A genetic screen unveils the polymerization cycle of the bacterial actin-like MreB

MreB, a bacterial actin homologue that organizes cell wall synthesis in most rod-shaped bacteria, assembles into membrane-associated filaments through mechanisms that have remained elusive despite decades of study. In particular, how ATP binding, ATP hydrolysis and membrane association are coordinated during the MreB polymerization cycle has remained unknown. Here, we combine genetics with systematic biochemical characterizations of purified MreB variants to decipher the key molecular steps governing the MreB assembly cycle. A genetic screen in Bacillus subtilis identified essential residues controlling filament assembly and membrane association. Systematic biochemical analyses of purified homologous MreB variants from Geobacillus stearothermophilus demonstrated that monomer-monomer interactions are required for both ATP hydrolysis and high-affinity membrane binding, whereas ATP binding, but not ATP hydrolysis, is sufficient to promote polymerization. Conversely, ATP hydrolysis destabilizes MreB polymers, promoting filament turnover. Together, these results support a model in which ATP-binding induces an early nucleation step that increases membrane affinity, membrane association promotes filament assembly, and subsequent ATP hydrolysis completes the cycle by driving polymer disassembly and turnover. These findings provide a mechanistic basis for understanding how MreB polymerization dynamics may regulate the spatial and temporal organization of bacterial cell wall growth.