Unusual properties of the fungal conventional kinesin neck domain from Neurospora crassa

A Kinesin Vdkin2 Required for Vacuole Formation, Mycelium Growth, and Penetration Structure Formation of Verticillium dahliae

The soil-borne vascular fungus Verticillium dahliae infects hundreds of dicotyledonous plants, causing severe wilt diseases. During the initial colonization, V. dahliae develops a penetration peg to enable infection of cotton roots. In some phytopathogenic fungi, vacuoles play a critical role in normal formation of the infection structure. Kinesin 2 protein is associated with vacuole formation in Ustilago maydis. To identify the function of vacuoles in the V. dahliae infection structure, we identified VdKin2, an ortholog of kinesin 2, in V. dahliae and investigated its function through gene knockout. VdKin2 mutants showed severe defects in virulence and were suppressed during initial infection and root colonization based on observation of green fluorescent protein-labeled V. dahliae. We also found that deletion of VdKin2 compromised penetration peg formation and the derived septin neck. Disruption strains were viable and showed normal microsclerotia formation, whereas mycelium growth and conidial production were reduced, with shorter and more branched hyphae. Furthermore, the VdKin2 mutant, unlike wild-type V. dahliae, lacked a large basal vacuole, accompanied by a failure to generate concentrated lipid droplets. Taken together, VdKin2 regulates vacuole formation by V. dahliae, which is required for conidiation, mycelium growth, and penetration structure formation during initial plant root infection.

Kinesin Motors in the Filamentous Basidiomycetes in Light of the Schizophyllum commune Genome

Kinesins are essential motor molecules of the microtubule cytoskeleton. All eukaryotic organisms have several genes encoding kinesin proteins, which are necessary for various cell biological functions. During the vegetative growth of filamentous basidiomycetes, the apical cells of long leading hyphae have microtubules extending toward the tip. The reciprocal exchange and migration of nuclei between haploid hyphae at mating is also dependent on cytoskeletal structures, including the microtubules and their motor molecules. In dikaryotic hyphae, resulting from a compatible mating, the nuclear location, synchronous nuclear division, and extensive nuclear separation at telophase are microtubule-dependent processes that involve unidentified molecular motors. The genome of Schizophyllum commune is analyzed as an example of a species belonging to the Basidiomycota subclass, Agaricomycetes. In this subclass, the investigation of cell biology is restricted to a few species. Instead, the whole genome sequences of several species are now available. The analyses of the mating type genes and the genes necessary for fruiting body formation or wood degrading enzymes in several genomes of Agaricomycetes have shown that they are controlled by comparable systems. This supports the idea that the genes regulating the cell biological process in a model fungus, such as the genes encoding kinesin motor molecules, are also functional in other filamentous Agaricomycetes.

Possible cold-adaptation for the fungal kinesin in compensation for thermal stability acquired by single amino acid substitution

The amino acid sequence of the motor domain of AnKinA, kinesin-1 from Aspergillus nidulans, growing optimally at 37°C, was compared with that of SbKin1, kinesin-1 from the snow mold Sclerotinia borealis. For cold-adaptation, some enzymes are thought to exhibit augmented protein structure flexibility, acquired most effectively by substituting a glycine residue for another amino acid residue. By the comparison described above, two glycine residues proximal to tightly bound ADP were identified in the SbKin1 motor domain, of which the corresponding residues of AnKinA were non-glycine ones (P60 and S323). We made AnKinA recombinant kinesin (AnKinA-WT (WT)) along with P60G and S323G mutants. From the basal ATPase activity (without microtubules), these kinesins showed similar characteristics in activation energies, while deviation from the linearity of the ATPase activity time-course was detected at 34°C for WT and P60G but at 24°C for S323G. The microtubule translocation velocity of WT, P60G or S323G exhibited an activation energy of 60, 58 or 53 kJ/mol, respectively; for S323G, the activation energy was lower and the velocity at low temperatures was higher than those for the other two. These results suggest that the point mutation S323G would offer possible cold-adaptation in compensation for thermal stability.

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 walks the line: single motors observed by atomic force microscopy

Motor proteins of the kinesin family move actively along microtubules to transport cargo within cells. How exactly a single motor proceeds on the 13 narrow lanes or protofilaments of a microtubule has not been visualized directly, and there persists controversy on the relative position of the two kinesin heads in different nucleotide states. We have succeeded in imaging Kinesin-1 dimers immobilized on microtubules with single-head resolution by atomic force microscopy. Moreover, we could catch glimpses of single Kinesin-1 dimers in their motion along microtubules with nanometer resolution. We find in our experiments that frequently both heads of one dimer are microtubule-bound at submicromolar ATP concentrations. Furthermore, we could unambiguously resolve that both heads bind to the same protofilament, instead of straddling two, and remain on this track during processive movement.

Autoinhibition of the kinesin-2 motor KIF17 via dual intramolecular mechanisms

Kinesin-2 motor KIF17 autoinhibition is visualized in vivo; in the absence of cargo, this homodimer’s C-terminal tail blocks microtubule binding, and a coiled-coil segment blocks motility.

Long-distance transport in cells is driven by kinesin and dynein motors that move along microtubule tracks. These motors must be tightly regulated to ensure the spatial and temporal fidelity of their transport events. Transport motors of the kinesin-1 and kinesin-3 families are regulated by autoinhibition, but little is known about the mechanisms that regulate kinesin-2 motors. We show that the homodimeric kinesin-2 motor KIF17 is kept in an inactive state in the absence of cargo. Autoinhibition is caused by a folded conformation that enables nonmotor regions to directly contact and inhibit the enzymatic activity of the motor domain. We define two molecular mechanisms that contribute to autoinhibition of KIF17. First, the C-terminal tail interferes with microtubule binding; and second, a coiled-coil segment blocks processive motility. The latter is a new mechanism for regulation of kinesin motors. This work supports the model that autoinhibition is a general mechanism for regulation of kinesin motors involved in intracellular trafficking events.

Mammalian Kinesin-3 motors are dimeric in vivo and move by processive motility upon release of autoinhibition

Kinesin-3 motors drive the transport of synaptic vesicles and other membrane-bound organelles in neuronal cells. In the absence of cargo, kinesin motors are kept inactive to prevent motility and ATP hydrolysis. Current models state that the Kinesin-3 motor KIF1A is monomeric in the inactive state and that activation results from concentration-driven dimerization on the cargo membrane. To test this model, we have examined the activity and dimerization state of KIF1A. Unexpectedly, we found that both native and expressed proteins are dimeric in the inactive state. Thus, KIF1A motors are not activated by cargo-induced dimerization. Rather, we show that KIF1A motors are autoinhibited by two distinct inhibitory mechanisms, suggesting a simple model for activation of dimeric KIF1A motors by cargo binding. Successive truncations result in monomeric and dimeric motors that can undergo one-dimensional diffusion along the microtubule lattice. However, only dimeric motors undergo ATP-dependent processive motility. Thus, KIF1A may be uniquely suited to use both diffuse and processive motility to drive long-distance transport in neuronal cells.

Properties of the kinesin-3 NcKin3 motor domain and implications for neck function

Members of the Kinesin-3 family are microtubule motors involved in the transport of membranous cargo. NcKin3 from the fungus Neurospora crassa is dimeric but inactivates one of its motor heads to generate nonprocessive motility. To determine how one of the heads is inactivated, we investigated truncated monomeric constructs. None of the constructs generated processive single-molecule motility, and multimotor velocities depended linearly on the number of residues remaining in the neck. The kinetic analysis suggests futile ATP hydrolysis cycles, because a representative monomer showed a faster ATP turnover than the dimer while supporting slower motility. The K(0.5,MT) was 70-fold lower, the microtubule-bound portion of the kinetic cycle eight-fold longer and the microtubule detachment rate almost 15-fold slower than that of the dimer. Moreover, the monomer's microtubule-dependent ADP release occurred three-fold to four-fold faster than k(cat) (125 versus 34 s(-1)), whereas phosphate release was approximately equally fast (29 s(-1)). A dimeric construct containing a structure-breaking insert between motor head and neck showed a similar behaviour. These data suggest that the heads of the wild-type NcKin3 motor are strictly coupled via the neck domain, and that the dimeric structure is required for proper detachment after one ATPase cycle. This is the first direct comparison of a monomeric Kinesin-3 with its dimeric full-length counterpart, and the kinetic changes observed here may also apply to other Kinesin-3 motors.

Dissection of kinesin's processivity

The protein family of kinesins contains processive motor proteins that move stepwise along microtubules. This mechanism requires the precise coupling of the catalytic steps in the two heads, and their precise mechanical coordination. Here we show that these functionalities can be uncoupled in chimera of processive and non-processive kinesins. A chimera with the motor domain of Kinesin-1 and the dimerization domain of a non-processive Kinesin-3 motor behaves qualitatively as conventional kinesin and moves processively in TIRF and bead motility assays, suggesting that spatial proximity of two Kinein-1 motor domains is sufficient for processive behavior. In the reverse chimera, the non-processive motor domains are unable to step along microtubules, despite the presence of the Kinesin-1 neck coiled coil. Still, ATP-binding to one head of these chimera induces ADP-release from the partner head, a characteristic feature of alternating site catalysis. These results show that processive movement of kinesin dimers requires elements in the motor head that respond to ADP-release and induce stepping, in addition to a proper spacing of the motor heads via the neck coiled coil.

Effect of spastic paraplegia mutations in KIF5A kinesin on transport activity

Hereditary spastic paraplegia (HSP) is a neurodegenerative disease caused by motoneuron degeneration. It is linked to at least 30 loci, among them SPG10, which causes dominant forms and originates in point mutations in the neuronal Kinesin-1 gene (KIF5A). Here, we investigate the motility of KIF5A and four HSP mutants. All mutations are single amino-acid exchanges and located in kinesin's motor or neck domain. The mutation in the neck (A361V) did not change the gliding properties in vitro, the others either reduced microtubule affinity or gliding velocity or both. In laser-trapping assays, none of the mutants moved more than a few steps along microtubules. Motility assays with mixtures of homodimeric wild-type, homodimeric mutant and heterodimeric wild-type/mutant motors revealed that only one mutant (N256S) reduces the gliding velocity at ratios present in heterozygous patients, whereas the others (K253N, R280C) do not. Attached to quantum dots as artificial cargo, mixtures involving N256S mutants produced slower cargo populations lagging behind in transport, whereas mixtures with the other mutants led to populations of quantum dots that rarely bound to microtubules. These differences indicate that the dominant inheritance of SPG10 is caused by two different mechanisms that both reduce the gross cargo flux, leading to deficient supply of the synapse.

Properties of the Kinesin-1 motor DdKif3 from Dictyostelium discoideum

The amoeba Dictyostelium discoideum possesses genes for 13 different kinesins. Here we characterize DdKif3, a member of the Kinesin-1 family. Kinesin-1 motors form homodimers that can move micrometer-long distances on microtubules using the energy derived from ATP hydrolysis. We expressed recombinant motors in Escherichia coli and tested them in different in vitro assays. Full-length and truncated Kif3 motors were active in gliding and ATPase assays. They showed a strong dependence on ionic strength. Like the full-length motor, the truncated DdKif3-592 motor (aa 1-592; comprising motor domain, neck, and partial stalk) reached its maximum speed of around 2.0micrcom s(-1) at a potassium acetate concentration of 200mM. The shortened DdKif3-342 motor (aa 1-342; comprising motor domain, partial neck) showed a high ATP turnover, comparable to that of the fungal Kinesin-1, Nkin. Results from the duty cycle calculations and gliding assays indicate that DdKif3 is a processive motor. A GFP-fusion protein revealed a mainly cytoplasmic localization of DdKif3. Immunofluorescence staining makes an association with the endoplasmic reticulum or mitochondria unlikely. Despite a similar phylogenetic distance to both metazoa and fungi, in terms of its biochemical properties DdKif3 revealed a closer similarity to fungal than animal kinesins.

Formation and reversible dissociation of coiled coil of peptide to the C-terminus of the HSV B5 protein: a time-resolved spectroscopic analysis

An understanding of the molecular mechanisms of the newly characterized herpes simplex virus (HSV) B5 protein is important to further elucidate the HSV cell entry and infection. The synthetic peptide of B5 (wtB5) was functionalized with the nonlinear optical chromophore cascade yellow and its molecular dynamics was probed at physiological and endosomal pH (pH 7.4 and 5.5, respectively). Steady-state CD spectroscopy was utilized to characterize the peptides at different pH. These spectra showed structural changes in the peptide with time measured over several days. Nonlinear optical measurements were carried out to probe the interactions and local environment of the labeled peptide, and the increase in the two-photon cross section of this system suggests an increase in chromophore-peptide interactions. Time-resolved fluorescence upconversion measurements reflected changes in the hydrophilic and hydrophobic local environments of the labeled peptide-chromophore system. Ultrafast depolarization measurements gave rotational correlation times indicative of a reversible change in the size of the peptide. The time-resolved results provide compelling evidence of a reversible dissociation of the coiled coils of the wtB5 peptide. This process was found to be pH-insensitive. The data from this unique combination of techniques provide an initial step to understanding the molecular dynamics of B5 and a framework for the development of novel imaging methods based on two-photon emission, as well as new therapeutics for HSV.

Tau protein binding forms a 1 nm thick layer along protofilaments without affecting the radial elasticity of microtubules

Tau is one of the most abundant microtubule-associated proteins involved in kinetic stabilization and bundling of axonal microtubules. Although intense research has revealed much about tau function and its involvement in Alzheimer's disease during the past years, it still remains unclear how exactly tau binds on microtubules and if the kinetic stabilization of microtubules by tau is accompanied, at least in part, by a mechanical reinforcement of microtubules. In this paper, we have used atomic force microscopy to address both aspects by visualizing and mechanically analyzing microtubules in the presence of native tau isoforms. We could show that tau at saturating concentrations forms a 1 nm thick layer around the microtubule, but leaves the protofilament structure well visible. The latter observation argues for tau binding mainly along and not across the protofilaments. The radial elasticity of microtubules was almost unaffected by tau, consistent with tau binding along the tops of the protofilaments. Tau did increase the resistance of microtubules against rupture. Finite-element calculations confirmed our findings.

Kinetic and mechanistic basis of the nonprocessive Kinesin-3 motor NcKin3

Kinesin-3 motors have been shown to transport cellular cargo along microtubules and to function according to mechanisms that differ from the conventional hand-over-hand mechanism. To find out whether the mechanisms described for Kif1A and CeUnc104 cover the full spectrum of Kinesin-3 motors, we characterize here NcKin3, a novel member of the Kinesin-3 family that localizes to mitochondria of ascomycetes. We show that NcKin3 does not move in a K-loop-dependent way as Kif1A or in a cluster-dependent way as CeUnc104. Its in vitro gliding velocity ranges between 0.30 and 0.64 mum/s and correlates positively with motor density. The processivity index (k(bi,ratio)) of approximately 3 reveals that not more than three ATP molecules are hydrolyzed per productive microtubule encounter. The NcKin3 duty ratio of 0.03 indicates that the motor spends only a minute fraction of the ATPase cycle attached to the filament. Unlike other Kinesin-3 family members, NcKin3 forms stable dimers, but only one subunit releases ADP in a microtubule-dependent fashion. Together, these data exclude a processive hand-over-hand mechanism of movement and suggest a power-stroke mechanism where nucleotide-dependent structural changes in a single motor domain lead to displacement of the motor along the filament. Thus, NcKin3 is the first plus end-directed kinesin motor that is dimeric but moves in a nonprocessive fashion to its destination.

Flexibility of the neck domain enhances Kinesin-1 motility under load

Kinesin-1 is a dimeric motor protein that moves stepwise along microtubules. A two-stranded alpha-helical coiled-coil formed by the neck domain links the two heads of the molecule, and forces the motor heads to alternate. By exchanging the particularly soft neck region of the conventional kinesin from the fungus Neurospora crassa with an artificial, highly stable coiled-coil we investigated how this domain affects motor kinetics and motility. Under unloaded standard conditions, both motor constructs developed the same gliding velocity. However, in a force-feedback laser trap the mutant showed increasing motility defects with increasing loads, and did not reach wild-type velocities and run lengths. The stall force dropped significantly from 4.1 to 3.0 pN. These results indicate the compliance of kinesin's neck is important to sustain motility under load, and reveal a so far unknown constrain on the imperfect coiled-coil heptad pattern of Kinesin-1. We conclude that coiled-coil structures, a motif encountered in various types of molecular motors, are not merely a clamp for linking two heavy chains to a functional unit but may have specifically evolved to allow motor progression in a viscous, inhomogeneous environment or when several motors attached to a transported vesicle are required to cooperate efficiently.

Feedback of the kinesin-1 neck-linker position on the catalytic site

Kinesin-1 motor proteins step along microtubules by a mechanism in which the heads cycle through microtubule-bound and unbound states in an interlaced fashion. An important contribution to head-head coordination arises from the action of the neck-linker that docks onto the core motor domain upon ATP binding. We show here that the docked neck-linker not only guides the microtubule-unbound head to the next microtubule binding site but also signals its position to the head to which it is attached. Cross-linking studies on mutated kinesin constructs reveal that residues at the interface motor core/docked neck-linker, among them most importantly a conserved tyrosine, are involved in this feedback. The primary effect of the docked neck-linker is a reduced microtubule binding affinity in the ADP state.

Review: regulation mechanisms of Kinesin-1

Kinesin-1 microtubule motors are common kinesin motors from protozoa, fungi and animals. They transport vesicular or particle cargo in a strictly regulated manner. The relatively well-studied tail inhibition mechanism is based on a conformational change that leads to an interaction of Kinesin-1's tail with the junction of neck and hinge regions. This folding causes a decrease in microtubule binding and motor activity. In fungal Kinesin-1 motors several lines of evidence suggest that a conserved tyrosine in the neck coiled-coil mediates this inhibition. In the active state, a region surrounding a conserved tryptophan in the hinge stabilises the neck coiled-coil, and prevents the tyrosine from inhibiting. Although animal and fungal Kinesin-1 motors are clearly homologous and function according to the same chemo-mechanical mechanism, they differ in their regulation. Unlike fungal Kinesin-1s, animal kinesins associate with light chains that are important for regulation and cargo interaction. Several proteins interacting with animal Kinesin-1 heavy or light chains are known, among them typical scaffolding proteins that seem to link Kinesin-1 to signalling pathways.

Efficient designs for powering microscale devices with nanoscale biomolecular motors

Current MEMS and microfluidic designs require external power sources and actuators, which principally limit such technology. To overcome these limitations, we have developed a number of microfluidic systems into which we can seamlessly integrate a biomolecular motor, kinesin, that transports microtubules by extracting chemical energy from its aqueous working environment. Here we establish that our microfabricated structures, the self-assembly of the bio-derived transducer, and guided, unidirectional transport of microtubules are ideally suited to create engineered arrays for efficiently powering nano- and microscale devices.

The E-hook of tubulin interacts with kinesin's head to increase processivity and speed

Kinesins are dimeric motor proteins that move processively along microtubules. It has been proposed that the processivity of conventional kinesins is increased by electrostatic interactions between the positively charged neck of the motor and the negatively charged C-terminus of tubulin (E-hook). In this report we challenge this anchoring hypothesis by studying the motility of a fast fungal kinesin from Neurospora crassa (NcKin). NcKin is highly processive despite lacking the positive charges in the neck. We present a detailed analysis of how proteolytic removal of the E-hook affects truncated monomeric and dimeric constructs of NcKin. Upon digestion we observe a strong reduction of the processivity and speed of dimeric motor constructs. Monomeric motors with truncated or no neck display the same reduction of microtubule gliding speed as dimeric constructs, suggesting that the E-hook interacts with the head only. The E-hook has no effect on the strongly bound states of NcKin as microtubule digestion does not alter the stall forces produced by single dimeric motors, suggesting that the E-hook affects the interaction site of the kinesin.ADP-head and the microtubule. In fact, kinetic and binding experiments indicate that removal of the E-hook shifts the binding equilibrium of the weakly attached kinesin.ADP-head toward a more strongly bound state, which may explain reduced processivity and speed on digested microtubules.

The complex interplay between the neck and hinge domains in kinesin-1 dimerization and motor activity

Kinesin-1 dimerizes via the coiled-coil neck domain. In contrast to animal kinesins, neck dimerization of the fungal kinesin-1 NcKin requires additional residues from the hinge. Using chimeric constructs containing or lacking fungal-specific elements, the proximal part of the hinge was shown to stabilize the neck coiled-coil conformation in a complex manner. The conserved fungal kinesin hinge residue W384 caused neck coiled-coil formation in a chimeric NcKin construct, including parts of the human kinesin-1 stalk. The stabilizing effect was retained in a NcKinW384F mutant, suggesting important pi-stacking interactions. Without the stalk, W384 was not sufficient to induce coiled-coil formation, indicating that W384 is part of a cluster of several residues required for neck coiled-coil folding. A W384-less chimera of NcKin and human kinesin possessed a non-coiled-coil neck conformation and showed inhibited activity that could be reactivated when artificial interstrand disulfide bonds were used to stabilize the neck coiled-coil conformation. On the basis of yeast two-hybrid data, we propose that the proximal hinge can bind kinesin's cargo-free tail domain and causes inactivation of kinesin by disrupting the neck coiled-coil conformation.

Microtubule-kinesin interface mutants reveal a site critical for communication

Strict coordination of the two motor domains of kinesin is required for driving the processive movement of organelles along microtubules. Glutamate 164 of the kinesin heavy chain was shown to be critical for kinesin function through in vivo genetics in Drosophila melanogaster. The mutant motor E164K exhibited reduced steady-state ATPase activity and higher affinity for both ATP and microtubules. Moreover, an alanine substitution at this position (E164A) caused similar defects. It became stalled on the microtubule and was unable to bind and hydrolyze ATP at the second motor domain. Glu(164), which has been conserved through evolution, is located at the motor-microtubule interface close to key residues on helix alpha12 of beta-tubulin. We explored further the contributions of Glu(164) to motor function using several site-directed mutant proteins: E164K, E164N, E164D, E164Q, and D165A. The results indicate that the microtubule-E164K complex can only bind and hydrolyze one ATP. ATP with increased salt was able to dissociate a population of E164K motors from the microtubule but could not dissociate E164A. We tested the basis of the stabilized microtubule interaction with E164K, E164N, and E164A. The results provide new insights about the motor-microtubule interface and the pathway of communication for processive motility.

A kinesin-like motor inhibits microtubule dynamic instability

The motility of molecular motors and the dynamic instability of microtubules are key dynamic processes for mitotic spindle assembly and function. We report here that one of the mitotic kinesins that localizes to chromosomes, Xklp1 from Xenopus laevis, could inhibit microtubule growth and shrinkage. This effect appeared to be mediated by a structural change in the microtubule lattice. We also found that Xklp1 could act as a fast, nonprocessive, plus end-directed molecular motor. The integration of the two properties, motility and inhibition of microtubule dynamics, in one molecule emphasizes the versatile properties of kinesin family members.

Cytoskeleton and motor proteins in filamentous fungi

In filamentous fungi, the actin cytoskeleton is required for polarity establishment and maintenance at hyphal tips and for formation of a contractile ring at sites of septation. Recently, formins have been identified as Arp (actin-related protein) 2/3-independent nucleators of actin polymerization, and filamentous fungi contain a single formin that localizes to both sites. Work on cytoplasmic dynein and members of the kinesin and myosin families of motors has continued to reveal new information regarding the function and regulation of motors as well as demonstrate the importance of microtubules in the long-distance transport of vesicles/organelles in the filamentous fungi.

A complete inventory of fungal kinesins in representative filamentous ascomycetes

Complete inventories of kinesins from three pathogenic filamentous ascomycetes, Botryotinia fuckeliana, Cochliobolus heterostrophus, and Gibberella moniliformis, are described. These protein sequences were compared with those of the filamentous saprophyte, Neurospora crassa and the two yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. Data mining and phylogenetic analysis of the motor domain yielded a constant set of 10 kinesins in the filamentous fungal species, compared with a smaller set in S. cerevisiae and S. pombe. The filamentous fungal kinesins fell into nine subfamilies when compared with well-characterized kinesins from other eukaryotes. A few putative kinesins (one in B. fuckeliana and two in C. heterostrophus) could not be defined as functional, due to unorthodox organization and lack of experimental data. The broad representation of filamentous fungal kinesins across most of the known subfamilies and the ease of gene manipulation make fungi ideal models for functional and evolutionary investigation of these proteins.

Molecular motors

Life implies movement. Most forms of movement in the living world are powered by tiny protein machines known as molecular motors. Among the best known are motors that use sophisticated intramolecular amplification mechanisms to take nanometre steps along protein tracks in the cytoplasm. These motors transport a wide variety of cargo, power cell locomotion, drive cell division and, when combined in large ensembles, allow organisms to move. Motor defects can lead to severe diseases or may even be lethal. Basic principles of motor design and mechanism have now been derived, and an understanding of their complex cellular roles is emerging.

Synthesis and conformational characterization of peptides related to the neck domain of a fungal kinesin

The Y362K mutation in the neck domain of conventional kinesin from Neurospora crassa provokes a significant reduction of the rate of movement along microtubules. Since the alpha-helical coiled-coil structure of the neck region is implicated in the mechanism of the processive movement of kinesins, a series of peptides related to the heptad region 338-379 of the wild-type and the variant fungal kinesinswere synthesized as monomers and as N-terminal disulfide dimers, crosslinked to favour self-association into coiled-coil structures entropically. A comparison of the dichroic properties of the peptides and the effects of trifluoroethanol and peptide concentration clearly confirmed the strong implication of the single point mutation in destabilizing the intrinsic propensity of the peptides to fold into the supercoiled conformation. That there is a correlation between the stability of the coiled-coil and rate of movement of the kinesin is confirmed.

Single fungal kinesin motor molecules move processively along microtubules

Conventional kinesins are two-headed molecular motors that move as single molecules micrometer-long distances on microtubules by using energy derived from ATP hydrolysis. The presence of two heads is a prerequisite for this processive motility, but other interacting domains, like the neck and K-loop, influence the processivity and are implicated in allowing some single-headed kinesins to move processively. Neurospora kinesin (NKin) is a phylogenetically distant, dimeric kinesin from Neurospora crassa with high gliding speed and an unusual neck domain. We quantified the processivity of NKin and compared it to human kinesin, HKin, using gliding and fluorescence-based processivity assays. Our data show that NKin is a processive motor. Single NKin molecules translocated microtubules in gliding assays on average 2.14 micro m (N = 46). When we tracked single, fluorescently labeled NKin motors, they moved on average 1.75 micro m (N = 182) before detaching from the microtubule, whereas HKin motors moved shorter distances (0.83 micro m, N = 229) under identical conditions. NKin is therefore at least twice as processive as HKin. These studies, together with biochemical work, provide a basis for experiments to dissect the molecular mechanisms of processive movement.

A conserved tyrosine in the neck of a fungal kinesin regulates the catalytic motor core

The neck domain of fungal conventional kinesins displays characteristic properties which are reflected in a specific sequence pattern. The exchange of the strictly conserved Tyr 362, not present in animals, into Lys, Cys or Phe leads to a failure to dimerize. The destabilizing effect is confirmed by a lower coiled-coil propensity of mutant peptides. Whereas the Phe substitution has only a structural effect, the Lys and Cys replacements lead to dramatic kinetic changes. The steady state ATPase is 4- to 7-fold accelerated, which may be due to a faster microtubule-stimulated ADP release rate. These data suggest that an inhibitory effect of the fungal neck domain on the motor core is mediated by direct interaction of the aromatic ring of Tyr 362 with the head, whereas the OH group is essential for dimerization. This is the first demonstration of a direct influence of the kinesin neck region in regulation of the catalytic activity.