Adhesion controls bacterial actin polymerization-based movement

Abstract

As part of its infectious life cycle, the bacterial pathogen Listeria monocytogenes propels itself through the host-cell cytoplasm by triggering the polymerization of host-cell actin near the bacterial surface, harnessing the activity of several cytoskeletal proteins used during actin-based cell crawling. To distinguish among several classes of biophysical models of actin-based bacterial movement, we used a high-throughput tracking technique to record the movement of many individual bacteria during temperature shifts. The speed of each bacterium varied strongly with temperature, closely following the Arrhenius rate law. Among bacteria, the prefactor A of the Arrhenius dependence unexpectedly varied exponentially with apparent activation energy, E a, over a wide range (8 -21 kcal͞mol), reminiscent of the ''rate compensation effect'' of classical catalytic reactions. Average E a were increased for mutant bacteria deficient in binding Ena͞VASP proteins and bacteria moving in diluted extract. These two effects were additive. The observed temperature and rate compensation effects are consistent with a class of simple kinetic models in which the bacterium advances through the thermally driven, cooperative breakage of groups of adhesive bonds on its surface. The estimated number of coupled adhesive bonds N on the bacterial surface varies between 10 and 40 bonds. In contrast to other models, this model correctly predicts an experimentally observed negative correlation between bacterial speed and actin gel density. The idea that speed depends on adhesion, rather than polymerization, suggests several alternative mechanisms by which known cytoskeletal regulatory proteins could control cellular movement.ActA ͉ Listeria ͉ motility M any types of eukaryotic cells, including immune system lymphocytes, neurons, and cancer cells, use the forces generated by polymerizing actin filament networks to change shape and to move. In the actin polymerization-based movement of intracellular bacterial pathogens such as Listeria monocytogenes, individual bacteria harness the same cytoskeletal machinery to move within the host-cell cytoplasm (1).Several classes of biophysical models have been proposed to explain the mechanism of actin polymerization-based movement. The models differ in which steps in the motility cycle are proposed to control the rate of bacterial movement.The ''tethered elastic Brownian ratchet'' model (2) proposes that a large force generated by the network of polymerizing actin filaments in the ''comet tail'' behind the bacterium continuously induces the breakage of individual adhesive bonds between the bacterium and the surrounding actin gel, pushing the bacterium forward. In this model, bacteria move at the speed at which propulsive and adhesive forces are balanced; this equilibrium velocity depends strongly on the specific properties of actin polymerization and adhesion. The related ''elastic gel '' model (3) proposes that the elastic expansion of the actin gel behind the bacterium also contributes to propulsion, ''s...

Force amplification response of actin filaments under confined compression

“…The ability of actin filaments to alter their elastic properties under compression constitutes a mechanism of actin-based mechanotransduction that arises from the intrinsic physicochemical properties of the intermonomer bonds, which enables filaments to both sense and respond directly to external forces without the assistance of ancillary mediating proteins or chemical signals. This previously unknown stiffening response of individual actin filaments may shed light on a number of reported phenomena such as (i) the apparent acceleration in growth velocity observed in actin networks that were previously subjected to large compressive loads (20) and (ii) the counterintuitive adhesion-controlled propulsion of actin (21). In addition, the compression-induced stiffening response of actin may potentially play a critical function in a number of cellular processes that govern cell locomotion, phagocytosis, and cytoskeletal rearrangements triggered by external forces.…”

A kinematic description of the trajectories of Listeria monocytogenes propelled by actin comet tails

“…2, respectively. In general, the propulsive force generated by the filaments is balanced either by the viscous drag from the surrounding fluid or by the frictional/adhesive force due to breaking of the bonds that tether some of the actin filaments to the surface of the bacterium (8,10,16). However, for bacteria moving in the cytoplasmic extract with a viscosity of Ϸ0.01 Pa⅐s, the drag force is Ϸ10 fN, which is much smaller than the forces that tether the filament network to the bacterial surface, which are in the range of few hundred piconewtons to a few nanonewtons (8,(11)(12)(13).…”

Curvature and torsion in growing actin networks

“…Several biophysical models have been developed to describe the connection between network growth and the generation of force [9][10][11][12]. Despite the extensive theoretical and experimental investigation of polymerization forces and velocities [13,14], less is understood about the changes in direction of network growth.…”