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

Modeling the motion of disease-associated KIF1A heterodimers

KIF1A is a member of the kinesin-3 motor protein family that transports synaptic vesicle precursors in axons. Mutations in the Kif1a gene cause neuronal diseases. Most patients are heterozygous and have both mutated and intact KIF1A alleles, suggesting that heterodimers composed of wild-type KIF1A and mutant KIF1A are likely involved in pathogenesis. In this study, we propose mathematical models to describe the motility of KIF1A heterodimers composed of wild-type KIF1A and mutant KIF1A. Our models precisely describe run length, run time, and velocity of KIF1A heterodimers using a few parameters obtained from two homodimers. The first model is a simple hand-over-hand model in which stepping and detachment rates from a microtubule of each head are identical to those in the respective homodimers. Although the velocities of heterodimers expected from this model were in good agreement with the experimental results, this model underestimated the run lengths and run times of some heterodimeric motors. To address this discrepancy, we propose the tethered-head affinity model, in which we hypothesize a tethered head, in addition to a microtubule-binding head, contributes to microtubule binding in a vulnerable one-head-bound state. The run lengths and run times of the KIF1A heterodimers predicted by the tethered-head affinity model matched well with experimental results, suggesting a possibility that the tethered head affects the microtubule binding of KIF1A. Our models provide insights into how each head contributes to the processive movement of KIF1A and can be used to estimate motile parameters of KIF1A heterodimers.

KIF3A accelerates KIF3C within the kinesin-2 heterodimer to generate symmetrical phosphate release rates for each processive step

Heterodimeric KIF3AC is a mammalian kinesin-2 that is highly expressed in the central nervous system and associated with vesicles in neurons. KIF3AC is an intriguing member of the kinesin-2 family because the intrinsic kinetics of KIF3A and KIF3C when expressed as homodimers and analyzed in vitro are distinctively different from each other. For example, the single-molecule velocities of the engineered homodimers KIF3AA and KIF3CC are 293 and 7.5 nm/s, respectively, whereas KIF3AC has a velocity of 186 nm/s. These results led us to hypothesize that heterodimerization alters the intrinsic catalytic properties of the two heads, and an earlier computational analysis predicted that processive steps would alternate between a fast step for KIF3A followed by a slow step for KIF3C resulting in asymmetric stepping. To test this hypothesis directly, we measured the presteady-state kinetics of phosphate release for KIF3AC, KIF3AA, and KIF3CC followed by computational modeling of the KIF3AC phosphate release transients. The results reveal that KIF3A and KIF3C retain their intrinsic ATP-binding and hydrolysis kinetics. Yet within KIF3AC, KIF3A activates the rate of phosphate release for KIF3C such that the coupled steps of phosphate release and dissociation from the microtubule become more similar for KIF3A and KIF3C. These coupled steps are the rate-limiting transition for the ATPase cycle suggesting that within KIF3AC, the stepping kinetics are similar for each head during the processive run. Future work will be directed to define how these properties enable KIF3AC to achieve its physiological functions.

The mechanochemistry of the kinesin-2 KIF3AC heterodimer is related to strain-dependent kinetic properties of KIF3A and KIF3C

KIF3AC is a mammalian neuron-specific kinesin-2 implicated in intracellular cargo transport. It is a heterodimer of KIF3A and KIF3C motor polypeptides which have distinct biochemical and motile properties as engineered homodimers. Single-molecule motility assays show that KIF3AC moves processively along microtubules at a rate faster than expected given the motility rates of the KIF3AA and much slower KIF3CC homodimers. To resolve the stepping kinetics of KIF3A and KIF3C motors in homo- and heterodimeric constructs and determine their transport potential under load, we assayed motor activity using interferometric scattering microscopy and optical trapping. The distribution of stepping durations of KIF3AC molecules is described by a rate (k ₁ = 11 s^-1) without apparent kinetic asymmetry. Asymmetry was also not apparent under hindering or assisting mechanical loads in the optical trap. KIF3AC shows increased force sensitivity relative to KIF3AA yet is more capable of stepping against mechanical load than KIF3CC. Interestingly, the behavior of KIF3C mirrors prior studies of kinesins with increased interhead compliance. Microtubule gliding assays containing 1:1 mixtures of KIF3AA and KIF3CC result in speeds similar to KIF3AC, suggesting the homodimers mechanically impact each other's motility to reproduce the behavior of the heterodimer. Our observations are consistent with a mechanism in which the stepping of KIF3C can be activated by KIF3A in a strain-dependent manner, similar to application of an assisting load. These results suggest that the mechanochemical properties of KIF3AC can be explained by the strain-dependent kinetics of KIF3A and KIF3C.

The ability of the kinesin-2 heterodimer KIF3AC to navigate microtubule networks is provided by the KIF3A motor domain

Heterodimeric kinesin family member KIF3AC is a mammalian kinesin-2 that is highly expressed in the central nervous system and has been implicated in intracellular transport. KIF3AC is unusual in that the motility characteristics of KIF3C when expressed as a homodimer are exceeding slow, whereas homodimeric KIF3AA, as well as KIF3AC, have much faster ATPase kinetics and single molecule velocities. Heterodimeric KIF3AC and homodimeric KIF3AA and KIF3CC are processive, although the run length of KIF3AC exceeds that of KIF3AA and KIF3CC. KIF3C is of particular interest because it exhibits a signature 25-residue insert of glycine and serine residues in loop L11 of its motor domain, and this insert is not present in any other kinesin, suggesting that it confers specific properties to mammalian heterodimeric KIF3AC. To gain a better understanding of the mechanochemical potential of KIF3AC, we pursued a single molecule study to characterize the navigation ability of KIF3AC, KIF3AA, and KIF3CC when encountering microtubule intersections. The results show that all three motors exhibited a preference to remain on the same microtubule when approaching an intersection from the top microtubule, and the majority of track switches occurred from the bottom microtubule onto the top microtubule. Heterodimeric KIF3AC and homodimeric KIF3AA displayed a similar likelihood of switching tracks (36.1 and 32.3%, respectively). In contrast, KIF3CC detached at intersections (67.7%) rather than switch tracks. These results indicate that it is the properties of KIF3A that contribute largely to the ability of KIF3AC to switch microtubule tracks to navigate intersections.

Resolving kinesin stepping: one head at a time

Kinesins are well known to power diverse long-range transport processes in virtually all eukaryotic cells. The ATP-dependent processive stepping as well as the regulation of kinesin' activity have, thus, been the focus of extensive studies over the past decades. It is widely accepted that kinesin motors can self-regulate their activity by suppressing the catalytic activity of the "heads." The distal random coil at the C terminus, termed "tail domain," is proposed to mediate this autoinhibition; however, a direct regulatory influence of the tail on the processive stepping of kinesin proved difficult to capture. Here, we simultaneously tracked the two distinct head domains in the kinesin-2 motor using dual-color super resolution microscopy (dcFIONA) and reveal for the first time their individual properties during processive stepping. We show that the autoinhibitory wild-type conformation selectively impacts one head in the heterodimer but not the other. Our results provide insights into the regulated kinesin stepping that had escaped experimental scrutiny so far.

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.

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.

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.

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.

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.