Many prokaryotic cells are encapsulated by a surface layer (S-layer) consisting of repeating units of S-layer proteins. S-layer proteins are a diverse class of molecules found in Gram-positive and Gram-negative bacteria and most archaea1–5. S-layers protect cells from the outside, provide mechanical stability and also play roles in pathogenicity. In situ structural information about this highly abundant class of proteins is scarce, so atomic details of how S-layers are arranged on the surface of cells have remained elusive. Here, using purified Caulobacter crescentus' sole S-layer protein RsaA, we obtained a 2.7 Å X-ray structure that shows the hexameric S-layer lattice. We also solved a 7.4 Å structure of the S-layer through electron cryotomography and sub-tomogram averaging of cell stalks. The X-ray structure was docked unambiguously into the electron cryotomography map, resulting in a pseudo-atomic-level description of the in vivo S-layer, which agrees completely with the atomic X-ray lattice model. The cellular S-layer atomic structure shows that the S-layer is porous, with a largest gap dimension of 27 Å, and is stabilized by multiple Ca2+ ions bound near the interfaces. This study spans different spatial scales from atoms to cells by combining X-ray crystallography with electron cryotomography and sub-nanometre-resolution sub-tomogram averaging.
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The authors thank M. Skehel and F. Begum (MRC Laboratory of Molecular Biology, LMB) for mass-spectrometric identification of proteins, M. Yu (LMB) for help with X-ray data collection, C. Savva (LMB) for help with cryo-EM data collection, S. Scheres (LMB) for help with RELION software, F. Schur and W. Wan (European Molecular Biology Laboratory, EMBL) for providing high-resolution image-processing code and scripts before publication and for advice on their implementation, F. van den Ent (LMB) for advice on protein purification and T. Darling and J. Grimmett (LMB) for help with high-performance computing. The authors also thank Y. Modis (Cambridge University) for pointing out the similarity to anti-freeze proteins. Part of this work was funded by the European Molecular Biology Organization (aALTF 778-2015 to T.A.M.B.), the Medical Research Council (U105184326 to J.L.), the Wellcome Trust (095514/Z/11/Z to J.L.) and the National Institutes of Health (GM51986 to Y.V.B.). This work was supported by iNEXT, project no. 1482, funded by the Horizon 2020 programme of the European Union.
The authors declare no competing financial interests.
Supplementary Tables 1 and 2; Supplementary Figures 1–7; legends for Supplementary Videos (PDF 2780 kb)
Whole cell cryo-ET of a C. crescentus cell (related to Figure 1). (AVI 21817 kb)
Cryo-ET of reconstituted 2D sheets of RsaA (related to Figure 2). (AVI 18787 kb)
The X-ray structure and map of a RsaA monomer (related to Figure 2). (MOV 35083 kb)
X-ray crystallography structure of the outer S-layer lattice (related to Figure 2). (MOV 35508 kb)
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Bharat, T., Kureisaite-Ciziene, D., Hardy, G. et al. Structure of the hexagonal surface layer on Caulobacter crescentus cells. Nat Microbiol 2, 17059 (2017). https://doi.org/10.1038/nmicrobiol.2017.59