Engineering a lever into the kinesin neck

Kinesin Motor Enzymology: Chemistry, Structure, and Physics of Nanoscale Molecular Machines

Molecular motors are enzymes that convert chemical potential energy into controlled kinetic energy for mechanical work inside cells. Understanding the biophysics of these motors is essential for appreciating life as well as apprehending diseases that arise from motor malfunction. This review focuses on kinesin motor enzymology with special emphasis on the literature that reports the chemistry, structure and physics of several different kinesin superfamily members.

Pressure-induced changes in the structure and function of the kinesin-microtubule complex

Kinesin-1 is an ATP-driven molecular motor that "walks" along a microtubule by working two heads in a "hand-over-hand" fashion. The stepping motion is well-coordinated by intermolecular interactions between the kinesin head and microtubule, and is sensitively changed by applied forces. We demonstrate that hydrostatic pressure works as an inhibitory action on kinesin motility. We developed a high-pressure microscope that enables the application of hydrostatic pressures of up to 200 MPa (2000 bar). Under high-pressure conditions, taxol-stabilized microtubules were shortened from both ends at the same speed. The sliding velocity of kinesin motors was reversibly changed by pressure, and reached half-maximal value at approximately 100 MPa. The pressure-velocity relationship was very close to the force-velocity relationship of single kinesin molecules, suggesting a similar inhibitory mechanism on kinesin motility. Further analysis showed that the pressure mainly affects the stepping motion, but not the ATP binding reaction. The application of pressure is thought to enhance the structural fluctuation and/or association of water molecules with the exposed regions of the kinesin head and microtubule. These pressure-induced effects could prevent kinesin motors from completing the stepping motion.

Structural model for strain-dependent microtubule activation of Mg-ADP release from kinesin

Mg-ADP release is considered to be a crucial process for the regulation and motility of kinesin. To gain insight into the structural basis of this process, we solved the atomic structures of kinesin superfamily protein-1A (KIF1A) during and after Mg(2+) release. On the basis of new structural and mutagenesis data, we propose a model mechanism for microtubule activation of Mg-ADP release from KIF1A. In our model, a specific interaction between loop L7 of KIF1A and beta-tubulin reconfigures the KIF1A active site by shifting the relative positions of switches I and II. This leads to the sequential release of a group of water molecules that sits over the Mg(2+) in the active site, followed by Mg(2+) and finally the ADP. We further propose that this set of events is linked to a strain-dependent docking of the neck linker to the motor core, which produces a two-step power stroke.

Active alignment of microtubules with electric fields

The direction of translocation of microtubules on a surface coated with kinesin is usually random. Here we demonstrate and quantify the rate at which externally applied electric fields can direct moving microtubules parallel to the field by deflecting their leading end toward the anode. Effects of electric field strength, kinesin surface density, and microtubule translocation speed on the rate of redirection of microtubules were analyzed statistically. Furthermore, we demonstrated that microtubules can be steered in any desired direction via manipulation of externally applied E-fields.

Single molecule thermodynamics in biological motors

Biological molecular machines use thermal activation energy to carry out various functions. The process of thermal activation has the stochastic nature of output events that can be described according to the laws of thermodynamics. Recently developed single molecule detection techniques have allowed each distinct enzymatic event of single biological machines to be characterized providing clues to the underlying thermodynamics. In this study, the thermodynamic properties in the stepping movement of a biological molecular motor have been examined. A single molecule detection technique was used to measure the stepping movements at various loads and temperatures and a range of thermodynamic parameters associated with the production of each forward and backward step including free energy, enthalpy, entropy and characteristic distance were obtained. The results show that an asymmetry in entropy is a primary factor that controls the direction in which the motor will step. The investigation on single molecule thermodynamics has the potential to reveal dynamic properties underlying the mechanisms of how biological molecular machines work.

Entropy rectifies the Brownian steps of kinesin

Kinesin is a stepping motor that successively produces forward and backward 8-nm steps along microtubules. Under physiological conditions, the steps powering kinesin's motility are biased in one direction and drive various biological motile processes. The physical mechanism underlying the unidirectional bias of the kinesin steps is not fully understood. Here we explored the mechanical kinetics and thermodynamics of forward and backward kinesin steps by analyzing their temperature and load dependence. Results show that the frequency asymmetry between forward and backward steps is produced by entropy. Furthermore, the magnitude of the entropic asymmetry is 6 k(B)T, more than three times greater than expected from a current model, in which a mechanical conformational change within the kinesin molecular structure directly biases the kinesin steps forward. We propose that the stepping direction of kinesin is preferably caused by an entropy asymmetry resulting from the compatibility between the kinesin and microtubule interaction based on their polar structures.

The human chromokinesin Kid is a plus end-directed microtubule-based motor

Kid is a kinesin-like DNA-binding protein known to be involved in chromosome movement during mitosis, although its actual motor function has not been demonstrated. Here, we describe the initial characterization of Kid as a microtubule-based motor using optical trapping microscopy. A bacterially expressed fusion protein consisting of a truncated Kid fragment (amino acids 1-388 or 1-439) is indeed an active microtubule motor with an average speed of approximately 160 nm/s, and the polarity of movement is plus end directed. We could not detect processive movement of either monomeric Kid or dimerizing chimeric Kid; however, low levels of processivity (a few steps) cannot be detected with our method. These results are consistent with Kid having a role in chromosome congression in vivo, where it would be responsible for the polar ejection forces acting on the chromosome arms.

Thermal activation of single kinesin molecules with temperature pulse microscopy

Conventional kinesin is a processive motor protein that keeps "walking" along a microtubule using chemical energy released by ATP hydrolysis. We previously studied the effects of temperature between 15 degrees and 35 degrees C on the moving velocity, force, and processivity of single kinesin molecules using a bead assay [Kawaguchi and Ishiwata, 2000b: Biochem Biophys Res Commun 272:895-899]. However, we could not examine the effects of temperature higher than 35 degrees C because of the thermal damage to proteins. Here, using temperature pulse microscopy (TPM) [Kato et al., 1999: Proc Natl Acad Sci USA 96:9602-9606], we could examine the temperature dependence of the gliding velocity of single kinesin molecules interacting with a microtubule above 35 degrees C up to 50 degrees C (instantaneously, approximately 60 degrees C), where the velocity reached 3.68 microm/s, the highest ever reported. The Arrhenius plot showed no breaks between 15 degrees and 50 degrees C with a unique activation energy of about 50 kJ/mol, suggesting that the molecular mechanism of kinesin motility is common over a broad temperature range including physiological temperature.

Conformational preferences of a synthetic 30mer peptide from the interface between the neck and stalk regions of kinesin

The conformation of a synthetic peptide, consisting of 30 amino acids spanning the neck and hinge regions of rat brain kinesin, was investigated by NMR spectroscopy. The peptide extends from K357 to D386 and has the sequence KSVIQHLEVELNRWRNGEAVPEDEQISAKD. A total of 82 distance range constraints and 23 dihedral angle constraints could be obtained from NOESY and E.COSY spectra, respectively. These were used to calculate 500 structures by applying the REDAC algorithm of the software package DYANA. The first half of the peptide matched the helical structure of the neck determined from an X-ray crystal structure of kinesin. This part normally dimerizes into a coiled-coil by virtue of a leucine zipper interaction, but it is alpha-helical even in the monomeric state. The second half (not visible in the X-ray structure because of disorder) contains locally defined structure elements (extended chain, helical loop) connected by flexible joints. This is consistent with the "hinge" function postulated for this domain which is important for kinesin's motility and orientation.

Temperature dependence of force, velocity, and processivity of single kinesin molecules

Using the bead assay in optical microscopy equipped with optical tweezers, we have examined the effect of temperature on the gliding velocity, force, and processivity of single kinesin molecules interacting with a microtubule between 15 and 35 degrees C. The gliding velocity increased with the Arrhenius activation energy of 50 kJ/mol, consistent with the temperature dependence of the microtubule-dependent ATPase activity. Also, the average run length, i.e., a measure of processivity of kinesin, increased on increasing temperature. On the other hand, the generated force was independent of temperature, 7.34 +/- 0.33 pN (average +/- S.D., n = 70). The gliding velocities decreased almost linearly with an increase in force irrespective of temperature, implying that the efficiency of mechano-chemical energy conversion is maintained constant in this temperature range. Thus, we suggest that the force generation is attributable to the temperature-insensitive nucleotide-binding state(s) and/or conformational change(s) of kinesin-microtubule complex, whereas the gliding velocity is determined by the ATPase rate.

The conformational cycle of kinesin

The stepping mechanism of kinesin can be thought of as a programme of conformational changes. We briefly review protein chemical, electron microscopic and transient kinetic evidence for conformational changes, and working from this evidence, outline a model for the mechanism. In the model, both kinesin heads initially trap Mg x ADP. Microtubule binding releases ADP from one head only (the trailing head). Subsequent ATP binding and hydrolysis by the trailing head progressively accelerate attachment of the leading head, by positioning it closer to its next site. Once attached, the leading head releases its ADP and exerts a sustained pull on the trailing head. The rate of closure of the molecular gate which traps ADP on the trailing head governs its detachment rate. A speculative but crucial coordinating feature is that this rate is strain sensitive, slowing down under negative strain and accelerating under positive strain.

Directional motility of kinesin motor proteins

Kinesin motor proteins are molecules capable of moving along microtubules. They share homology in the so-called core motor domain which acts as a microtubule-dependent ATPase. The surprising finding that different members of the superfamily move in opposite directions along microtubules despite their close similarity has stimulated intensive research on the determinants of motor directionality. This article reviews recent biophysical, biochemical, structural and mutagenic studies that contributed to the elucidation of the mechanisms that cause directional motion of kinesin motor proteins.

Kinesins and microtubules: their structures and motor mechanisms

Atomic resolution three-dimensional structures of two oppositely directed kinesin motors - conventional kinesin and non-claret disjunctional (ncd) protein - are now available in their functional dimeric form. A detailed model of the microtubule has also been recently obtained by docking the 3.7 A structure of tubulin into a 20 A map of the microtubule. Recent structural studies of kinesin motors and their microtubule tracks are contributing to our current understanding of kinesin motor mechanisms.