Control of septum thickness by the curvature of SepF polymers

Abstract

Gram-positive bacteria divide by forming a thick cross wall. How the thickness of this septal wall is controlled is unknown. In this type of bacteria, the key cell division protein FtsZ is anchored to the cell membrane by two proteins, FtsA and/or SepF. We have isolated SepF homologs from different bacterial species and found that they all polymerize into large protein rings with diameters varying from 19 to 44 nm. Interestingly, these values correlated well with the thickness of their septa. To test whether ring diameter determines septal thickness, we tried to construct different SepF chimeras with the purpose to manipulate the diameter of the SepF protein ring. This was indeed possible and confirmed that the conserved core domain of SepF regulates ring diameter. Importantly, when SepF chimeras with different diameters were expressed in the bacterial hostBacillus subtilis, the thickness of its septa changed accordingly. These results strongly support a model in which septal thickness is controlled by curved molecular clamps formed by SepF polymers attached to the leading edge of nascent septa. This also implies that the intrinsic shape of a protein polymer can function as a mold to shape the cell wall.

FtsZ-Ring Regulation and Cell Division Are Mediated by Essential EzrA and Accessory Proteins ZapA and ZapJ in Streptococcus pneumoniae

“…FtsA and SepF have been characterized previously in Spn ( Mura et al, 2016 ; Perez et al, 2019 ). FtsA is essential and always co-localizes with FtsZ in Spn , whereas Δ sepF deletion mutants form elongated, widened cells lacking septal constrictions, suggesting that SepF polymers mediate FtsZ-ring closure during septation, possibly by acting as a curved clamp at the leading edge of the closing septum ( Wenzel et al, 2021 ). Despite these phenotypes, Δ sepF mutants grow, similarly, to WT in BHI broth, and the cell morphology defects of Δ sepF mutants are largely reversed by FtsA overexpression ( Mura et al, 2016 ).…”

Structural Insights into the Interaction between Bacillus subtilis SepF Assembly and FtsZ by Solid-State NMR Spectroscopy

“…A previous study suggested that septal thickness is controlled by curved molecular clamps formed by SepF polymers. The stacking of SepF polymers could be meaningful in septa on the longitudinal direction …”

“…Similarly, the depletion of SepF in Haloferax volcanii or Mycobacterium tuberculosis results in the formation of large cells or defective growth. Further studies indicated an additional function of SepF in Bacillus subtilis to shape the cell wall due to its intrinsic property of forming ring structures . In M. tuberculosis , SepF could also function as a link between cell division and peptidoglycan synthesis as a consequence of the interaction with MurG …”

Integrative illustration of a JCVI-syn3A minimal cell

“…In order to highlight the shape of the cleavage furrow, the ftsZ filaments are depicted perpendicular to the plane of the page. Recent study of sepF indicates that it forms rings with the membrane-interacting surface on the inner face [ 14 ], so it is depicted as forming a C-shaped assembly cupping the membrane. Uncharacterized protein JCVISYN3A_0239 was predicted to have a single transmembrane segment and several spectrin repeats, and was included in a speculative interaction with ftsZ/ftsA similar to EzrA [ 15 ].…”