ATP-driven membrane binding and polymerization of bacterial actin MreB promotes local membrane fluidization

The bacterial actin homolog MreB plays a key role in rod cell shape determination. We recently showed that MreB from the Gram-positive bacterium Geobacillus stearothermophilus (MreB^Gs) polymerizes into straight pairs of protofilaments in the presence of both ATP and a lipid surface. Membrane interaction is thought to be mediated by electrostatic interactions with anionic lipids, with final anchoring relying on two spatially close hydrophobic motifs that protrude from the MreB^Gs monomers, forming a putative membrane-insertion domain. Here, we determined the binding properties of ATP and ADP to MreB^Gs using fluorescence anisotropy and monitored ATP-mediated binding and polymer formation on lipid bilayers using liposome-binding assays and atomic force microscopy, respectively. Finally, we used solid-state NMR to visualize the interaction between the membrane and MreB^Gs at the atomic level. Our findings reveal that divalent cations are required for nucleotide binding and that, unlike eukaryotic actin, MreB^Gs has similar affinity for both ATP and ADP. We also show that monomeric MreB^Gs establishes peripheral contacts with the membrane likely through electrostatic interactions, while Mg⋅ATP-induced MreB^Gs filaments insert into the lipid bilayer without interfering with the membrane lamellar phase and have a significant local fluidifying effect.