AimB is a small protein regulator of cell size and MreB assembly

The MreB actin-like cytoskeleton assembles into dynamic polymers that coordinate cell shape in many bacteria. In contrast to most other cytoskeletons, few MreB interacting proteins have been well characterized. Here we identify a small protein from Caulobacter crescentus, AimB, as an Assembly Inhibitor of MreB. AimB overexpression mimics inhibition of MreB polymerization, leading to increased cell width and MreB delocalization. Molecular dynamics simulations suggest that AimB binds MreB at its monomer-monomer protofilament interaction cleft. We validate this model through functional analysis of point mutants in both AimB and MreB, photo-crosslinking studies with site-specific unnatural amino acids, and species-specific activity of AimB. Together, our findings indicate that AimB promotes MreB dynamics by inhibiting monomer-monomer assembly interactions, representing a new mechanism for regulating actin-like polymers and the first identification of a non-toxin MreB assembly inhibitor.