Kinesin-1 tail autoregulation and microtubule-binding regions function in saltatory transport but not ooplasmic streaming

The FHA domain is essential for autoinhibition of KIF1A/UNC-104 proteins

KIF1A/UNC-104 proteins, which are members of the kinesin superfamily of motor proteins, play a pivotal role in the axonal transport of synaptic vesicles and their precursors. Drosophila melanogaster UNC-104 (DmUNC-104) is a relatively recently discovered Drosophila kinesin. Although some point mutations that disrupt synapse formation have been identified, the biochemical properties of the DmUNC-104 protein have not been investigated. Here, we prepared recombinant full-length DmUNC-104 protein and determined its biochemical features. We analyzed the effect of a previously identified missense mutation in the forkhead-associated (FHA) domain, called bristly (bris). The bris mutation strongly promoted the dimerization of DmUNC-104 protein, whereas wild-type DmUNC-104 was a mixture of monomers and dimers. We further tested the G618R mutation near the FHA domain, which was previously shown to disrupt the autoinhibition of Caenorhabditis elegans UNC-104. The biochemical properties of the G618R mutant recapitulated those of the bris mutant. Finally, we found that disease-associated mutations also promote the dimerization of DmUNC-104. Collectively, our results suggest that the FHA domain is essential for autoinhibition of KIF1A/UNC-104 proteins, and that abnormal dimerization of KIF1A might be linked to human diseases.

Autoinhibition and activation of kinesin-1 and their involvement in amyotrophic lateral sclerosis

Kinesin-1, composed of kinesin heavy chain and kinesin light chain, is a founding member of kinesin superfamily and transports various neuronal cargos. Kinesin-1 is one of the most abundant ATPases in the cell and thus need to be tightly regulated to avoid wastage of energy. It has been well established that kinesin-1 is regulated by the autoinhibition mechanism. This review focuses on the recent researches that have contributed to the understanding of mechanisms for the autoinhibition of kinesin-1 and its unlocking. Recent electron microscopic studies have shown an unanticipated structure of autoinhibited kinesin-1. Biochemical reconstitution have revealed detailed molecular mechanisms how the autoinhibition is unlocked. Importantly, misregulation of kinesin-1 is emerging as one of the major causes of amyotrophic lateral sclerosis.

Dissection and Direct Imaging of Axonal Transport in Drosophila Segmental Nerves

For neurons, especially those with long axons, the forceful transport of mitochondria, vesicles, and other cytoplasmic components by cytoskeletal motors is vital. Defects in cytoplasmic transport machinery cause a degradation of signaling capacity that is most severe for neurons with the longest axons. In humans, with motor axons up to a meter long, even a mild mutation in one copy of the gene that codes for kinesin-1, the primary anterograde axonal transport motor, can cause spastic paraplegia and other distal neuropathies.To address questions about the molecular mechanisms of organelle movement, we turned to Drosophila as a model system, because it offered rigorous genetic and molecular approaches to the identification and inhibition of specific elements of transport machinery. However, methods for direct observation of organelle transport were largely lacking. We describe here an approach that we developed for imaging the transport behaviors of specific organelles in the long motor axons of larvae. It is straightforward, the equipment is commonly available, and it provides a powerful tool for studying the contributions of specific proteins to organelle transport mechanisms.

Sequences in the stalk domain regulate auto-inhibition and ciliary tip localization of the immotile kinesin-4 KIF7

The kinesin-4 member KIF7 plays critical roles in Hedgehog signaling in vertebrate cells. KIF7 is an atypical kinesin as it binds to microtubules but is immotile. We demonstrate that, like conventional kinesins, KIF7 is regulated by auto-inhibition, as the full-length protein is inactive for microtubule binding in cells. We identify a segment, the inhibitory coiled coil (inhCC), that is required for auto-inhibition of KIF7, whereas the adjacent regulatory coiled coil (rCC) that contributes to auto-inhibition of the motile kinesin-4s KIF21A and KIF21B is not sufficient for KIF7 auto-inhibition. Disease-associated mutations in the inhCC relieve auto-inhibition and result in strong microtubule binding. Surprisingly, uninhibited KIF7 proteins did not bind preferentially to or track the plus ends of growing microtubules in cells, as suggested by previous in vitro work, but rather bound along cytosolic and axonemal microtubules. Localization to the tip of the primary cilium also required the inhCC, and could be increased by disease-associated mutations regardless of the auto-inhibition state of the protein. These findings suggest that loss of KIF7 auto-inhibition and/or altered cilium tip localization can contribute to the pathogenesis of human disease.

Optical flow analysis reveals that Kinesin-mediated advection impacts the orientation of microtubules in the Drosophila oocyte

The orientation of microtubule (MT) networks is exploited by motors to deliver cargoes to specific intracellular destinations and is thus essential for cell polarity and function. Reconstituted in vitro systems have largely contributed to understanding the molecular framework regulating the behavior of MT filaments. In cells, however, MTs are exposed to various biomechanical forces that might impact on their orientation, but little is known about it. Oocytes, which display forceful cytoplasmic streaming, are excellent model systems to study the impact of motion forces on cytoskeletons in vivo. Here we implement variational optical flow analysis as a new approach to analyze the polarity of MTs in the Drosophila oocyte, a cell that displays distinct Kinesin-dependent streaming. After validating the method as robust for describing MT orientation from confocal movies, we find that increasing the speed of flows results in aberrant plus end growth direction. Furthermore, we find that in oocytes where Kinesin is unable to induce cytoplasmic streaming, the growth direction of MT plus ends is also altered. These findings lead us to propose that cytoplasmic streaming - and thus motion by advection – contributes to the correct orientation of MTs in vivo. Finally, we propose a possible mechanism for a specialized cytoplasmic actin network (the actin mesh) to act as a regulator of flow speeds to counteract the recruitment of Kinesin to MTs.

Autoinhibition of kinesin-1 is essential to the dendrite-specific localization of Golgi outposts

Neuronal polarity relies on the axon- or dendrite-specific localization of cargo by molecular motors such as kinesin-1. This study shows how autoinhibition regulates both kinesin-1 activity and localization to keep dendritic Golgi outposts from entering axons.

Neuronal polarity relies on the selective localization of cargo to axons or dendrites. The molecular motor kinesin-1 moves cargo into axons but is also active in dendrites. This raises the question of how kinesin-1 activity is regulated to maintain the compartment-specific localization of cargo. Our in vivo structure–function analysis of endogenous Drosophila melanogaster kinesin-1 reveals a novel role for autoinhibition in enabling the dendrite-specific localization of Golgi outposts. Mutations that disrupt kinesin-1 autoinhibition result in the axonal mislocalization of Golgi outposts. Autoinhibition also regulates kinesin-1 localization. Uninhibited kinesin-1 accumulates in axons and is depleted from dendrites, correlating with the change in outpost distribution and dendrite growth defects. Genetic interaction tests show that a balance of kinesin-1 inhibition and dynein activity is necessary to localize Golgi outposts to dendrites and keep them from entering axons. Our data indicate that kinesin-1 activity is precisely regulated by autoinhibition to achieve the selective localization of dendritic cargo.

Microtubule-microtubule sliding by kinesin-1 is essential for normal cytoplasmic streaming in Drosophila oocytes

Cytoplasmic streaming in Drosophila oocytes is a microtubule-based bulk cytoplasmic movement. Streaming efficiently circulates and localizes mRNAs and proteins deposited by the nurse cells across the oocyte. This movement is driven by kinesin-1, a major microtubule motor. Recently, we have shown that kinesin-1 heavy chain (KHC) can transport one microtubule on another microtubule, thus driving microtubule-microtubule sliding in multiple cell types. To study the role of microtubule sliding in oocyte cytoplasmic streaming, we used a Khc mutant that is deficient in microtubule sliding but able to transport a majority of cargoes. We demonstrated that streaming is reduced by genomic replacement of wild-type Khc with this sliding-deficient mutant. Streaming can be fully rescued by wild-type KHC and partially rescued by a chimeric motor that cannot move organelles but is active in microtubule sliding. Consistent with these data, we identified two populations of microtubules in fast-streaming oocytes: a network of stable microtubules anchored to the actin cortex and free cytoplasmic microtubules that moved in the ooplasm. We further demonstrated that the reduced streaming in sliding-deficient oocytes resulted in posterior determination defects. Together, we propose that kinesin-1 slides free cytoplasmic microtubules against cortically immobilized microtubules, generating forces that contribute to cytoplasmic streaming and are essential for the refinement of posterior determinants.

Cytoplasmic Streaming in the Drosophila Oocyte

Objects are commonly moved within the cell by either passive diffusion or active directed transport. A third possibility is advection, in which objects within the cytoplasm are moved with the flow of the cytoplasm. Bulk movement of the cytoplasm, or streaming, as required for advection, is more common in large cells than in small cells. For example, streaming is observed in elongated plant cells and the oocytes of several species. In the Drosophila oocyte, two stages of streaming are observed: relatively slow streaming during mid-oogenesis and streaming that is approximately ten times faster during late oogenesis. These flows are implicated in two processes: polarity establishment and mixing. In this review, I discuss the underlying mechanism of streaming, how slow and fast streaming are differentiated, and what we know about the physiological roles of the two types of streaming.

JIP3 Activates Kinesin-1 Motility to Promote Axon Elongation

Kinesin-1 is a molecular motor responsible for cargo transport along microtubules and plays critical roles in polarized cells, such as neurons. Kinesin-1 can function as a dimer of two kinesin heavy chains (KHC), which harbor the motor domain, or as a tetramer in combination with two accessory light chains (KLC). To ensure proper cargo distribution, kinesin-1 activity is precisely regulated. Both KLC and KHC subunits bind cargoes or regulatory proteins to engage the motor for movement along microtubules. We previously showed that the scaffolding protein JIP3 interacts directly with KHC in addition to its interaction with KLC and positively regulates dimeric KHC motility. Here we determined the stoichiometry of JIP3-KHC complexes and observed approximately four JIP3 molecules binding per KHC dimer. We then determined whether JIP3 activates tetrameric kinesin-1 motility. Using an in vitro motility assay, we show that JIP3 binding to KLC engages kinesin-1 with microtubules and that JIP3 binding to KHC promotes kinesin-1 motility along microtubules. We tested the in vivo relevance of these findings using axon elongation as a model for kinesin-1-dependent cellular function. We demonstrate that JIP3 binding to KHC, but not KLC, is essential for axon elongation in hippocampal neurons as well as axon regeneration in sensory neurons. These findings reveal that JIP3 regulation of kinesin-1 motility is critical for axon elongation and regeneration.

Unique function of Kinesin Kif5A in localization of mitochondria in axons

Mutations in Kinesin proteins (Kifs) are linked to various neurological diseases, but the specific and redundant functions of the vertebrate Kifs are incompletely understood. For example, Kif5A, but not other Kinesin-1 heavy-chain family members, is implicated in Charcot-Marie-Tooth disease (CMT) and Hereditary Spastic Paraplegia (HSP), but the mechanism of its involvement in the progressive axonal degeneration characteristic of these diseases is not well understood. We report that zebrafish kif5Aa mutants exhibit hyperexcitability, peripheral polyneuropathy, and axonal degeneration reminiscent of CMT and HSP. Strikingly, although kif5 genes are thought to act largely redundantly in other contexts, and zebrafish peripheral neurons express five kif5 genes, kif5Aa mutant peripheral sensory axons lack mitochondria and degenerate. We show that this Kif5Aa-specific function is cell autonomous and is mediated by its C-terminal tail, as only Kif5Aa and chimeric motors containing the Kif5Aa C-tail can rescue deficits. Finally, concurrent loss of the kinesin-3, kif1b, or its adaptor kbp, exacerbates axonal degeneration via a nonmitochondrial cargo common to Kif5Aa. Our results shed light on Kinesin complexity and reveal determinants of specific Kif5A functions in mitochondrial transport, adaptor binding, and axonal maintenance.

Human CFEOM1 mutations attenuate KIF21A autoinhibition and cause oculomotor axon stalling

The ocular motility disorder "Congenital fibrosis of the extraocular muscles type 1" (CFEOM1) results from heterozygous mutations altering the motor and third coiled-coil stalk of the anterograde kinesin, KIF21A. We demonstrate that Kif21a knockin mice harboring the most common human mutation develop CFEOM. The developing axons of the oculomotor nerve's superior division stall in the proximal nerve; the growth cones enlarge, extend excessive filopodia, and assume random trajectories. Inferior division axons reach the orbit but branch ectopically. We establish a gain-of-function mechanism and find that human motor or stalk mutations attenuate Kif21a autoinhibition, providing in vivo evidence for mammalian kinesin autoregulation. We identify Map1b as a Kif21a-interacting protein and report that Map1b⁻/⁻ mice develop CFEOM. The interaction between Kif21a and Map1b is likely to play a critical role in the pathogenesis of CFEOM1 and highlights a selective vulnerability of the developing oculomotor nerve to perturbations of the axon cytoskeleton.

The auto-inhibitory domain and ATP-independent microtubule-binding region of Kinesin heavy chain are major functional domains for transport in the Drosophila germline

The major motor Kinesin-1 provides a key pathway for cell polarization through intracellular transport. Little is known about how Kinesin works in complex cellular surroundings. Several cargos associate with Kinesin via Kinesin light chain (KLC). However, KLC is not required for all Kinesin transport. A putative cargo-binding domain was identified in the C-terminal tail of fungal Kinesin heavy chain (KHC). The tail is conserved in animal KHCs and might therefore represent an alternative KLC-independent cargo-interacting region. By comprehensive functional analysis of the tail during Drosophila oogenesis we have gained an understanding of how KHC achieves specificity in its transport and how it is regulated. This is, to our knowledge, the first in vivo structural/functional analysis of the tail in animal Kinesins. We show that the tail is essential for all functions of KHC except Dynein transport, which is KLC dependent. These tail-dependent KHC activities can be functionally separated from one another by further characterizing domains within the tail. In particular, our data show the following. First, KHC is temporally regulated during oogenesis. Second, the IAK domain has an essential role distinct from its auto-inhibitory function. Third, lack of auto-inhibition in itself is not necessarily detrimental to KHC function. Finally, the ATP-independent microtubule-binding motif is required for cargo localization. These results stress that two unexpected highly conserved domains, namely the auto-inhibitory IAK and the auxiliary microtubule-binding motifs, are crucial for transport by Kinesin-1 and that, although not all cargos are conserved, their transport involves the most conserved domains of animal KHCs.

The molecular basis for kinesin functional specificity during mitosis

Microtubule-based motor proteins play key roles during mitosis to assemble the bipolar spindle, define the cell division axis, and align and segregate the chromosomes. The majority of mitotic motors are members of the kinesin superfamily. Despite sharing a conserved catalytic core, each kinesin has distinct functions and localization, and is uniquely regulated in time and space. These distinct behaviors and functional specificity are generated by variations in the enzymatic domain as well as the non-conserved regions outside of the kinesin motor domain and the stalk. These flanking regions can directly modulate the properties of the kinesin motor through dimerization or self-interactions, and can associate with extrinsic factors, such as microtubule or DNA binding proteins, to provide additional functional properties. This review discusses the recently identified molecular mechanisms that explain how the control and functional specification of mitotic kinesins is achieved. © 2013 Wiley Periodicals, Inc.

Loss of syd-1 from R7 neurons disrupts two distinct phases of presynaptic development

Genetic analyses in both worm and fly have identified the RhoGAP-like protein Syd-1 as a key positive regulator of presynaptic assembly. In worm, loss of syd-1 can be fully rescued by overexpressing wild-type Liprin-α, suggesting that the primary function of Syd-1 in this process is to recruit Liprin-α. We show that loss of syd-1 from Drosophila R7 photoreceptors causes two morphological defects that occur at distinct developmental time points. First, syd-1 mutant R7 axons often fail to form terminal boutons in their normal M6 target layer. Later, those mutant axons that do contact M6 often project thin extensions beyond it. We find that the earlier defect coincides with a failure to localize synaptic vesicles, suggesting that it reflects a failure in presynaptic assembly. We then analyze the relationship between syd-1 and Liprin-α in R7s. We find that loss of Liprin-α causes a stronger early R7 defect and provide a possible explanation for this disparity: we show that Liprin-α promotes Kinesin-3/Unc-104/Imac-mediated axon transport independently of Syd-1 and that Kinesin-3/Unc-104/Imac is required for normal R7 bouton formation. Unlike loss of syd-1, loss of Liprin-α does not cause late R7 extensions. We show that overexpressing Liprin-α partly rescues the early but not the late syd-1 mutant R7 defect. We therefore conclude that the two defects are caused by distinct molecular mechanisms. We find that Trio overexpression rescues both syd-1 defects and that trio and syd-1 have similar loss- and gain-of-function phenotypes, suggesting that the primary function of Syd-1 in R7s may be to promote Trio activity.

Kinesin tail domains are intrinsically disordered

Kinesin motor proteins transport a wide variety of molecular cargoes in a spatially and temporally regulated manner. Kinesin motor domains, which hydrolyze ATP to produce a directed mechanical force along a microtubule, are well conserved throughout the entire superfamily. Outside of the motor domains, kinesin sequences diverge along with their transport functions. The nonmotor regions, particularly the tails, respond to a wide variety of structural and molecular cues that enable kinesins to carry specific cargoes in response to particular cellular signals. Here, we demonstrate that intrinsic disorder is a common structural feature of kinesins. A bioinformatics survey of the full-length sequences of all 43 human kinesins predicts that significant regions of intrinsically disordered residues are present in all kinesins. These regions are concentrated in the nonmotor domains, particularly in the tails and near sites for ligand binding or post-translational modifications. In order to experimentally verify these predictions, we expressed and purified the tail domains of kinesins representing three different families (Kif5B, Kif10, and KifC3). Circular dichroism and NMR spectroscopy experiments demonstrate that the isolated tails are disordered in vitro, yet they retain their functional microtubule-binding activity. On the basis of these results, we propose that intrinsic disorder is a common structural feature that confers functional specificity to kinesins.

Three routes to suppression of the neurodegenerative phenotypes caused by kinesin heavy chain mutations

Kinesin-1 is a motor protein that moves stepwise along microtubules by employing dimerized kinesin heavy chain (Khc) subunits that alternate cycles of microtubule binding, conformational change, and ATP hydrolysis. Mutations in the Drosophila Khc gene are known to cause distal paralysis and lethality preceded by the occurrence of dystrophic axon terminals, reduced axonal transport, organelle-filled axonal swellings, and impaired action potential propagation. Mutations in the equivalent human gene, Kif5A, result in similar problems that cause hereditary spastic paraplegia (HSP) and Charcot-Marie-Tooth type 2 (CMT2) distal neuropathies. By comparing the phenotypes and the complementation behaviors of a large set of Khc missense alleles, including one that is identical to a human Kif5A HSP allele, we identified three routes to suppression of Khc phenotypes: nutrient restriction, genetic background manipulation, and a remarkable intramolecular complementation between mutations known or likely to cause reciprocal changes in the rate of microtubule-stimulated ADP release by kinesin-1. Our results reveal the value of large-scale complementation analysis for gaining insight into protein structure-function relationships in vivo and point to possible paths for suppressing symptoms of HSP and related distal neuropathies.

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.