Actin-like MreB proteins: from single molecules to cell morphogenesis
How cells control their shape is a long-standing question in cell biology. In bacteria, the external peptidoglycan cell wall -one of the most prominent targets for antibiotics- is the primary determinant of cell shape. Many rod-shaped bacteria elongate their sidewalls by the action of cell wall synthesizing machineries that are associated to circumferentially-moving nanofilaments of MreB, the structural homolog of eukaryotic actin. The prevailing model is that actin-like MreB assemblies are used as platforms that restrict the diffusion and orient the motion of peptidoglycan-synthesizing enzymes in the membrane, thereby patterning new cell wall insertion and controlling cylindrical expansion. We use cutting-edge fluorescence microscopy techniques combined with automated single particle and single molecule tracking to monitor the dynamics of MreB isoforms, as a proxy for cell wall synthesis, in live cells of the model Gram-positive bacterium Bacillus subtilis. We also use super-resolution techniques to elucidate the nanostructure and orientation of MreB cortical assemblies in the cell. In parallel, we are using biochemical and single filament imaging approaches to investigate the polymerization properties of recombinant MreB of B. subtilis in vitro, to fill in the gap between structure and cellular function. Based on our results, we have recently proposed a model for the mechanism of MreB polymerization, which displays some actin-like and some unexpected actin-unlike properties.