The bacterial actin-like cell division protein FtsA forms curved antiparallel double filaments upon binding of FtsN

Cell growth and division of walled bacteria depend on the synthesis and remodelling of peptidoglycan (PG). These activities are carried out by two multiprotein complexes, the elongasome and the divisome during cell elongation and division, respectively. Filaments of tubulin-like FtsZ form the cytoplasmic scaffold for divisome assembly, the Z-ring. In E. coli , the actin homologue FtsA anchors the Z-ring to the membrane and recruits downstream divisome components, including bitopic FtsN. FtsN is recruited late and activates the periplasmic PG synthase FtsWI. To start unravelling the activation mechanism involving FtsA and FtsN, we showed that E. coli FtsA forms antiparallel double filaments on lipid monolayers when also binding FtsN’s cytoplasmic tail, and that Vibrio maritimus FtsA crystallised as an equivalent double filament. We structurally located the FtsA-FtsN interaction site in FtsA’s IA-IC interdomain cleft and confirmed FtsA double filament formation in vivo using site-specific cysteine cross-linking. FtsA-FtsN double filaments reconstituted on and in liposomes preferred negative Gaussian curvature, as was previously shown for the elongasome’s actin, MreB. MreB filaments serve as curvature-sensing “rudders”, orienting insertion of PG around the cell’s circumference. We propose that curved antiparallel FtsA double filaments function similarly in the divisome: FtsA filaments, together with dynamic FtsZ filaments orient and concentrate cell-constricting septal PG synthesis in the division plane. ### Competing Interest Statement The authors have declared no competing interest.