Kinesin-2 KIF3AB exhibits novel ATPase characteristics

Pathogenic mutations in the kinesin-3 motor KIF1A diminish force generation and movement through allosteric mechanisms

The kinesin-3 motor KIF1A functions in neurons, where its fast and superprocessive motility facilitates long-distance transport, but little is known about its force-generating properties. Using optical tweezers, we demonstrate that KIF1A stalls at an opposing load of ~3 pN but more frequently detaches at lower forces. KIF1A rapidly reattaches to the microtubule to resume motion due to its class-specific K-loop, resulting in a unique clustering of force generation events. To test the importance of neck linker docking in KIF1A force generation, we introduced mutations linked to human neurodevelopmental disorders. Molecular dynamics simulations predict that V8M and Y89D mutations impair neck linker docking. Indeed, both mutations dramatically reduce the force generation of KIF1A but not the motor’s ability to rapidly reattach to the microtubule. Although both mutations relieve autoinhibition of the full-length motor, the mutant motors display decreased velocities, run lengths, and landing rates and delayed cargo transport in cells. These results advance our understanding of how mutations in KIF1A can manifest in disease.

Force Dependence of Velocity and Run Length of Kinesin-1, Kinesin-2 and Kinesin-5 Family Molecular Motors

Kinesin-1, kinesin-2 and kinesin-5 are three families of a superfamily of motor proteins; which can walk processively on microtubule filaments by hydrolyzing ATP. It was experimentally shown that while the three kinesin dimers show similar feature on the force dependence of velocity, they show rather different features on the force dependence of run length. However, why the three families of kinesins show these rather different features is unclear. Here, we computationally studied the movement dynamics of the three dimers based on our proposed model. The simulated results reproduce well the available experimental data on the force dependence of velocity and run length. Moreover, the simulated results on the velocity and run length for the three dimers with altered neck linker lengths are also in quantitative agreement with the available experimental data. The studies indicate that the three families of kinesins show much similar movement mechanism and the rather different features on the force dependence of run length arise mainly from the difference in rate constants of the ATPase activity and neck linker docking. Additionally, the asymmetric (limping) movement dynamics of the three families of homodimers with and without altered neck linker lengths are studied, providing predicted results.

Interaction of heterotrimeric kinesin-II with IFT-B-connecting tetramer is crucial for ciliogenesis

Intraflagellar transport (IFT) is crucial for the assembly and maintenance of cilia and is mediated by IFT particles containing IFT-A and IFT-B complexes. IFT-B powered by heterotrimeric kinesin-II and IFT-A powered by the dynein-2 complex are responsible for anterograde and retrograde protein trafficking, respectively. However, little is known about the molecular basis of the trafficking of these IFT particles regulated by kinesin and dynein motors. Using the visible immunoprecipitation assay, we identified in this study a three-to-four protein interaction involving the kinesin-II trimer KIF3A-KIF3B-KAP3 and the IFT-B-connecting tetramer IFT38-IFT52-IFT57-IFT88; among the kinesin-II subunits, KIF3B contributed mainly to IFT-B binding. Furthermore, we showed that the ciliogenesis defect of KIF3B-knockout cells can be rescued by the exogenous expression of wild-type KIF3B but not by that of its mutant compromised with respect to IFT-B binding. Thus, interaction of heterotrimeric kinesin-II with the IFT-B-connecting tetramer is crucial for ciliogenesis via the powering of IFT particles to move in the anterograde direction.

Kinesin-2 heterodimerization alters entry into a processive run along the microtubule but not stepping within the run

Heterodimeric KIF3AC and KIF3AB, two members of the mammalian kinesin-2 family, generate force for microtubule plus end-directed cargo transport. However, the advantage of heterodimeric kinesins over homodimeric ones is not well-understood. We showed previously that microtubule association for entry into a processive run was similar in rate for KIF3AC and KIF3AB at ∼7.0 μm^-1 s^-1 Yet, for engineered homodimers of KIF3AA and KIF3BB, this constant is significantly faster at 11 μm^-1 s^-1 and much slower for KIF3CC at 2.1 μm^-1 s^-1 These results led us to hypothesize that heterodimerization of KIF3A with KIF3C and KIF3A with KIF3B altered the intrinsic catalytic properties of each motor domain. Here, we tested this hypothesis by using presteady-state stopped-flow kinetics and mathematical modeling. Surprisingly, the modeling revealed that the catalytic properties of KIF3A and KIF3B/C were altered upon microtubule binding, yet each motor domain retained its relative intrinsic kinetics for ADP release and subsequent ATP binding and the nucleotide-promoted transitions thereafter. These results are consistent with the interpretation that for KIF3AB, each motor head is catalytically similar and therefore each step is approximately equivalent. In contrast, for KIF3AC the results predict that the processive steps will alternate between a fast step for KIF3A followed by a slow step for KIF3C resulting in asymmetric stepping during a processive run. This study reveals the impact of heterodimerization of the motor polypeptides for microtubule association to start the processive run and the fundamental differences in the motile properties of KIF3C compared with KIF3A and KIF3B.

An allosteric propofol-binding site in kinesin disrupts kinesin-mediated processive movement on microtubules

Microtubule-based molecular motors mediate transport of intracellular cargo to subdomains in neurons. Previous evidence has suggested that the anesthetic propofol decreases the average run-length potential of the major anterograde transporters kinesin-1 and kinesin-2 without altering their velocity. This effect on kinesin has not been observed with other inhibitors, stimulating considerable interest in the underlying mechanism. Here, we used a photoactive derivative of propofol, meta-azipropofol (AziPm), to search for potential propofol-binding sites in kinesin. Single-molecule motility assays confirmed that AziPm and propofol similarly inhibit kinesin-1 and kinesin-2. We then applied AziPm in semiquantitative radiolabeling and MS microsequencing assays to identify propofol-binding sites within microtubule-kinesin complexes. The radiolabeling experiments suggested preferential AziPm binding to the ATP-bound microtubule-kinesin complex. The photolabeled residues were contained within the kinesin motor domain rather than at the motor domain-β-tubulin interface. No residues within the P-loop of kinesin were photolabeled, indicating an inhibitory mechanism that does not directly affect ATPase activity and has an effect on run length without changing velocity. Our results also indicated that when the kinesin motor interacts with the microtubule during its processive run, a site forms in kinesin to which propofol can then bind and allosterically disrupt the kinesin-microtubule interaction, resulting in kinesin detachment and run termination. The discovery of the propofol-binding allosteric site in kinesin may improve our understanding of the strict coordination of the motor heads during the processive run. We hypothesize that propofol's potent effect on intracellular transport contributes to various components of its anesthetic action.

Kinesin-2 motors: Kinetics and biophysics

Kinesin-2s are major transporters of cellular cargoes. This subfamily contains both homodimeric kinesins whose catalytic domains result from the same gene product and heterodimeric kinesins with motor domains derived from two different gene products. In this Minireview, we focus on the progress to define the biochemical and biophysical properties of the kinesin-2 family members. Our understanding of their mechanochemical capabilities has been advanced by the ability to identify the kinesin-2 genes in multiple species, expression and purification of these motors for single-molecule and ensemble assays, and development of new technologies enabling quantitative measurements of kinesin activity with greater sensitivity.

Altered chemomechanical coupling causes impaired motility of the kinesin-4 motors KIF27 and KIF7

Despite their shared ability to regulate microtubule polymerization dynamics, kinesin-4 motors display dramatically different motility properties ranging from fast processive motility to no movement. Yue et al. demonstrate that for KIF7 and KIF27, altered chemomechanical coupling results in immotile behavior and slow processive movement, respectively.

Kinesin-4 motors play important roles in cell division, microtubule organization, and signaling. Understanding how motors perform their functions requires an understanding of their mechanochemical and motility properties. We demonstrate that KIF27 can influence microtubule dynamics, suggesting a conserved function in microtubule organization across the kinesin-4 family. However, kinesin-4 motors display dramatically different motility characteristics: KIF4 and KIF21 motors are fast and processive, KIF7 and its Drosophila melanogaster homologue Costal2 (Cos2) are immotile, and KIF27 is slow and processive. Neither KIF7 nor KIF27 can cooperate for fast processive transport when working in teams. The mechanistic basis of immotile KIF7 behavior arises from an inability to release adenosine diphosphate in response to microtubule binding, whereas slow processive KIF27 behavior arises from a slow adenosine triphosphatase rate and a high affinity for both adenosine triphosphate and microtubules. We suggest that evolutionarily selected sequence differences enable immotile KIF7 and Cos2 motors to function not as transporters but as microtubule-based tethers of signaling complexes.

Homodimeric Kinesin-2 KIF3CC Promotes Microtubule Dynamics

KIF3C is one subunit of the functional microtubule-based kinesin-2 KIF3AC motor, an anterograde cargo transporter in neurons. However, KIF3C has also been implicated as an injury-specific kinesin that is a key regulator of axonal growth and regeneration by promoting microtubule dynamics for reorganization at the neuronal growth cone. To test its potential role as a modulator of microtubule dynamics in vitro, an engineered homodimeric KIF3CC was incorporated into a dynamic microtubule assay and examined by total internal reflection fluorescence microscopy. The results reveal that KIF3CC is targeted to the microtubule plus-end, acts as a potent catastrophe factor through an increase in microtubule catastrophe frequency, and does so by elimination of the dependence of the catastrophe rate on microtubule lifetime. Moreover, KIF3CC accelerates the catastrophe rate without altering the microtubule growth rate. Therefore, the ATP-promoted KIF3CC mechanism of catastrophe is different from the well-described catastrophe factors kinesin-13 MCAK and kinesin-8 Kip3/KIF18A. The properties of KIF3CC were not shared by heterodimeric KIF3AC and required the unique KIF3C-specific sequence extension in loop L11 at the microtubule interface. At the microtubule plus-end, the presence of KIF3CC resulted in modulation of the tapered structure typically seen in growing dynamic microtubules to microtubule blunt plus-ends. Overall our results implicate homodimeric KIF3CC as a unique promoter of microtubule catastrophe and substantiate its physiological role in cytoskeletal remodeling.

Common general anesthetic propofol impairs kinesin processivity

Propofol is the most widely used i.v. general anesthetic to induce and maintain anesthesia. It is now recognized that this small molecule influences ligand-gated channels, including the GABA_A receptor and others. Specific propofol binding sites have been mapped using photoaffinity ligands and mutagenesis; however, their precise target interaction profiles fail to provide complete mechanistic underpinnings for the anesthetic state. These results suggest that propofol and other common anesthetics, such as etomidate and ketamine, may target additional protein networks of the CNS to contribute to the desired and undesired anesthesia end points. Some evidence for anesthetic interactions with the cytoskeleton exists, but the molecular motors have received no attention as anesthetic targets. We have recently discovered that propofol inhibits conventional kinesin-1 KIF5B and kinesin-2 KIF3AB and KIF3AC, causing a significant reduction in the distances that these processive kinesins can travel. These microtubule-based motors are highly expressed in the CNS and the major anterograde transporters of cargos, such as mitochondria, synaptic vesicle precursors, neurotransmitter receptors, cell signaling and adhesion molecules, and ciliary intraflagellar transport particles. The single-molecule results presented show that the kinesin processive stepping distance decreases 40-60% with EC₅₀ values <100 nM propofol without an effect on velocity. The lack of a velocity effect suggests that propofol is not binding at the ATP site or allosteric sites that modulate microtubule-activated ATP turnover. Rather, we propose that a transient propofol allosteric site forms when the motor head binds to the microtubule during stepping.

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.

Kinesin-2 KIF3AC and KIF3AB Can Drive Long-Range Transport along Microtubules

Mammalian KIF3AC is classified as a heterotrimeric kinesin-2 that is best known for organelle transport in neurons, yet in vitro studies to characterize its single molecule behavior are lacking. The results presented show that a KIF3AC motor that includes the native helix α7 sequence for coiled-coil formation is highly processive with run lengths of ∼1.23 μm and matching those exhibited by conventional kinesin-1. This result was unexpected because KIF3AC exhibits the canonical kinesin-2 neck-linker sequence that has been reported to be responsible for shorter run lengths observed for another heterotrimeric kinesin-2, KIF3AB. However, KIF3AB with its native neck linker and helix α7 is also highly processive with run lengths of ∼1.62 μm and exceeding those of KIF3AC and kinesin-1. Loop L11, a component of the microtubule-motor interface and implicated in activating ADP release upon microtubule collision, is significantly extended in KIF3C as compared with other kinesins. A KIF3AC encoding a truncation in KIF3C loop L11 (KIF3ACΔL11) exhibited longer run lengths at ∼1.55 μm than wild-type KIF3AC and were more similar to KIF3AB run lengths, suggesting that L11 also contributes to tuning motor processivity. The steady-state ATPase results show that shortening L11 does not alter kcat, consistent with the observation that single molecule velocities are not affected by this truncation. However, shortening loop L11 of KIF3C significantly increases the microtubule affinity of KIF3ACΔL11, revealing another structural and mechanistic property that can modulate processivity. The results presented provide new, to our knowledge, insights to understand structure-function relationships governing processivity and a better understanding of the potential of KIF3AC for long-distance transport in neurons.

Family-specific Kinesin Structures Reveal Neck-linker Length Based on Initiation of the Coiled-coil

Kinesin-1, -2, -5, and -7 generate processive hand-over-hand 8-nm steps to transport intracellular cargoes toward the microtubule plus end. This processive motility requires gating mechanisms to coordinate the mechanochemical cycles of the two motor heads to sustain the processive run. A key structural element believed to regulate the degree of processivity is the neck-linker, a short peptide of 12-18 residues, which connects the motor domain to its coiled-coil stalk. Although a shorter neck-linker has been correlated with longer run lengths, the structural data to support this hypothesis have been lacking. To test this hypothesis, seven kinesin structures were determined by x-ray crystallography. Each included the neck-linker motif, followed by helix α7 that constitutes the start of the coiled-coil stalk. In the majority of the structures, the neck-linker length differed from predictions because helix α7, which initiates the coiled-coil, started earlier in the sequence than predicted. A further examination of structures in the Protein Data Bank reveals that there is a great disparity between the predicted and observed starting residues. This suggests that an accurate prediction of the start of a coiled-coil is currently difficult to achieve. These results are significant because they now exclude simple comparisons between members of the kinesin superfamily and add a further layer of complexity when interpreting the results of mutagenesis or protein fusion. They also re-emphasize the need to consider factors beyond the kinesin neck-linker motif when attempting to understand how inter-head communication is tuned to achieve the degree of processivity required for cellular function.

Fast or Slow, Either Head Can Start the Processive Run of Kinesin-2 KIF3AC

Mammalian KIF3AC contains two distinct motor polypeptides and is best known for its role in organelle transport in neurons. Our recent studies showed that KIF3AC is as processive as conventional kinesin-1, suggesting that their ATPase mechanochemistry may be similar. However, the presence of two different motor polypeptides in KIF3AC implies that there must be a cellular advantage for the KIF3AC heterodimer. The hypothesis tested was whether there is an intrinsic bias within KIF3AC such that either KIF3A or KIF3C initiates the processive run. To pursue these experiments, a mechanistic approach was used to compare the pre-steady-state kinetics of KIF3AC to the kinetics of homodimeric KIF3AA and KIF3CC. The results indicate that microtubule collision at 11.4 μM(-1) s(-1) coupled with ADP release at 78 s(-1) are fast steps for homodimeric KIF3AA. In contrast, KIF3CC exhibits much slower microtubule association at 2.1 μM(-1) s(-1) and ADP release at 8 s(-1). For KIF3AC, microtubule association at 6.6 μM(-1) s(-1) and ADP release at 51 s(-1) are intermediate between the constants for KIF3AA and KIF3CC. These results indicate that either KIF3A or KIF3C can initiate the processive run. Surprisingly, the kinetics of the initial event of microtubule collision followed by ADP release for KIF3AC is not equivalent to 1:1 mixtures of KIF3AA plus KIF3CC homodimers at the same motor concentration. These results reveal that the intermolecular communication within the KIF3AC heterodimer modulates entry into the processive run regardless of whether the run is initiated by the KIF3A or KIF3C motor domain.

The Mechanochemical Cycle of Mammalian Kinesin-2 KIF3A/B under Load

The response of motor proteins to external loads underlies their ability to work in teams and determines the net speed and directionality of cargo transport. The mammalian kinesin-2, KIF3A/B, is a heterotrimeric motor involved in intraflagellar transport and vesicle motility in neurons. Bidirectional cargo transport is known to result from the opposing activities of KIF3A/B and dynein bound to the same cargo, but the load-dependent properties of kinesin-2 are poorly understood. We used a feedback-controlled optical trap to probe the velocity, run length, and unbinding kinetics of mouse KIF3A/B under various loads and nucleotide conditions. The kinesin-2 motor velocity is less sensitive than kinesin-1 to external forces, but its processivity diminishes steeply with load, and the motor was observed occasionally to slip and reattach. Each motor domain was characterized by studying homodimeric constructs, and a global fit to the data resulted in a comprehensive pathway that quantifies the principal force-dependent kinetic transitions. The properties of the KIF3A/B heterodimer are intermediate between the two homodimers, and the distinct load-dependent behavior is attributable to the properties of the motor domains and not to the neck linkers or the coiled-coil stalk. We conclude that the force-dependent movement of KIF3A/B differs significantly from conventional kinesin-1. Against opposing dynein forces, KIF3A/B motors are predicted to rapidly unbind and rebind, resulting in qualitatively different transport behavior from kinesin-1.

Processivity of the kinesin-2 KIF3A results from rear head gating and not front head gating

The kinesin-2 family motor KIF3A/B works together with dynein to bidirectionally transport intraflagellar particles, melanosomes, and neuronal vesicles. Compared with kinesin-1, kinesin-2 is less processive, and its processivity is more sensitive to load, suggesting that processivity may be controlled by different gating mechanisms. We used stopped-flow and steady-state kinetics experiments, along with single-molecule and multimotor assays to characterize the entire kinetic cycle of a KIF3A homodimer that exhibits motility similar to that of full-length KIF3A/B. Upon first encounter with a microtubule, the motor rapidly exchanges both mADP and mATP. When adenosine 5'-[(β,γ)-imido]triphosphate was used to entrap the motor in a two-head-bound state, exchange kinetics were unchanged, indicating that rearward strain in the two-head-bound state does not alter nucleotide binding to the front head. A similar lack of front head gating was found when intramolecular strain was enhanced by shortening the neck linker domain from 17 to 14 residues. In single-molecule assays in ADP, the motor dissociates at 2.1 s(-1), 20-fold slower than the stepping rate, demonstrating the presence of rear head gating. In microtubule pelleting assays, the KD(Mt) is similar in ADP and ATP. The data and accompanying simulations suggest that, rather than KIF3A processivity resulting from strain-dependent regulation of nucleotide binding (front head gating), the motor spends a significant fraction of its hydrolysis cycle in a low affinity state but dissociates only slowly from this state. This work provides a mechanism to explain differences in the load-dependent properties of kinesin-1 and kinesin-2.