Bactofilins form non-polar filaments that bind to membranes directly

Bactofilins are small beta-helical proteins that form cytoskeletal filaments in a range of bacteria. Bactofilins have diverse functions: filaments in Caulobacter crescentus are involved in cell stalk formation whereas Myxococcus xanthus filaments aid chromosome segregation and motility. The precise molecular architecture of bactofilin filaments has remained unclear. Here we revealed by sequence analyses and electron microscopy that in addition to wide distribution across bacteria and archaea, bactofilins are also present in a few eukaryotic cells such as oomycetes. The sole bactofilin from Thermus thermophilus was demonstrated to form constitutive filaments and cryo-EM analysis revealed that protofilaments formed through end-to-end association of the beta-helical domains. Using a nanobody against Thermus bactofilin we determined the near-atomic filament structure, showing that the filaments are non-polar, with subunits arranged head-to-head and tail-to-tail. A polymerisation-impaired mutant F105R, that disrupts one of the two protofilament interfaces, enabled crystallisation. The crystal structure also revealed non-polar protofilaments, and the dominance of the beta-stacking interface that formed despite the inhibiting mutation. To confirm the generality of the lack of polarity, we performed co-evolutionary analysis of a large set of sequences. Finally, using Thermus bactofilin, we determined that the N-terminal disordered tail of the protein is responsible for direct binding to lipid membranes both on liposomes and by electron cryotomography in E. coli cells. The tail is conserved, suggesting that membrane binding is likely a general feature of these very common but only recently discovered filaments of the prokaryotic cytoskeleton.