Effects of Polymerization and Nucleotide Identity on the Conformational Dynamics of the Bacterial Actin Homolog MreB

The assembly of cytoskeletal proteins underpins many cellular processes, from growth and division in single cells to muscle contraction in animals. In eukaryotes, dynamic actin networks maintain cell shape and drive motility, and the bulk properties of the actin network are regulated through the conformational changes of individual subunits. In bacteria, the actin structural homolog MreB forms filaments colocalized with the cell-wall synthesis machinery to regulate rod-shaped growth and contribute to cellular stiffness through unknown mechanisms. Like actin, MreB polymerizes in the presence of ATP, and polymerization promotes nucleotide hydrolysis. However, it is unclear if other similarities exist between MreB and actin since the two proteins share low sequence identity and have differentiated cellular roles. Here, we use all-atom molecular dynamics simulations to reveal surprising parallels between the structural dynamics of MreB and actin. We observe that MreB exhibits actin-like polymerization-dependent structural changes, wherein polymerization induces flattening of the MreB subunits, which restructures the nucleotide-binding pocket to favor hydrolysis. The MreB filament bending is nucleotide-dependent, with hydrolyzed polymers favoring a straighter conformation. We used steered simulations to demonstrate a coupling between intersubunit bending and the degree of flattening of each subunit, suggesting cooperative bending along a filament. Taken together, our results provide molecular-scale insight into the diversity of structural states of MreB and the relationships among polymerization, hydrolysis, and filament properties, which may be applicable to other members of the broad actin family.