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

GpsB interacts with FtsZ in multiple species and may serve as an accessory Z-ring anchor

Bacterial cytokinesis commences when a tubulin-like GTPase, FtsZ, forms a Z-ring to mark the division site. Synchronized movement of Z-ring filaments and peptidoglycan synthesis along the axis of division generates a division septum to separate the daughter cells. Thus, FtsZ needs to be linked to the peptidoglycan synthesis machinery. GpsB is a highly conserved protein among species of the Firmicutes phylum known to regulate peptidoglycan synthesis. Previously, we showed that Staphylococcus aureus GpsB directly binds to FtsZ by recognizing a signature sequence in its C-terminal tail (CTT) region. As the GpsB recognition sequence is also present in Bacillus subtilis, we speculated that GpsB may interact with FtsZ in this organism. Earlier reports revealed that disruption of gpsB and ftsA or gpsB and ezrA is deleterious. Given that both FtsA and EzrA also target the CTT of FtsZ for interaction, we hypothesized that in the absence of other FtsZ partners, GpsB-FtsZ interaction may become apparent. Our data confirm that is the case, and reveal that GpsB interacts with FtsZ in multiple species and stimulates the GTPase activity of the latter. Moreover, it appears that GpsB may serve as an accessory Z-ring anchor such as when FtsA, one of the main anchors, is absent.

Tubules, Rods, and Spirals: Diverse Modes of SepF-FtsZ Assembling

Z-ring formation by FtsZ, the master assembler of the divisome, is a key step in bacterial cell division. Membrane anchoring of the Z-ring requires the assistance of dedicated Z-ring binding proteins, such as SepF and FtsA. SepF participates in bundling and membrane anchoring of FtsZ in gram-positive bacteria. We report in vitro biophysical studies of the interactions between FtsZ and a cytoplasmic component of cognate SepF from three different bacteria: Mycobacterium tuberculosis, Staphylococcus aureus, and Enterococcus gallinarum. While the cytosolic domain of SepF from M. tuberculosis is primarily a dimer, those from S. aureus and E. gallinarum polymerize to form ring-like structures. Mycobacterial SepF helps in the bundling of FtsZ filaments to form thick filaments and large spirals. On the other hand, ring-forming SepF from the Firmicutes bundle FtsZ into tubules. Our results suggest that the oligomeric form of SepF directs how it bundles FtsZ filaments.

Molecular basis for curvature formation in SepF polymerization

The self-assembly of proteins into curved structures plays an important role in many cellular processes. One good example of this phenomenon is observed in the septum-forming protein (SepF), which forms polymerized structures with uniform curvatures. SepF is essential for regulating the thickness of the septum during bacteria cell division. In Bacillus subtilis, SepF polymerization involves two distinct interfaces, the β-β and α-α interfaces, which define the assembly unit and contact interfaces, respectively. However, the mechanism of curvature formation in this step is not yet fully understood. In this study, we employed solid-state NMR (SSNMR) to compare the structures of cyclic wild-type SepF assemblies with linear assemblies resulting from a mutation of G137 on the β-β interface. Our results demonstrate that while the sequence differences arise from the internal assembly unit, the dramatic changes in the shape of the assemblies depend on the α-α interface between the units. We further provide atomic-level insights into how the angular variation of the α2 helix on the α-α interface affects the curvature of the assemblies, using a combination of SSNMR, cryo-electron microscopy, and simulation methods. Our findings shed light on the shape control of protein assemblies and emphasize the importance of interhelical contacts in retaining curvature.