also reveal key differences in the ATPase cycles of Kar3Vik1 and Kar3Cik1, supporting the fact that these two motors have distinctive biological functions.

Kar3Vik1, a member of the kinesin-14 superfamily, shows a novel kinesin microtubule binding pattern

Kinesin-14 motors generate microtubule minus-end-directed force used in mitosis and meiosis. These motors are dimeric and operate with a nonprocessive powerstroke mechanism, but the role of the second head in motility has been unclear. In Saccharomyces cerevisiae, the Kinesin-14 Kar3 forms a heterodimer with either Vik1 or Cik1. Vik1 contains a motor homology domain that retains microtubule binding properties but lacks a nucleotide binding site. In this case, both heads are implicated in motility. Here, we show through structural determination of a C-terminal heterodimeric Kar3Vik1, electron microscopy, equilibrium binding, and motility that at the start of the cycle, Kar3Vik1 binds to or occludes two αβ-tubulin subunits on adjacent protofilaments. The cycle begins as Vik1 collides with the microtubule followed by Kar3 microtubule association and ADP release, thereby destabilizing the Vik1-microtubule interaction and positioning the motor for the start of the powerstroke. The results indicate that head-head communication is mediated through the adjoining coiled coil.

Microtubule capture by mitotic kinesin centromere protein E (CENP-E)

Centromere protein E, CENP-E, is a kinetochore-associated kinesin-7 that establishes the microtubule-chromosome linkage and transports monooriented chromosomes to the spindle equator along kinetochore fibers of already bioriented chromosomes. As a processive kinesin, CENP-E uses a hand-over-hand mechanism, yet a number of studies suggest that CENP-E exhibits mechanistic differences from other processive kinesins that may be important for its role in chromosome congression. The results reported here show that association of CENP-E with the microtubule is unusually slow at 0.08 μM(-1) s(-1) followed by slow ADP release at 0.9 s(-1). ATP binding and hydrolysis are fast with motor dissociation from the microtubule at 1.4 s(-1), suggesting that CENP-E head detachment from the microtubule, possibly controlled by phosphate release, determines the rate of stepping during a processive run because the rate of microtubule gliding corresponds to 1.4 steps/s. We hypothesize that the unusually slow CENP-E microtubule association step favors CENP-E binding of stable microtubules over dynamic ones, a mechanism that would bias CENP-E binding to kinetochore fibers.

Unconventional functions of microtubule motors

With the functional characterization of proteins advancing at fast pace, the notion that one protein performs different functions - often with no relation to each other - emerges as a novel principle of how cells work. Molecular motors are no exception to this new development. Here, we provide an account on recent findings revealing that microtubule motors are multifunctional proteins that regulate many cellular processes, in addition to their main function in transport. Some of these functions rely on their motor activity, but others are independent of it. Of the first category, we focus on the role of microtubule motors in organelle biogenesis, and in the remodeling of the cytoskeleton, especially through the regulation of microtubule dynamics. Of the second category, we discuss the function of microtubule motors as static anchors of the cargo at the destination, and their participation in regulating signaling cascades by modulating interactions between signaling proteins, including transcription factors. We also review atypical forms of transport, such as the cytoplasmic streaming in the oocyte, and the movement of cargo by microtubule fluctuations. Our goal is to provide an overview of these unexpected functions of microtubule motors, and to incite future research in this expanding field.