Front Head Research Articles

Molecular Simulation Study of Kinesin: Coupling between ATPase Domain Conformational Change and Mechanical Stepping

Conventional two-headed kinesin is a motor protein that moves unidirectionally by stepping in hand-over-hand manner (akin to human walking) along microtubule (MT) driven by ATP hydrolysis free energy. In the absence of MT, X-ray crystallography revealed primarily two conformations of the head, ATPase domain; “T” structure preferred with a bound ATP and “D” structure preferred with a bound ADP.However, relations among the ATPase conformations, stepping motion, and type of bound nucleotide are still rather unclear. Here, we investigated the coupling mechanism between the enzyme structure and mechanical stepping. For this purpose, we performed molecular dynamics simulations with coarse-grained structure-based models. In particular, to investigate structural preference between T and D in front and rear heads bound on MT, we applied multiple-basin energy landscape model (Okazaki et al., 2006).Through simulations, we found the followings. (1) Enzyme structure can regulate its affinity to MT by the difference in the contact surface area: “T” structure has higher affinity to MT than “D”, which is consistent with experiments. (2) The internal-strain between two heads can regulate the ATPase structural preference: The rear head with forward-directed neck-linker prefer T structure, while the front head with backward-directed neck-linker prefer D structure.

Direct Observation of Individual Kinesin Head Motions

Optical-trapping assays for kinesin typically involve attaching a bead to the common stalk of the protein, whose motions report the average position of the molecule. Individual displacements produced by the separate heads therefore remain unresolved. We developed a novel assay for tracking a single head of Kinesin-1 while under controlled loads, by attaching a bead to one of the two head domains via a short (70 bp) DNA tether. This assay can directly report binding of the tethered head to the microtubule. Under hindering loads, we observed steps of ∼16 nm, as anticipated for heads moving in a hand-over-hand walk. Under assisting loads, we observed large jumps (>16-nm) in displacement at the start of step dwells, as load pulled the rear head forward beyond its partner head by ∼4 nm. Torque generated between the points of head attachment of the DNA and the neck linker tends to rotate the head, explaining this ‘overshoot’ feature. The durations of overshoots depend on the ATP concentration, implying that ATP binding to the new rear head allows the front (DNA-linked) head to rotate back to its normal orientation and bind the microtubule. To directly test whether one head is free to diffuse about its bound partner head between steps, we applied rapidly oscillating (hindering and assisting) loads to kinesin during stepping and measured the time-dependent difference between forward and rearward displacements of the bead. The magnitude of this signal varied between two discrete values, corresponding to those intervals when the bead-linked head adopted bound and unbound states. The existence of an unbound state disfavors models where one head docks against its partner between steps. We conclude that internal strain, generated whenever both heads bind the microtubule, is responsible for gating the kinetic cycle to ensure kinesin processivity.

Operation of an ITER relevant inspection robot on Tore Supra tokamak

Robotic operations are one of the major maintenance challenges for ITER and future fusion reactors. CEA has developed a multipurpose carrier able to realize deployments in the plasma vessel without breaking the Ultra High Vacuum (UHV) and temperature conditioning. A 6 years R&D programme was jointly conducted by CEA-LIST Interactive Robotics Unit and the Institute for Magnetic Fusion Research (IRFM) in order to demonstrate the feasibility and reliability of an in-vessel inspection robot relevant to ITER requirements. The Articulated Inspection Arm robot (AIA) is an 8-m long multilink carrier with a payload up to 10 kg operable between plasma under tokamak conditioning environment; its geometry allows a complete close inspection of Plasma Facing Components (PFCs) of the Tore Supra vessel. Different tools are being developed by CEA to be plugged at the front head of the carrier. The diagnostic presently in operation consists in a viewing system offering accurate visual inspection of PFCs. Leak detection of first wall based on helium sniffing and laser compact system for carbon co-deposited layers characterizations or treatments are also considered for demonstration. In April 2008, the AIA robot equipped with its vision diagnostic has realized a complete deployment into Tore Supra and the first closed inspection of the vessel under UHV conditions. During the upcoming experimental campaign, the same operation will be performed under relevant conditions (10 −6 Pa and 120 °C) after a conditioning phase at 200 °C to avoid outgassing pollution of the chamber. This paper describes the different steps of the project development, robot capabilities with the present operations conducted on Tore Supra and future requirements for making the robot a tool for tokamak routine operation.

Images of Barnacle Goose Branta leucopsis hybrids – a photo documentation of some crosses with different Anser species

This article presents full-body and close-up photographs of hybrids between Barnacle Goose and some Anser goose species. The second parent was in one case a Snow Goose, in five cases a Greylag Goose, in one case a domestic Swan Goose and in one case a Bar-headed Goose. Parentage is considered proven in the Snow Goose and the Greylag Goose hybrids. These hybrids are all rare. The number of individuals is in the magnitude of the estimated total Swedish populations and this is apparently the first report of a Barnacle Goose x domestic Swan Goose hybrid in Sweden. The bill colour pattern varied markedly between the five Greylag Goose hybrid siblings but little from one year to another. In the autumn, Greylag Goose hybrids and to some degree also the Swan Goose hybrid developed a transient white area of the front head next to the bill but a darker and browner colour of the pale cheek area. The various expressions of features inherited from the two parent species are exemplified in the Greylag Goose hybrids.

Front propagation in a laminar cellular flow: Shapes, velocities, and least time criterion

We experimentally investigate the propagation of chemical fronts in steady laminar cellular flows at large Péclet numbers and large Damköhler numbers. Fronts are generated in an aqueous solution by an autocatalytic oxydoreduction reaction. They propagate in a channel in which a chain of counter-rotative parallel vortices is induced by electroconvection. We first accurately determine the form, the dynamics and the mean velocity of these fronts in the whole Hele-Shaw regime of the flow. We then address the modeling of the evolution of their mean velocity with the flow amplitude. The structure of the front wakes yields us to reject an effective reaction-diffusion wave as a relevant model for large-scale front propagation. On the other hand, analysis of the role of front heads brings us to introduce a kinematic model at the vortex scale for uncovering the front dynamics. This model addresses the propagation of the front leading point in a chain of vortices whose field is modeled by a two-dimensional solid rotation complemented by a boundary layer. Interestingly, it sensitively relies on the effective trajectory followed by the front leading point. To account for this, a competition is worked out among a one-parameter family of potential trajectories. The actual trajectory is then selected as the fastest one with quite a good agreement with measurements and observations. In particular, the measured effective front velocities are well recovered from the model, including their intrinsic dependence on the boundary layer width. Accordingly, effective front propagation in a laminar steadily stirred medium is thus understood from an optimization principle similar to the Fermat principle of ray propagation in heterogeneous media.

Management Factors Associated with Impaired Locomotion in Dairy Cows in England and Wales

Forty-nine farms in England and Wales were visited on 4 occasions between February 2003 and March 2004. A total of 21,693 scores of locomotion were assigned to 7,722 cattle. Locomotion was assessed on a 3-point scale by observing the posture of a cow's back while standing and walking (1=sound, 2=not sound, 3=lame). Data on measurable factors potentially associated with locomotion were collected from all farms using direct observations of the farm environment and a comprehensive farmer interview. The mean herd locomotion score was 1.77±0.02. There was no significant difference in mean herd locomotion scores between 5 herds housed in straw yards (1.72±0.02) and 44 herds housed in free stalls (1.78±0.02), possibly because of lack of power. A GLM was produced using data from the 44 herds housed in free stalls, with the mean farm locomotion score of all cows examined on all 4 visits as the outcome variable. Factors associated with an elevated locomotion score were dry cows kept in straw yards compared with free stalls (increase in locomotion score=0.06±0.03), pregnant heifers kept with milking cows in winter compared with being kept with dry cows (increase in locomotion score=0.09±0.03), aisle widths of <3m compared with widths of ≥3m (increase in locomotion score=0.06±0.02), a curb height of ≤15cm compared with a height of >15cm (increase in locomotion score=0.07±0.03), routine trimming of hooves of all cows by a hoof trimmer or by the farmer compared with no routine hoof trimming (increase in locomotion score=0.18±0.04 and 0.13±0.03 respectively), feeding corn silage to milking cows compared with feeding other forage types (increase in locomotion score=0.10±0.03), and the use of automatic scrapers in the free-stall barn compared with tractor scrapers (increase in locomotion score=0.10±0.03). These variables were correlated with many other management variables. The use of automatic scrapers was correlated with the use of sawdust on rubber mats in free stalls. Curb height was negatively correlated with increasing distance of the neck rail from the front (head end) of the free stall. These putative risk factors support the hypothesis that locomotion score is linked to management factors; in particular, the combination of sawdust on rubber mats with automatic scrapers was associated with elevated locomotion scores.

Backsteps induced by nucleotide analogs suggest the front head of kinesin is gated by strain

The two-headed kinesin motor harnesses the energy of ATP hydrolysis to take 8-nm steps, walking processively along a microtubule, alternately stepping with each of its catalytic heads in a hand-over-hand fashion. Two persistent challenges for models of kinesin motility are to explain how the two heads are coordinated ("gated") and when the translocation step occurs relative to other events in the mechanochemical reaction cycle. To investigate these questions, we used a precision optical trap to measure the single-molecule kinetics of kinesin in the presence of substrate analogs beryllium fluoride or adenylyl-imidodiphosphate. We found that normal stepping patterns were interspersed with long pauses induced by analog binding, and that these pauses were interrupted by short-lived backsteps. After a pause, processive stepping could only resume once the kinesin molecule took an obligatory, terminal backstep, exchanging the positions of its front and rear heads, presumably to allow release of the bound analog from the new front head. Preferential release from the front head implies that the kinetics of the two heads are differentially affected when both are bound to the microtubule, presumably by internal strain that is responsible for the gating. Furthermore, we found that ATP binding was required to reinitiate processive stepping after the terminal backstep. Together, our results support stepping models in which ATP binding triggers the mechanical step and the front head is gated by strain.

On the hand-over-hand mechanism of kinesin

We present here a simple theoretical model for conventional kinesin. The model reproduces the hand-over-hand mechanism for kinesin walking to the plus end of a microtubule. A large hindering force induces kinesin to walk slowly to the minus end, again by a hand-over-hand mechanism. Good agreement is obtained between the calculated and experimental results on the external force dependence of the walking speed, the forward/backward step ratio, and dwell times for both forward and backward steps. The model predicts that both forward and backward motions of kinesin take place at the same chemical state of the motor heads, with the front head being occupied by an ATP (or ADP,Pi) and the rear being occupied by an ADP. The direction of motion is a result of the competition between the power stroke produced by the front head and the external load. The other predictions include the external force dependence of the chemomechanical coupling ratio (e.g., the stepping distance/ATP ratio) and the walking speed of kinesin at force ranges that have not been tested by experiments. The model predicts that the chemomechanical coupling remains tight in a large force range. However, when the external force is very large (e.g., approximately 18 pN), kinesin slides in an inchworm fashion, and the translocation of kinesin becomes loosely coupled to ATP turnovers.

Cellular automaton modelling of surface discharge dynamics for EMC problems

Surface charge distribution on aircraft windshields and front head shields of missiles is highly inhomogeneous and it may cause creeping discharge appearance. The discharge produces strong electromagnetic emission, which get into the bodies through apertures. A fractal model based on a cellular automaton network reproducing the dynamics of surface electrical charge distribution is proposed. The model allows us to obtain both integrated characteristics of the discharge and its local properties. The latter is of extremely importance for EMC applications.

Load-dependent kinetics of myosin-V can explain its high processivity

Recent studies provide strong evidence that single myosin class V molecules transport vesicles and organelles processively along F-actin, taking several 36-nm steps, 'hand over hand', for each diffusional encounter. The mechanisms regulating myosin-V's processivity remain unknown. Here, we have used an optical-tweezers-based transducer to measure the effect of load on the mechanical interactions between rabbit skeletal F-actin and a single head of mouse brain myosin-V, which produces its working stroke in two phases. We found that the lifetimes of the first phase of the working stroke changed exponentially and about 10-fold over a range of pushing and pulling forces of +/- 1.5 pN. Stiffness measurements suggest that intramolecular forces could approach 3.6 pN when both heads are bound to F-actin, in which case extrapolation would predict the detachment kinetics of the front head to slow down 50-fold and the kinetics of the rear head to accelerate respectively. This synchronizing effect on the chemo-mechanical cycles of the heads increases the probability of the trail head detaching first and causes a strong increase in the number of forward steps per diffusional encounter over a system with no strain dependence.

A Novel Leucodystrophy in a Dog

A diffuse, bilaterally symmetrical leucoencephalopathy was observed in a 2-month-old female crossbred puppy with a clinical history of progressive tetraparesis with front limb hypermetria, head tremor and seizures. Severe myelinolytic lesions with significant macrophage infiltration were confined to the white matter, mainly of the cerebellum and spinal cord. Moderate loss of myelin with severe gliosis predominated in the cerebrum. Axonal degeneration and axonal loss accompanied myelin degeneration. This disease was classified as a leucodystrophy. The clinical signs and certain features of the lesions (morphology and distribution), differed from those in previously described degenerative myelinolytic diseases in animals. The possible occurrence of the disorder in a littermate suggested a genetic basis.

The multiple effects of ketamine on electroencephalographic activity and behavior in WAG/Rij rats

The effects of ketamine, a noncompetitive antagonist at the NMDA receptor, were studied on the EEG and in the open field in a genetic animal model of generalized absence epilepsy—the WAG/Rij rat strain. Animals of this strain display spontaneous occurring generalized spike-wave discharges (SWDs) in the EEG. Ketamine was systemically administered in a dose range from 3 to 30 mg/kg. Biphasic effects of ketamine were observed in the EEG. The first phase was a dose-dependent suppression of SWDs, followed by a second phase characterized by the facilitation of SWDs. This increase was expressed first as an increased number of SWDs, and later on as a significant prolongation of individual discharges and decrease in frequency of SWDs. An obvious amplitude modulation of the discharges was also found. During the period of suppression of SWDs, a new phenomenon was observed: quasi-periodic groups of spikes or wave spikes, with an internal frequency of 4–5 Hz and a periodicity of about 5 s. That quasi-periodic activity vanished a few minutes prior to the recovery of the classical SWDs. However, a specific 5-s amplitude modulation of SWDs remained present in the recovery period. The propensity of that specific ketamine-induced activity was found to be correlated with propensity of SWDs in background EEGs of drug-free animals. Ketamine also produced a dose-related initial behavioral excitation, a decrease of muscle tone in hind quarters, followed by front quarters and head, and an absence of locomotor activity. However, the time course of the behavioral changes cannot explain the effects on the EEG. It can be concluded that ketamine has more effects on the EEG than previously assumed which cannot be explained by a simple blockade of the NMDA receptor. It is proposed that the obtained specific dynamics of SWDs' frequency may be caused by changes in the activity of the thalamo–cortical pacemaker that is generating SWDs.

Parasitism, condition and number of front head breeding tubercles in roach (Rutilus rutilus L.)

Abstract – In a previous study on the male roach, Rutilus rutilus, we found a positive correlation between sexual ornamentation, breeding tubercles (BTs) on lateral sides and parasite resistance. As a continuation of that study, we examined another measure of sexual ornamentation – number of BTs on front head – of male roach from five populations. BTs on head correlated positively with fish size in two populations and with condition in one population, but not with parasite resistance (proportion of dead parasites) in any of the populations. Moreover, head BTs correlated negatively with the number of two parasite species (Rhipidocotyle campanula and Myxobolus mülleri) in two populations, but positively with the number of a third parasite (Raphidascarus acus) in one population. In addition, a negative correlation with spleen size was found in one population. The present results suggest that head BTs of male roach – although the patterns vary between populations – may signal condition and parasite load, but not resistance, in contrast to lateral BTs. Therefore, our results are in line with the hypothesis of multiple‐message sexual ornaments.

The sting at the end of the tail: Damaging winds associated with extratropical cyclones

AbstractStrong surface winds often accompany the low‐level jets that occur along the cold fronts of extratropical cyclones, but there is evidence that the strongest surface winds occur in a distinctly different part of a certain class of cyclone. The most damaging extratropical cyclones go through an evolution that involves the formation of a bent‐back front and cloud head separated from the main polar‐front cloud band by a dry slot. When the cyclone attains its minimum central pressure, the trailing tip of the cloud head bounding the bent‐back front forms a hook which goes on to encircle a seclusion of warm air. The most damaging winds occur near the tip of this hook—the sting at the end of the tail.Observations of the Great Storm of October 1987 in south‐east England are re‐examined in some detail to study this phenomenon. The cloud head is shown to have a banded structure consistent with the existence of multiple mesoscale slantwise circulations. Air within these circulations leaves the hooked tip of the cloud head (and enters the dry slot) much faster than the rate of travel of the cloud‐head tip, implying rapid evaporation and diabatic cooling immediately upwind of the area of damaging surface winds. The circumstantial evidence from the observational study leads one to hypothesize that the mesoscale circulations and the associated evaporative heat sinks may play an active role in strengthening the damaging winds. Regardless of how important this role may be, the evolution of the cloud pattern seen in satellite imagery is a useful tool for nowcasting the occurrence and location of the worst winds. Copyright © 2004 Royal Meteorological Society

Effect of the thigh-cuffs on the carotid artery diameter jugular vein section and facial skin edema: HDT study.

Objective: To evaluate the distal arterial, venous and skin changes in a group using thigh cuffs during daytime and in a control group. Method: Cardiac, arterial, venous parameters were measured by echography and Doppler. Skin thickness was measured by high frequency echography. Results & discussion: Head down position induced plasma volume reduction, increased cerebral resistance, reduced lower limb resistance. The jugular vein increased whereas the femoral and popliteal veins decreased. All these changes were already observed in previous HDT. Common carotid diameter decreased, Front head skin thickness increased and Tibial skin thickness decreased. Eight hours with thigh cuffs increased the cardiac and carotid sizes which is in agreement with the plasma volume increase. Conversely they reduced the cerebral vascular resistance, jugular section and front head edema which may explain the sensation of comfort reported by the subjects. At the lower limb level the thigh cuffs restored the skin thickness to pre-HDT level but enlarged markedly the femoral and popliteal veins. HR, BP, CO, TPR did not change.

Dynamics of action potential head-tail interaction during reentry in cardiac tissue: ionic mechanisms.

In a sufficiently short reentry pathway, the excitation wave front (head) propagates into tissue that is partially refractory (tail) from the previous action potential (AP). We incorporate a detailed mathematical model of the ventricular myocyte into a one-dimensional closed pathway to investigate the effects of head-tail interaction and ion accumulation on the dynamics of reentry. The results were the following: 1) a high degree of head-tail interaction produces oscillations in several AP properties; 2) Ca(2+)-transient oscillations are in phase with AP duration oscillations and are often of greater magnitude; 3) as the wave front propagates around the pathway, AP properties undergo periodic spatial oscillations that produce complicated temporal oscillations at a single site; 4) depending on the degree of head-tail interaction, intracellular [Na(+)] accumulation during reentry either stabilizes or destabilizes reentry; and 5) elevated extracellular [K(+)] destabilizes reentry by prolonging the tail of postrepolarization refractoriness.

Thermodynamic and Vertical Velocity Structure of Two Gust Fronts Observed with a Wind Profiler/RASS during MCTEX

High-time-resolution wind profiler/RASS observations are used to describe the vertical velocity, temperature, and reflectivity fields of two gust fronts in detail. The first was a freely propagating gust front and the second interacted with a rain cell near the profiler site. The first of these shows a large updraft confined to the warm air ahead of the front. This updraft coincided with the (nonhydrostatic) pressure jump. The vertical motions within the gust front were an order of magnitude smaller. The updraft impinging on the top of the boundary layer excited a clear gravity wave signature in the free troposphere. The interaction of the vertical circulation and the weakly precipitating cloud in the second case coincided with explosive growth of the cell with reflectivities increasing from ;30 dBZ to .50 dBZ in 6 min. A descending reflectivity core was observed at this time. Precipitation loading played a significant role in a downdraft behind the gust front head leading to adiabatic warming as no evidence of evaporative cooling in the downdraft was seen. A distinct clear air peak was visible in the profiler Doppler spectra even during the heavy rain.

Effects of head cooling on sweat rate in exercising subjects wearing protective clothing and mask for pesticide.

This study examined the effects of the head cooling on the sweat rates in 6 healthy female subjects at an ambient temperature of 28 degrees C and a relative humidity of 60%. The exercising subjects wore protective clothing and a mask for pesticide. Head cooling was made by putting frozen gel strips on the parietal and the front head which were held by a net-bandage (with cooling). On separate days, the same experiments were made without head cooling (without cooling). The total weight of the frozen gel strips was 250 g. After the subjects rested for 15 min in a chair, they repeated three times the work/rest schedule of 15 min exercise on a bicycle ergometer (50 W) followed by 5 min rest. Frozen gel strips were put on the head just before the first exercise bout. Main results were summarized as follows: 1) The forearm sweat rate, heart rate and the changes in Tre from the initial level (delta Tre) were significantly lower "with cooling" than "without cooling". 2) The amount of sweat adhered to the clothing ensemble was significantly lower "with cooling" than "without cooling". 3) The changes in Tty from the initial level (delta Tty) tended to be clearly lower "with cooling" than "without cooling". 4) Skin temperatures of the forehead and the cheek under a mask tended to be lower "with cooling" than "without cooling". 5) Subjective sensation ratings seemed to be improved in most subjects "with cooling". Thus, it was concluded that head cooling seemed to be more effective in reducing thermal strain in working subjects wearing the protective clothing and a mask for pesticide at hot environments.

Stability of Low-Crested Rubble-Mound Breakwater Heads

Three-dimensional physical models of low-crested, detached rubble-mound breakwaters were built, with their individual sections representing the following components of the breakwater: front slope, crest, back slope, total section (combination of the three previous sections), front head, and back head. The stability of the armor units in each of these sections was assessed, for different freeboards, using irregular waves. In a 1992 paper, Vidal et al. presented the stability criteria for the different trunk sections of this model breakwater. This paper focusses on test results from the head sections. A comparison between the stabilities of the head and trunk sections is also discussed. The stability of the back head (BH) section is found to be highly freeboard-dependent. For fully submerged conditions, the back head is one of the most stable sections of the breakwater. On the other hand, when the breakwater is nonovertopping, it is the least stable section. The front head (FH) section's behavior is similar...

The Oklahoma Squall Line of 19 May 1977. Part II: Mechanisms for Maintenance of the Region of Strong Convection

Mechanisms for maintenance of the strong convection along the leading edge of a broad squall line that occurred in Oklahoma on 19 May 1977 are investigated. The findings are based upon analysis of data from a surveillance radar, a surface mesonetwork, Doppler radars, proximity soundings and aircraft data, and upon the results of a two-dimensional, cloud-scale numerical simulation. The detailed results of the multiple Doppler analysis are contained in the Part I paper reporting results of research on this squall line. It is found that at a preferred location along the squall line, an area of intense convection is maintained over a long time period. A meso-β scale organized structure, which includes an area of low pressure near the southeast edge of the intense convection and an associated area of convergence extending to the east, promotes the formation of small showers in short line segments. These showers, due to their differing motion from elements within the main line, merge with the line to the north of the mesolow, resulting in maintenance of the strong area of convection. The observed meso-β structure on this day is believed to be made possible by a deep low-level layer of weak vertical wind shear and high water-vapor content. At other locations along the line, the numerical simulation indicates an unsteady behavior in the maintenance of squall line convection by gust frontal convergence. Perturbations in the vertical motion field are periodically initiated by either (i) enhanced convergence at the gust front resulting from diverging downdrafts at locations farther to the west, or (ii) Kelvin-Helmholtz instability produced at the gust front head. These perturbations move westward relative to the gust front above the low-level cold air and periodically invigorate the main region of updrafts located a few tens of kilometers west of the gust front. Low-level updrafts, forced by diverging surface outflow from weak downdrafts, occasionally interact with the translating perturbations to increase their amplitude. The existence of the westward-moving perturbations is tentatively substantiated by the presence of similar structures in the analyzed Doppler wind fields. Greater time resolution in Doppler data, in combination with more comprehensive surface and upper air data ahead of squall lines of this type, would aid in confirming the reported structures.