Membrane binding properties of the cytoskeletal protein bactofilin

Bactofilins are a widespread family of cytoskeletal proteins that are essential for bacterial morphogenesis, chromosome organization, and motility. They assemble into non-polar filaments independently of nucleotides and typically associate with the cytoplasmic membrane. Their membrane interaction is thought to involve a short N-terminal peptide, but the underlying mechanism is unclear. Here, we clarify the complete membrane-targeting sequence (MTS) of the Caulobacter crescentus bactofilin BacA and identify residues critical for its function. Using molecular dynamics simulations, we show that its affinity for membranes arises from hydrophobic residue-driven water exclusion and electrostatic interactions with negatively charged phospholipid headgroups. Bioinformatic analysis suggests that this mode of membrane binding is conserved across diverse bacterial phyla. Importantly, we observe that BacA polymerization and membrane binding stimulate each other, and both of these processes are necessary for recruiting the membrane-bound client protein PbpC, a cell wall synthase that interacts with BacA via its N-terminal cytoplasmic region. PbpC can functionally replace the MTS of BacA when overproduced, demonstrating that client proteins contribute to the bactofilin-membrane association. Thus, bactofilin assembly and localization are determined by a complex interplay of different factors, thereby enabling the adaptation of these processes to the needs of the systems they control.