Molecular Mechanism of the SepF N‑Terminal Conformational Switch in Ring Assembly and Membrane Anchoring.

Septum-forming protein (SepF) self-assembles into ring-shaped structures during bacterial cell division, acting as a bridge that anchors the Z-ring to the cell membrane and thereby facilitates membrane remodeling and constriction. In this study, we elucidate how salt ions regulate the ring assembly of SepF and its membrane anchoring using solid-state NMR (ssNMR) spectroscopy in combination with biochemical methods. We show that the N-terminal fragment of SepF (residues 1-12) is highly dynamic and disordered under salt-free conditions, which inhibits SepF ring assembly. In the presence of NaCl, this N-terminal fragment of SepF undergoes a conformational transition to a less flexible and more ordered state, enabling ring assembly and enhancing its membrane anchoring. In contrast, a SepF mutant that assembles into noncurved fibrils exhibits impaired membrane recruitment capability, highlighting the crucial role of assembly curvature in membrane remodeling. Based on these findings, we propose a mechanistic model in which the coordination of ring assembly and membrane anchoring is critical for membrane remodeling, with NaCl-dependent modulation of the N-terminal fragment playing a central role. Our work provides atomic-level insights into how SepF responds to environmental factors to regulate its conformation and assembly curvature, advancing our understanding of the functional regulation of supramolecular machinery in bacterial cell division.