The bacterial actin homologue MreB forms membrane-associated filaments and is integral for determining rod cell shape, which is suggested to be due to its role in assembling the enzymes involved in cell wall synthesis. However, the factors that determine MreB membrane localization and its interactions with the cell wall synthesis enzymes are poorly understood. Schirner et al. used a combination of small-molecule inhibitors and gene deletions to show that MreB membrane localization depends on the presence of the peptidoglycan precursor lipid II in Bacillus subtilis. Treatment of B. subtilis with cell wall synthesis inhibitors resulted in the disassembly of MreB filaments and the distribution of MreB throughout the cell. The authors propose that MreB binds to intracellular lipid II, which enables filament formation and the assembly of the cell wall synthesis machinery. Thus, these findings suggest a mechanistic link between the metabolic state of the cell (that is, the availability of peptidoglycan precursors) and cell growth.