Plus and minus ends of microtubule respond asymmetrically to kinesin binding by a long range directionally driven allosteric mechanism

Many members in the kinesin superfamily walk predominantly towards the plus end of the microtubule (MT) in a hand-over-hand manner. Despite great progress in elucidating the mechanism of stepping kinetics, the origin of stepping directionality is not fully understood. To provide quantitative insights into this important issue, we represent the structures of conventional kinesin (Kin1), MT, and the Kin1-MT complex using the elastic network model, and calculate the residue-dependent responses to a local perturbation in these constructs. Fluctuations in the residues in the β domain of the α/β-tubulin are distinct from the α domain. Surprisingly, the Kin1-induced asymmetry, which is more pronounced in α/β-tubulin in the plus end of MT than in the minus end, propagates spatially across multiple α/β-tubulin dimers. Kin1 binding expands the MT lattice by mechanical stresses, resulting in a transition in the cleft of α/β tubulin dimer between a closed (CC for closed cleft) state (not poised for Kin1 to bind) to an open (OC for open cleft) binding competent state. The long-range asymmetric responses in the MT, leading to the creation of OC states with high probability in several α/β dimers on the plus end of the bound Kin1, is needed for the motor to take multiple steps towards the plus end of the MT. Reciprocally, kinesin binding to the MT stiffens the residues in the MT binding region, induces correlations between switches I and II in the motor, and enhances fluctuations in ADP and the residues in the binding pocket. Our findings explain both the directionality of stepping and MT effects on a key step in the catalytic cycle of Kin1.

Multi-Scale Coarse Grained Model for the Stepping of Molecular Motors with application to Kinesin.

Conventional kinesin, a motor protein that transports cargo within cells, walks by taking multiple steps towards the plus end of the microtubule (MT). While significant progress has been made in understanding the details of the walking mechanism of kinesin there are many unresolved issues. From a computational perspective, a central challenge is the large size of the system, which limits the scope of time scales accessible in standard computer simulations. Here, we create a general multi-scale coarse-grained model for motors that enables us to simulate the stepping process of motors on polar tracks (actin and MT) with focus on kinesin. Our approach greatly shortens the computation times without a significant loss in detail, thus allowing us to better describe the molecular basis of the stepping kinetics. The small number of parameters, which are determined by fitting to experimental data, allows us to develop an accurate method that may be adopted to simulate stepping in other molecular motors. The model enables us to simulate a large number of steps, which was not possible previously. We show in agreement with experiments that due to the docking of the neck linker (NL) of kinesin, sometimes deemed as the power stroke, the space explored diffusively by the tethered head is severely restricted allowing the step to be in a tens of microseconds. We predict that increasing the interaction strength between the NL and the motor head, achievable by mutations in the NL, decreases the stepping time but reaches a saturation value. Furthermore, the full 3-dimensional dynamics of the cargo are fully resolved in our model, contributing to the predictive power and allowing us to study the important aspects of cargo-motor interactions.

Effects of Gold Nanoparticles on the Stepping Trajectories of Kinesin

Substantial increase in the temporal resolution of the stepping of dimeric molec- ular motors is possible by tracking the position of a large gold nanoparticle (GNP) attached to a labeled site on one of the heads. This technique was used to measure the stepping trajectories of conventional kinesin (Kin1) using the time dependent position of the GNP as a proxy. The trajectories revealed that the detached head always passes to the right of the head that is tightly bound to the microtubule (MT) during a step. In interpreting the results of such experiments, it is implicitly assumed that the GNP does not significantly alter the diffusive motion of the detached head. We used coarse-grained simulations of a system consisting of the MT-Kin1 complex with and without attached GNP to investigate how the stepping trajectories are affected. The two significant findings are: (1) The GNP does not faithfully track the position of the stepping head. (2) The rightward bias is typically exaggerated by the GNP. Both these findings depend on the precise residue position to which the GNP is attached. Surprisingly, we predict that the stepping trajectories of kinesin are not significantly affected if, in addition to the GNP, a 1 μm diameter cargo is attached to the coiled coil. Our simulations suggest the effects of the large probe have to be considered when inferring the stepping mechanisms using GNP tracking experiments.

Postsynaptic density protein 95 (PSD-95) is transported by KIF5 to dendritic regions

Postsynaptic density protein 95 (PSD-95) is a pivotal postsynaptic scaffolding protein in excitatory neurons. Although the transport and regulation of PSD-95 in synaptic regions is well understood, dendritic transport of PSD-95 before synaptic localization still remains to be clarified. To evaluate the role of KIF5, conventional kinesin, in the dendritic transport of PSD-95 protein, we expressed a transport defective form of KIF5A (ΔMD) that does not contain the N-terminal motor domain. Expression of ΔMD significantly decreased PSD-95 level in the dendrites. Consistently, KIF5 was associated with PSD-95 in in vitro and in vivo assays. This interaction was mediated by the C-terminal tail regions of KIF5A and the third PDZ domain of PSD-95. Additionally, the ADPDZ3 (the association domain of NMDA receptor and PDZ3 domain) expression significantly reduced the levels of PSD-95, glutamate receptor 1 (GluA1) in dendrites. The association between PSD-95 and KIF5A was dose-dependent on Staufen protein, suggesting that the Staufen plays a role as a regulatory role in the association. Taken together, our data suggest a new mechanism for dendritic transport of the AMPA receptor-PSD-95.

How kinesin waits for ATP affects the nucleotide and load dependence of the stepping kinetics

Conventional kinesin, responsible for directional transport of cellular vesicles, takes multiple nearly uniform 8.2-nm steps by consuming one ATP molecule per step as it walks toward the plus end of the microtubule (MT). Despite decades of intensive experimental and theoretical studies, there are gaps in the elucidation of key steps in the catalytic cycle of kinesin. How the motor waits for ATP to bind to the leading head is controversial. Two experiments using a similar protocol have arrived at different conclusions. One asserts that kinesin waits for ATP in a state with both the heads bound to the MT, whereas the other shows that ATP binds to the leading head after the trailing head detaches. To discriminate between the 2 scenarios, we developed a minimal model, which analytically predicts the outcomes of a number of experimental observable quantities (the distribution of run length, the distribution of velocity [P(v)], and the randomness parameter) as a function of an external resistive force (F) and ATP concentration ([T]). The differences in the predicted bimodality in P(v) as a function of F between the 2 models may be amenable to experimental testing. Most importantly, we predict that the F and [T] dependence of the randomness parameters differ qualitatively depending on the waiting states. The randomness parameters as a function of F and [T] can be quantitatively measured from stepping trajectories with very little prejudice in data analysis. Therefore, an accurate measurement of the randomness parameter and the velocity distribution as a function of load and nucleotide concentration could resolve the apparent controversy.

Chromokinesins NOD and KID Use Distinct ATPase Mechanisms and Microtubule Interactions to Perform a Similar Function

Chromokinesins NOD and KID have similar DNA binding domains and functions during cell division, while their motor domain sequences show significant variations. It has been unclear whether these motors have similar structure, chemistry, and microtubule interactions necessary to follow a similar mechanism of force mediation. We used biochemical rate measurements, cosedimentation, and structural analysis to investigate the ATPase mechanisms of the NOD and KID core domains. These experiments and analysis revealed that NOD and KID have different ATPase mechanisms, microtubule interactions, and catalytic domain structures. The ATPase cycles of NOD and KID have different rate limiting steps. The ATPase rate of NOD was robustly stimulated by microtubules albeit its microtubule affinity was weakened in all nucleotide bound states. KID bound microtubules tightly in all nucleotide states and remained associated with the microtubule for more than 100 cycles of ATP hydrolysis before dissociating. The structure of KID was most similar to conventional kinesin (KIF5). Key differences in the microtubule binding region and allosteric communication pathway between KID and NOD are consistent with our biochemical data. Our results support the model that NOD and KID utilize distinct mechanistic pathways to achieve the same function during cell division.

Common general anesthetic propofol impairs kinesin processivity

Propofol is the most widely used i.v. general anesthetic to induce and maintain anesthesia. It is now recognized that this small molecule influences ligand-gated channels, including the GABAA receptor and others. Specific propofol binding sites have been mapped using photoaffinity ligands and mutagenesis; however, their precise target interaction profiles fail to provide complete mechanistic underpinnings for the anesthetic state. These results suggest that propofol and other common anesthetics, such as etomidate and ketamine, may target additional protein networks of the CNS to contribute to the desired and undesired anesthesia end points. Some evidence for anesthetic interactions with the cytoskeleton exists, but the molecular motors have received no attention as anesthetic targets. We have recently discovered that propofol inhibits conventional kinesin-1 KIF5B and kinesin-2 KIF3AB and KIF3AC, causing a significant reduction in the distances that these processive kinesins can travel. These microtubule-based motors are highly expressed in the CNS and the major anterograde transporters of cargos, such as mitochondria, synaptic vesicle precursors, neurotransmitter receptors, cell signaling and adhesion molecules, and ciliary intraflagellar transport particles. The single-molecule results presented show that the kinesin processive stepping distance decreases 40–60% with EC50 values <100 nM propofol without an effect on velocity. The lack of a velocity effect suggests that propofol is not binding at the ATP site or allosteric sites that modulate microtubule-activated ATP turnover. Rather, we propose that a transient propofol allosteric site forms when the motor head binds to the microtubule during stepping.