Molecular mechanism of cell curvature in the spiral-shaped bacterium Rhodospirillum rubrum
Final Report Abstract
Bacteria come is a variety of different shapes, which increase their fitness in the environmental niches they inhabit. Apart from the typical rod and spherical morphologies, spiral shapes are widespread in many bacterial lineages, but the mechanisms underlying their establishment are still incompletely understood. In this project, we have identified a novel and conserved system mediating cell curvature in the spiral-shaped bacterium Rhodospirillum rubrum and likely many of its relatives. We show that, in this species, the outer-membrane porins Por39/41 form a helical ribbon-like assembly in the outer curve of the cell that recruits a cell wall-binding outermembrane lipoprotein (PapS). We demonstrate that these porin-PapS assemblies entrap the cell wall biosynthetic complexes mediating lateral cell growth, thereby inducing a relative increase in the amount of cell wall incorporated in their vicinity compared to other cell wall regions, thus distorting the cell cylinder into a helical shape. These findings reveal a striking degree of organization in the outer membrane and demonstrate a so-far unprecedented regulatory effect of outer membrane proteins on the dynamics of the cell wall biosynthetic machinery in the inner membrane, which reverses the typical inside-to-outside hierarchy of regulatory processes in cell shape determination. Interestingly, we found that the activity of this primary curvature-inducing system is counteracted by a second morphogenetic module composed of the cytoskeletal bactofilin and the cell wall hydrolase LmdC. Our results show that bactofilin- LmdC complexes localize to the inner curve of the cell, where they stimulate cell wall growth, thereby reducing the overall degree of cell curvature. Bioinformatic studies revealed that the bactofilin-LmdC module is conserved in a large number of bacterial species. To further investigate its morphogenetic role, we additionally studied its function in the stalked budding bacterium Hyphomonas neptunium, a relative of R. rubrum with a very different kind of cell shape. We found that bactofilin-LmdC assemblies have a key morphogenetic role in this species, with their absence leading to severe cell shape defects or amorphous growth. Our results suggest that these assemblies may relax the cell wall to facilitate the introduction of high cell curvature and, in addition, serve as diffusion barriers that limit the cell wall biosynthetic machinery to specific growth zones. Finally, our work identified an additional component of the PapS system (CsgP) that is critical for its function in R. rubrum but not in more distantly related species. Moreover, we identified a third protein (CcfM) that curved cells independently of PapS when overproduced. Together, our findings demonstrate that the curved cell shape of R. rubrum is established by the interplay of three different systems, which may allow the dynamic adjustment of its curvature to changes in its environment.
Publications
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A dynamic bactofilin cytoskeleton cooperates with an M23 endopeptidase to control bacterial morphogenesis. eLife, 12.
Pöhl, Sebastian; Osorio-Valeriano, Manuel; Cserti, Emöke; Harberding, Jannik; Hernandez-Tamayo, Rogelio; Biboy, Jacob; Sobetzko, Patrick; Vollmer, Waldemar; Graumann, Peter L. & Thanbichler, Martin
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An outer membrane porin-lipoprotein complex modulates elongasome movement to establish cell curvature in Rhodospirillum rubrum. Nature Communications, 15(1).
Pöhl, Sebastian; Giacomelli, Giacomo; Meyer, Fabian M.; Kleeberg, Volker; Cohen, Eli J.; Biboy, Jacob; Rosum, Julia; Glatter, Timo; Vollmer, Waldemar; van Teeseling, Muriel C. F.; Heider, Johann; Bramkamp, Marc & Thanbichler, Martin
