Session F43: Focus Session: Motor dynamics---from Single Molecules to Cells I

Molecular Motors from DNA

DNA is a wonderful material for nanoscale construction: its self-assembly can be programmed by making use of its information-carrying capability and its hybridization or hydrolysis can be used as to provide energy for synthetic molecular machinery. With DNA it is possible to design and build three-dimensional scaffolds, to attach molecular components to them with sub-nanometre precision--and then to make them move. I shall describe our work on autonomous, biomimetic molecular motors powered by chemical fuels and the use of synthetic molecular machinery to control covalent chemical synthesis. I shall demonstrate bipedal motors whose operation depends on the coordination of the chemomechanical cycles of two separate catalytic centres and burnt bridges motors that can be programmed to navigate networks of tracks. I shall also discuss the use of kinesin motor proteins to power synthetic devices.   

Remote control of molecular motors using light-activated gearshifting

Engineering molecular motors with dynamically controllable properties will allow selective perturbation of mechanical processes in vivo and provide sophisticated components for directed nanoscale transport in vitro. We previously constructed myosin motors that respond to a change in [Ca++] by reversing their direction of motion along the polarized actin filament [1]. To expand the potential applications of controllable molecular motors, we have now developed myosins that shift gears in response to blue light illumination. Light is a versatile control signal that can be readily modulated in time and space, and is generally orthogonal to cellular signaling. Using structure-guided protein engineering, we have incorporated LOV photoreceptor domains into the lever arms of chimeric myosins, resulting in motors that robustly speed up, slow down, or switch directions upon illumination. These genetically encoded motors should be directly deployable inside living cells. Our successful designs include constructs based on two different myosin classes, and we show that optical velocity control can be implemented in motors that move at microns/sec speeds, enabling practical biological and bioengineering applications.\\[4pt] [1] Chen, L., Nakamura, M., Schindler,T.D., and Bryant Z. (2012). Nat. Nanotechnol. 7, 252-6.   

Anomalous dynamics in intracellular transport

This talk will describe quantitative analyses of particle tracking data for systems with cytoskeletally associated molecular motors to better understand the motions contributing to intracellular transport and, more generally, means for characterizing systems far from equilibrium. In particular, we have studied the motions of insulin-containing vesicles (granules) in a pancreatic beta cell line. We find subdiffusive behavior with correlations in both space and time. These data can be modeled by subordinating an ergodic random walk process to a non-ergodic one. We relate the dynamics to the underlying microtubule structure by imaging in the presence of the drug vinblastine. Our results provide a simple physical mechanism for how diverse pools of insulin granules and, in turn, biphasic secretion could arise. Time permitting, these dynamics will be compared with those of actomyosin assemblies.   

Morphogenetic Functions of Actomyosin

Morphogenesis refers to the generation of form in Biology. Much is known about molecular mechanisms of regulation, but little is known about the physical mechanisms by which an unpatterned blob of cells develops into a fully structured and formed organism. The actomyosin cortex is a thin layer underneath the cellular membrane that can self contract, which drives many of the large-scale morphogenetic rearrangements that are observed during development. How this cortex reshapes and deforms, and how such morphogenetic processes couple to regulatory biochemical pathways is largely unknown. I will discuss two emergent physical activities of the actomyosin cytoskeleton, an active contractile tension and an active torque, both of which can serve to drive flows and large-scale chiral rotations of the actomyosin cytoskeleton. Discussing two biological examples, polarization of the Caenorhabditis elegans zygote and epiboly during zebrafish gastrulation, I will illustrate how active tension drive flows, how molecular constituents of the cortex affect flows, and how morphogenetic patterns can be formed by coupling regulatory biochemistry to active cortical mechanics. A particular focus will be the investigation of how active chiral torques drive chiral flow, and the resulting functions of such chiral activities of the actomyosin cytoskeleton for left-right symmetry breaking in development.   

Force-balance model of suppression of multipolar division in cancer cells with extra centrosomes

Cancer cells often possess extra centrosomes which have the potential to cause cell death due to catastrophic multipolar division. Many cancer cells, however, are able to escape multipolar mitosis by clustering the extra centrosomes to form bipolar spindles. The mechanism of centrosome clustering is therefore of great interest to the development of anti-cancer drugs because the de-clustering of extra centrosomes provides an appealing way to eliminate cancer cells while keeping healthy cells intact. We present a physical model assuming 1) dynamic centrosomal microtubules interact with chromosomes by both pushing on chromosome arms and pulling along kinetochores; 2) these microtubules interact with force generators associated with actin/adhesion structures at the cell boundary; and 3) motors act on anti-parallel microtubules from different centrosomes. We find via computer simulations that chromosomes tend to aggregate near the cell center while centrosomes can be either clustered to form bipolar spindles or scattered to form multipolar spindles, depending on the strengths of relative forces, cell shape and adhesion geometry. The model predictions agree with data from cells plated on adhesive micropatterns and from biochemically or genetically perturbed cells. Furthermore, our model is able to explain various microtubule distributions in interphase cells on patterned substrates.