The bacterium Listeria monocytogenes uses the energy of the actin polymerization to propel itself through infected tissues. In steady state, it continuously adds new polymerized filaments to its surface, pushing on its tail, which is made from previously cross-linked actin filaments. In this paper we introduce an elastic model to describe how the addition of actin filaments to the tail results in the propulsive force on the bacterium. Filament growth on the bacterial surface produces stresses that are relieved at the back of the bacterium as it moves forward. The model leads to a natural competition between growth from the sides and growth from the back of the bacterium, with different velocities and strengths for each. This competition can lead to the periodic motion observed in a Listeria mutant.

We report biophysical experiments performed on the bacterium Listeria monocytogenes, a model system to study actin-based motility. Using optical tweezers and electrophoresis experiments, we find that the bacterium is firmly attached to its tail, and we demonstrate that the tail responds as an elastic gel when deformed. We have measured its elastic modulus at a value of 10(3)-10(4) Pa, which is 10 times higher than the rigidity of the eukaryotic cytoplasm. These results demonstrate that the bacterium and its tail form a very robust system, consistent with the steadyness of the motion observed in vivo. We propose an elastic model for the propulsion mechanism which takes into account the connection and thus the interaction between the actin filaments. It provides a generic description of the various aspects of actin-tail based movements.

It has been proposed that microtubules (MTs) participate in skeletal muscle cell differentiation. However, it is still unclear how this happens. To examine whether MTs could participate directly in the organization of thick and thin filaments into sarcomeres, we observed the concomitant reorganization and dynamics of MTs with the behavior of sarcomeric actin and myosin by time-lapse confocal microscopy. Using green fluorescent protein (GFP)-EB1 protein to label MT plus ends, we determined that MTs become organized into antiparallel arrays along fusing myotubes. Their dynamics and orientation was found to be different across the thickness of the myotubes. We observed fast movements of Dsred-myosin along GFP-MTs. Comparison of GFP-EB1 and Dsred-myosin dynamics revealed that myosin moved toward MT plus ends. Immuno-electron microscopy experiments confirmed that myosin was actually associated with MTs in myotubes. Finally, we confirmed that MTs were required for the stabilization of myosin-containing elements prior to incorporation into mature sarcomeres. Collectively, our results strongly suggest that MTs become organized into a scaffold that provides directional cues for the positioning and organization of myosin filaments during sarcomere formation.