Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session V7: Piconewtons and Nanometers: The Physics of Molecular Motors |
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Sponsoring Units: DBP Chair: Ned Wingreen, Princeton University Room: Portland Ballroom 254 |
Thursday, March 18, 2010 8:00AM - 8:36AM |
V7.00001: In vitro motility assays and single molecule analyses reveal functional structural transitions in the molecular motor myosin Invited Speaker: The molecular basis of how myosin motors work has been significantly advanced by laser trap and other single molecule studies of myosins V and VI. Myosin V moves processively by stepping arm-over-arm, walking along the 36-nm pseudo-repeat of an actin filament by swinging its long lever arms through an angle of $\sim 70 ^{\circ}$, and hydrolyzing one ATP per step. Compared to the laser trap, we have improved time resolution to submilliseconds by tracking single gold nanoparticle-myosin V conjugates using darkfield imaging, and have directly observed the behavior of the unbound head as the motor translocates. We have also developed a technique called single-molecule high resolution co-localization (SHREC), which allows simultaneous co-localization of two chromatically differing fluorophores only 10 nm apart. We used SHREC to directly observe myosin V molecules walking hand-over-hand. Myosin VI, a considerably different myosin family member, has been the biggest challenge to the lever arm hypothesis of myosin movement. It has a very short light chain binding domain (the conventional lever arm). Nevertheless, the molecule surprisingly steps processively 36 nm along an actin filament. Furthermore, myosin VI moves in the opposite direction to that of myosin II and myosin V. We now understand how this marvelous molecular motor achieves these feats. [Preview Abstract] |
Thursday, March 18, 2010 8:36AM - 9:12AM |
V7.00002: Modeling of Single Molecule Cytoplasmic Dynein Invited Speaker: A living cell has an infrastructure much like that of a city. We will describe the transportation system that consists of roads (filaments) and molecular motors (proteins) that haul cargo along these roads. Dynein is one type of motor protein that walks along microtubules towards the nucleus of the cell. Dynein is more complicated in its structure and function than other motors. Experiments have found that, unlike other motors, dynein can take different size steps along microtubules depending on load and ATP concentration. We use Monte Carlo simulations to model the molecular motor function of cytoplasmic dynein at the single molecule level. The theory relates dynein's enzymatic properties to its mechanical force production. Our simulations reproduce the main features of recent single molecule experiments. We make testable predictions that should guide future experiments related to dynein function. [Preview Abstract] |
Thursday, March 18, 2010 9:12AM - 9:48AM |
V7.00003: Molecular approach to intracellular cargo transport Invited Speaker: Landmark discoveries in the study of cytoplasmic motors have been made through advances in single molecule biophysics and detailed mechanistic models exist for kinesin and dynein. However, the function of motors in physiological conditions has not been carefully tested. In cells, more than few dyneins can attach to the same cargo and interact with the opposite polarity motors of kinesin. To study the molecular crosstalk between the motors, we have used intraflagellar transport (IFT) in \textit{Chlamydomonas reinhardtii} as a model system. Ultrahigh spatio-temporal tracking of single cargo movement showed that IFT particles move for long distances unidirectionally with 8 nm increments, agreeing with measured step sizes of kinesin and dynein. To measure how many motors transport each cargo, we have linked large polystyrene beads to internal IFT particles through a transmembrane protein. Force measurements indicated that, on average, 3-4 motors transport cargoes in each direction. The results showed that IFT motors are tightly coordinated and might be involved in recycling each other to the appropriate end of the flagellum. [Preview Abstract] |
Thursday, March 18, 2010 9:48AM - 10:24AM |
V7.00004: Steps, shot noise and diffusion in the bacterial flagellar motor Invited Speaker: Many bacteria like {\em E. coli} swim by virtue of small rotary motors that drive rotation of helical flagella. Each motor is powered by a transmembrane proton flux passing through the motor. This flux is converted into torque with near-perfect efficiency by a mechanism whose details remain largely unknown. First I will describe the important biophysical properties of the motor, as measured in experiments, including the recent observation of a stepping behaviour at low speeds. I will then present a simple physical model that allows us to explain most of these data, but also to make new predictions. In particular, I will show how steps can be interpreted as barrier-crossing events in a corrugated energy landscape. Then I will show how to use our model to calculate the effect of shot noise (due to the discrete nature of the energy source--the protons) on motor diffusivity, and thus propose experiments to measure the proton cooperativity in the torque generation process. [Preview Abstract] |
Thursday, March 18, 2010 10:24AM - 11:00AM |
V7.00005: Bacterial chromosomal segregation by ParM polymerization Invited Speaker: |
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