Despite years of intensive research, much remains to be discovered to understand the regulatory networks coordinating bacterial cell growth and division. The mechanisms by which Streptococcus pneumoniae achieves its characteristic ellipsoid-cell shape remain largely unknown. In this study, we analyzed the interplay of the cell division paralogs DivIVA and GpsB with the ser/thr kinase StkP. We observed that the deletion of divIVA hindered cell elongation and resulted in cell shortening and rounding. By contrast, the absence of GpsB resulted in hampered cell division and triggered cell elongation. Remarkably, ΔgpsB elongated cells exhibited a helical FtsZ pattern instead of a Z-ring, accompanied by helical patterns for DivIVA and peptidoglycan synthesis. Strikingly, divIVA deletion suppressed the elongated phenotype of ΔgpsB cells. These data suggest that DivIVA promotes cell elongation and that GpsB counteracts it. Analysis of protein-protein interactions revealed that GpsB and DivIVA do not interact with FtsZ but with the cell division protein EzrA, which itself interacts with FtsZ. In addition, GpsB interacts directly with DivIVA. These results are consistent with DivIVA and GpsB acting as a molecular switch to orchestrate peripheral and septal PG synthesis and connecting them with the Z-ring via EzrA. The cellular co-localization of the transpeptidases PBP2x and PBP2b as well as the lipid-flippases FtsW and RodA in ΔgpsB cells further suggest the existence of a single large PG assembly complex. Finally, we show that GpsB is required for septal localization and kinase activity of StkP, and therefore for StkP-dependent phosphorylation of DivIVA. Altogether, we propose that the StkP/DivIVA/GpsB triad finely tunes the two modes of peptidoglycan (peripheral and septal) synthesis responsible for the pneumococcal ellipsoid cell shape.

SummaryEukaryotic-like serine/threonine-kinases are involved in the regulation of a variety of physiological processes in bacteria. In Streptococcus pneumoniae, deletion of the single serine/threonine-kinase gene stkP results in an aberrant cell morphology suggesting that StkP participates in pneumococcus cell division. To understand the function of StkP, we have engineered various pneumococcus strains expressing truncated or kinase-dead forms of StkP. We show that StkP kinase activity, but also its extracellular and cytoplasmic domains per se, are required for pneumococcus cell division. Indeed, we observe that mutant cells show round or elongated shapes with nonfunctional septa and a chain phenotype, delocalized sites of peptidoglycan synthesis and diffused membrane StkP localization. To gain understanding of the underlying StkP-mediated regulatory mechanism, we show that StkP specifically phosphorylates in vivo the cell division protein DivIVA on threonine 201. Pneumococcus cells expressing non-phosphorylatable DivIVA-T201A possess an elongated shape with a polar bulge and aberrant spatial organization of nascent peptidoglycan. This brings the first evidence of the importance of StkP in relationship to the phosphorylation of one of its substrates in cell division. It is concluded that StkP is a multifunctional protein that plays crucial functions in pneumococcus cell shape and division.

Background:The YvcK protein is essential for Bacillus subtilis growth on gluconeogenic conditions; its overproduction rescues an mreB mutant. Results: PrkC phosphorylates YvcK; this phosphorylation is not required for growth on gluconeogenic conditions but is necessary for mreB rescue. Conclusion: YvcK phosphorylation is specifically involved in B. subtilis morphogenesis. Significance: This phosphorylation-based regulatory mechanism could be widespread in bacteria.