Molecular mechanism of cell curvature in the spiral-shaped bacterium Rhodospirillum rubrum

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Bacteria show a high diversity of cell shapes, which in most cases are determined by the peptidoglycan cell wall. Previous work has revealed that the prototypical rod-shaped morphologies of standard model organisms are generated by two major peptidoglycan biosynthetic complexes, the elongasome and the divisome. In these complexes, synthetic, lytic and regulatory proteins are combined to coordinately remodel the peptidoglycan layer in order to generate the cylindrical and spherical elements of the cell, respectively. However, the mechanisms underlying the establishment of more complex morphologies are still poorly understood. Widespread variations of the common rod shape are curved or spiral morphologies, which arise by differential growth of the rod-shaped peptidoglyan layer at the inner and outer curvature. Curved shapes are commonly found among bacteria, and it appears that the underlying shape-generating mechanisms differ fundamentally between different lineages. However, even for well-investigated systems, the precise modes of action of the shape determinants identified are still unclear. Here, we propose to study the establishment of cell curvature in the spiral-shaped bacterium Rhodospirillum rubrum. In preliminary work, we have identified a novel peptidoglycan-binding outer-membrane lipoprotein, PapS, which forms a filament-like structure at the outer curvature of the R. rubrum cell and is essential for its spiral morphology. PapS is highly conserved among the members of the Rhodospirillaceae, a family comprising a large number of curved and spiral-shaped species, suggesting that it represents a key cell shape determinant in this lineage. We propose to unravel the function of PapS using a combination of cell biological, genetic, biochemical and biophysical approaches. These studies will provide insight into a novel and widespread pathway of shape determination in bacteria and thus significantly further our understanding of bacterial cell wall biosynthesis and morphogenesis.