Methylomirabilis bacteria perform anaerobic methane oxidation coupled to nitrite reduction via an intra-aerobic pathway, producing carbon dioxide and dinitrogen gas as end products. These Gram-negative bacteria possess an atypical polygonal cell shape with sharp ridges that run along the cell length. Previously, a putative surface protein layer (S-layer) was observed as the outermost cell layer of these bacteria. We hypothesized that the Methylomirabilis S-layer is the determining factor for their polygonal cell shape. Here we tested this hypothesis by isolation and characterization of the Methylomirabilis S-layer. We identified the S-layer protein and characterized the S-layer structure via electron tomography and subtomogram averaging of negative-stained S-layer patches. Cryo-electron tomography showed that the S-layer consisted of multiple “tectonic” plates that partially overlapped each other, giving rise to the unique polygonal cell shape.
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