Preparation and characterization of a novel rice plant-specific kinesin

Kinesin is an ATP-driven motor protein that plays important physiological roles in intracellular transport, mitosis and meiosis, control of microtubule dynamics, and signal transduction. The kinesin family is classified into subfamilies. Kinesin species derived from vertebrates have been well characterized. In contrast, plant kinesins have yet to be adequately characterized. In this study, we expressed the motor domain of a novel rice plant-specific kinesin, K16, in Escherichia coli, and then determined its enzymatic characteristics and compared them with those of kinesin 1. Our findings demonstrated that the rice kinesin motor domain has different enzymatic properties from those of well known kinesin 1.     

Temperature dependent properties of a kinesin-3 motor protein from Thermomyces lanuginosus

Kinesins are cytoskeletal motor proteins that share a common mechanochemical motor domain, and are responsible for trafficking macromolecules. Here we report the cloning and characterization of a monomeric, kinesin-3 (TKIN) from Thermomyces lanuginosus. TKIN displayed a maximum rate of ATP hydrolysis at approximately 55 degrees C; the K(m)(ATP) was also significantly greater at 50 degrees C. Gliding motility rates reached a maximum of 5.5 microms(-1) at 45 degrees C, which is among the highest rates reported for kinesin. Arrhenius energy barriers were calculated to be approximately 103 kJmol(-1), nearly twofold greater than other mesophilic kinesin motors. The enthalpy of activation and entropy activation of TKIN were also significantly greater when compared to other mesophilic kinesins. A thermally induced aggregation of TKIN, which could be moderated by the addition of ATP, was observed at temperatures above 45 degrees C. Together, these results illustrate the kinetic response and stability of this unique motor protein at elevated temperatures.     

Characterization of a novel rice kinesin O12 with a calponin homology domain

Genomic analysis predicted that the rice (Oryza sativa var. japonica) genome encodes at least 41 kinesin-like proteins including the novel kinesin O12, which is classified as a kinesin-14 family member. O12 has a calponin homology (CH) domain that is known as an actin-binding domain. In this study, we expressed the functional domains of O12 in Escherichia coli and determined its enzymatic characteristics compared with other kinesins. The microtubule-dependent ATPase activity of recombinant O12 containing the motor and CH domains was significantly reduced in the presence of actin. Interestingly, microtubule-dependent ATPase activity of the motor domain was also affected by actin in the absence of the CH domain. Our findings suggest that the motor activity of the rice plant-specific kinesin O12 may be regulated by actin.     

Intermolecular cross-linking of a novel rice Kinesin k16 motor domain with a photoreactive ATP derivative

A fluorescent photoreactive ATP derivative, 2'(3')-O-(4-benzoylbenzoyl)-1,N(6)-etheno-ATP (Bz(2)-epsilonATP), was synthesized and reacted with the rice kinesin K16 motor domain (K16MD). In the presence of ADP or ATP, UV irradiation of the K16MD solution containing Bz(2)-epsilonATP resulted in a new 100 kDa band, which was an intermolecular cross-linked product of motor domains. In contrast, no cross-linking was observed in the absence of nucleotides. For the motor domain of mouse brain kinesin and skeletal muscle myosin subfragment-1, no such intermolecular photo cross-linking by Bz(2)-epsilonATP was observed. Our results indicate that Bz(2)-epsilonATP acts unusually as a photoreactive crosslinker to detect conformational changes in K16MD induced by nucleotide binding resulting in the formation of dimers.     

Conformational change of the loop L5 in rice kinesin motor domain induced by nucleotide binding

Loop L5 of kinesin is located near the ATPase site, in common with kinesins of various animal species. The rice plant-specific kinesin K16 also has a corresponding loop that is slightly shorter than that of mouse brain kinesin. The present study was designed to monitor conformational changes in loop L5 during ATP hydrolysis. For this purpose, we introduced one reactive cysteine into the L5 of rice kinesin and modified it with fluorescent probes. The cysteine in L5 was labeled with a fluorescent probe 2-(4'(iodoacetamide) anilino-naphthalene-6-sulfonic acid sodium salt) [IAANS]. IAANS was incorporated into L5 at an almost equimolar ratio in the absence of nucleotides. In contrast, the incorporated amount was reduced to 0.62 and 0.32 mol IAANS/mol motor domain in the presence of ATP and ADP, respectively. Upon nucleotide addition, the fluorescent intensity of IAANS incorporated into L5 was significantly reduced to 63% and 51% for ATP and ADP, respectively. These results suggest that L5 of rice kinesin significantly changes its conformation during ATP hydrolysis.     

An internal motor kinesin is associated with the Golgi apparatus and plays a role in trichome morphogenesis in Arabidopsis

Members of the kinesin superfamily are microtubule-based motor proteins that transport molecules/organelles along microtubules. We have identified similar internal motor kinesins, Kinesin-13A, from the cotton Gossypium hirsutum and Arabidopsis thaliana. Their motor domains share high degree of similarity with those of internal motor kinesins of animals and protists in the MCAK/Kinesin13 subfamily. However, no significant sequence similarities were detected in sequences outside the motor domain. In Arabidopsis plants carrying the T-DNA knockout kinesin-13a-1 and kinesin-13a-2 mutations at the Kinesin-13A locus, >70% leaf trichomes had four branches, whereas wild-type trichomes had three. Immunofluorescent results showed that AtKinesin-13A and GhKinesin-13A localized to entire Golgi stacks. In both wild-type and kinesin-13a mutant cells, the Golgi stacks were frequently associated with microtubules and with actin microfilaments. Aggregation/clustering of Golgi stacks was often observed in the kinesin-13a mutant trichomes and other epidermal cells. This suggested that the distribution of the Golgi apparatus in cell cortex might require microtubules and Kinesin-13A, and the organization of Golgi stacks could play a regulatory role in trichome morphogenesis. Our results also indicate that plant kinesins in the MCAK/Kinesin-13 subfamily have evolved to take on different tasks than their animal counterparts.     

The kinesin-13 MCAK has an unconventional ATPase cycle adapted for microtubule depolymerization

Unlike other kinesins, members of the kinesin-13 subfamily do not move directionally along microtubules but, instead, depolymerize them. To understand how kinesins with structurally similar motor domains can have such dissimilar functions, we elucidated the ATP turnover cycle of the kinesin-13, MCAK. In contrast to translocating kinesins, ATP cleavage, rather than product release, is the rate-limiting step for ATP turnover by MCAK; unpolymerized tubulin and microtubules accelerate this step. Further, microtubule ends fully activate the ATPase by accelerating the exchange of ADP for ATP. This tuning of the cycle adapts MCAK for its depolymerization activity: lattice-stimulated ATP cleavage drives MCAK into a weakly bound nucleotide state that reaches microtubule ends by diffusion, and end-specific acceleration of nucleotide exchange drives MCAK into a strongly bound state that promotes depolymerization. This altered cycle accounts well for the different mechanical behaviour of this kinesin, which depolymerizes microtubules from their ends, compared to translocating kinesins that walk along microtubules. Thus, the kinesin motor domain is a nucleotide-dependent engine that can be differentially tuned for transport or depolymerization functions.     

Processive movement of single kinesins on crowded microtubules visualized using quantum dots

Kinesin-1 is a processive molecular motor transporting cargo along microtubules. Inside cells, several motors and microtubule-associated proteins compete for binding to microtubules. Therefore, the question arises how processive movement of kinesin-1 is affected by crowding on the microtubule. Here we use total internal reflection fluorescence microscopy to image in vitro the runs of single quantum dot-labelled kinesins on crowded microtubules under steady-state conditions and to measure the degree of crowding on a microtubule at steady-state. We find that the runs of kinesins are little affected by high kinesin densities on a microtubule. However, the presence of high densities of a mutant kinesin that is not able to step efficiently reduces the average speed of wild-type kinesin, while hardly changing its processivity. This indicates that kinesin waits in a strongly bound state on the microtubule when encountering an obstacle until the obstacle unbinds and frees the binding site for kinesin's next step. A simple kinetic model can explain quantitatively the behaviour of kinesin under both crowding conditions.     

The family-specific K-loop influences the microtubule on-rate but not the superprocessivity of kinesin-3 motors

The kinesin-3 family (KIF) is one of the largest among the kinesin superfamily and an important driver of a variety of cellular transport events. Whereas all kinesins contain the highly conserved kinesin motor domain, different families have evolved unique motor features that enable different mechanical and functional outputs. A defining feature of kinesin-3 motors is the presence of a positively charged insert, the K-loop, in loop 12 of their motor domains. However, the mechanical and functional output of the K-loop with respect to processive motility of dimeric kinesin-3 motors is unknown. We find that, surprisingly, the K-loop plays no role in generating the superprocessive motion of dimeric kinesin-3 motors (KIF1, KIF13, and KIF16). Instead, we find that the K-loop provides kinesin-3 motors with a high microtubule affinity in the motor''s ADP-bound state, a state that for other kinesins binds only weakly to the microtubule surface. A high microtubule affinity results in a high landing rate of processive kinesin-3 motors on the microtubule surface. We propose that the family-specific K-loop contributes to efficient kinesin-3 cargo transport by enhancing the initial interaction of dimeric motors with the microtubule track.     

Functional analysis of human microtubule-based motor proteins, the kinesins and dyneins, in mitosis/cytokinesis using RNA interference

Microtubule (MT)-based motor proteins, kinesins and dyneins, play important roles in multiple cellular processes including cell division. In this study, we describe the generation and use of an Escherichia coli RNase III-prepared human kinesin/dynein esiRNA library to systematically analyze the functions of all human kinesin/dynein MT motor proteins. Our results indicate that at least 12 kinesins are involved in mitosis and cytokinesis. Eg5 (a member of the kinesin-5 family), Kif2A (a member of the kinesin-13 family), and KifC1 (a member of the kinesin-14 family) are crucial for spindle formation; KifC1, MCAK (a member of the kinesin-13 family), CENP-E (a member of the kinesin-7 family), Kif14 (a member of the kinesin-3 family), Kif18 (a member of the kinesin-8 family), and Kid (a member of the kinesin-10 family) are required for chromosome congression and alignment; Kif4A and Kif4B (members of the kinesin-4 family) have roles in anaphase spindle dynamics; and Kif4A, Kif4B, MKLP1, and MKLP2 (members of the kinesin-6 family) are essential for cytokinesis. Using immunofluorescence analysis, time-lapse microscopy, and rescue experiments, we investigate the roles of these 12 kinesins in detail.     

The crystal structure of the minus-end-directed microtubule motor protein ncd reveals variable dimer conformations

BACKGROUND: The kinesin superfamily of microtubule-associated motor proteins are important for intracellular transport and for cell division in eukaryotes. Conventional kinesins have the motor domain at the N terminus of the heavy chain and move towards the plus end of microtubules. The ncd protein is necessary for chromosome segregation in meiosis. It belongs to a subfamily of kinesins that have the motor domain at the C terminus and move towards the minus end of microtubules. RESULTS: The crystal structure of dimeric ncd has been obtained at 2.9 A resolution from crystals with the C222(1) space group, with two independent dimers per asymmetric unit. The motor domains in these dimers are not related by crystallographic symmetry and the two ncd dimers have significantly different conformations. An alpha-helical coiled coil connects, and interacts with, the motor domains. CONCLUSIONS: The ncd protein has a very compact structure, largely due to extended interactions of the coiled coil with the head domains. Despite this, we find that the overall conformation of the ncd dimer can be rotated by as much as 10 degrees away from that of the twofold-symmetric archetypal ncd. The crystal structures of conventional kinesin and of ncd suggest a structural rationale for the reversal of the direction of movement in chimeric kinesins.     

Essential kinesins: characterization of Caenorhabditis elegans KLP-15

Kinesins form a superfamily of molecular motors involved in cell division and intracellular transport. Twenty kinesins have been found in the Caenorhabditis elegans genome, and four of these belong to the kinesin-14 subfamily, i.e., kinesins with C-terminal motor domains. Three of these kinesin-14s, KLP-15, KLP-16, and KLP-17, form a distinct subgroup in which KLP-15 and KLP-16 are more than 90% identical and appear to be related by a relatively recent gene duplication. They are essential for meiotic spindle organization and chromosome segregation, and are mostly expressed in the germline. With 587 amino acids each, they are among the smallest kinesins known. Using bacterially expressed KLP-15 constructs with different length extensions preceding the motor domain, we have determined in vitro the following characteristic properties: ATPase activity, microtubule binding, oligomeric state, microtubule gliding activity, and direction of movement. The constructs exhibit a monomer-dimer equilibrium that depends on the length of the predicted alpha-helical coiled-coil region preceding the motor domain. The longest construct with the complete coiled-coil domain is a stable dimer, and the shortest construct with only seven amino acids preceding the motor domain is a monomer. In microtubule gliding assays, the monomer is immobile whereas the fully dimeric KLP-15 construct supports gliding at 2.3 microm/min and moves toward microtubule minus ends, like other members of the kinesin-14 subfamily studied to date.     

Subunits interactions in kinesin motors

Kinesins form a large and diverse superfamily of proteins involved in numerous important cellular processes. The majority of them are molecular motors moving along microtubules. Conversion of chemical energy into mechanical work is accomplished in a sequence of events involving both biochemical and conformational alternation of the motor structure called the mechanochemical cycle. Different members of the kinesin superfamily can either perform their function in large groups or act as single molecules. Conventional kinesin, a member of the kinesin-1 subfamily, exemplifies the second type of motor which requires tight coordination of the mechanochemical cycle in two identical subunits to accomplish processive movement toward the microtubule plus end. Recent results strongly support an asymmetric hand-over-hand model of "walking" for this protein. Conformational strain between two subunits at the stage of the cycle where both heads are attached to the microtubule seems to be a major factor in intersubunit coordination, although molecular and kinetic details of this phenomenon are not yet deciphered. We discuss also current knowledge concerning intersubunit coordination in other kinesin subfamilies. Members of the kinesin-3 class use at least three different mechanisms of movement and can translocate in monomeric or dimeric forms. It is not known to what extent intersubunit coordination takes place in Ncd, a dimeric member of the kinesin-14 subfamily which, unlike conventional kinesin, exercises a power-stroke toward the microtubule minus end. Eg5, a member of the kinesin-5 subfamily is a homotetrameric protein with two kinesin-1-like dimeric halves controlled by their relative orientation on two microtubules. It seems that diversity of subunit organization, quaternary structures and cellular functions in the kinesin superfamily are reflected also by the divergent extent and mechanism of intersubunit coordination during kinesin movement along microtubules.     

Functions of the Arabidopsis kinesin superfamily of microtubule-based motor proteins

Plants possess a large number of microtubule-based kinesin motor proteins. While the kinesin-2, 3, 9, and 11 families are absent from land plants, the kinesin-7 and 14 families are greatly expanded. In addition, some kinesins are specifically present only in land plants. The distinctive inventory of plant kinesins suggests that kinesins have evolved to perform specialized functions in plants. Plants assemble unique microtubule arrays during their cell cycle, including the interphase cortical microtubule array, preprophase band, anastral spindle and phragmoplast. In this review, we explore the functions of plant kinesins from a microtubule array viewpoint, focusing mainly on Arabidopsis kinesins. We emphasize the conserved and novel functions of plant kinesins in the organization and function of the different microtubule arrays.     

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

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.     

The mechanism, function and regulation of depolymerizing kinesins during mitosis

Kinesins are motor proteins that use the hydrolysis of ATP to do mechanical work. Most of these motors translocate cargo along the surface of the microtubule (MT). However, a subfamily of these motors (Kin-I kinesins) can destabilize MTs directly from their ends. This distinct ability makes their activity crucial during mitosis, when reordering of the MT cytoskeleton is most evident. Recently, much work has been done to elucidate the structure and mechanism of depolymerizing kinesins, particularly those of the mammalian kinesin mitotic centromere-associated kinesin (MCAK). In addition, new regulatory factors have been discovered that shed light on the regulation and precise role of Kin-I kinesins during mitosis.     

Kinesin’s Neck-Linker Determines its Ability to Navigate Obstacles on the Microtubule Surface

The neck-linker is a structurally conserved region among most members of the kinesin superfamily of molecular motor proteins that is critical for kinesin’s processive transport of intracellular cargo along the microtubule surface. Variation in the neck-linker length has been shown to directly modulate processivity in different kinesin families; for example, kinesin-1, with a shorter neck-linker, is more processive than kinesin-2. Although small differences in processivity are likely obscured in vivo by the coupling of most cargo to multiple motors, longer and more flexible neck-linkers may allow different kinesins to navigate more efficiently around the many obstacles, including microtubule-associated proteins (MAPs), that are found on the microtubule surface within cells. We hypothesize that, due to its longer neck-linker, kinesin-2 can more easily navigate obstacles (e.g., MAPs) on the microtubule surface than kinesin-1. We used total internal reflection fluorescence microscopy to observe single-molecule motility from different kinesin-1 and kinesin-2 neck-linker chimeras stepping along microtubules in the absence or presence of two Tau isoforms, 3RS-Tau and 4RL-Tau, both of which are MAPs that are known to differentially affect kinesin-1 motility. Our results demonstrate that unlike kinesin-1, kinesin-2 is insensitive to the presence of either Tau isoform, and appears to have the ability to switch protofilaments while stepping along the microtubule when challenged by an obstacle, such as Tau. Thus, although kinesin-1 may be more processive, the longer neck-linker length of kinesin-2 allows it to be better optimized to navigate the complex microtubule landscape. These results provide new insight, to our knowledge, into how kinesin-1 and kinesin-2 may work together for the efficient delivery of cargo in cells.     

Structural links to kinesin directionality and movement

The kinesin motor proteins generate directional movement along microtubules and are involved in many vital processes, including cell division, in eukaryotes. The kinesin superfamily is characterized by a conserved motor domain of approximately 320 residues. Dimeric constructs of N and C class kinesins, with the motor domains at opposite ends of the heavy chain, move towards microtubule plus and minus ends, respectively. Their crystal structures differ mainly in the region linking the motor domain core to the alpha-helical coiled coil dimerization domain. Chimeric kinesins show that regions outside of the motor domain core determine the direction of movement and mutations in the linker region have a strong effect on motility. Recent work on chimeras and mutants is discussed in a structural context giving insights to possible molecular mechanisms of kinesin directionality and motility.