Control of septum thickness by a large protein ring

Cell division in bacteria is initiated by polymerization of the tubulin homologue FtsZ at midcell. In Gram-positive bacteria FtsZ polymers are anchored to the cell membrane by the protein SepF. Purified Bacillus subtilis SepF polymerizes into large rings with an inner diameter of ~40 nm. The biological function of this ring formation is unclear. In this study we have isolated SepF homologues from five different bacterial species and showed that the ability to form rings is conserved. In addition, we found a correlation between the diameter of SepF rings and the thickness of cell division septa. By constructing SepF chimeras, we were able to show that the conserved core domain determines both SepF ring diameter and septum thickness. Our results support a model in which SepF polymers form arc-shaped molecular clamps on the leading edge of nascent septa, thereby determining septal thickness. This suggests that the intrinsic form of a protein polymer can function as a mold to shape the cell wall.