Editing S-layer (section)

==S-layer structure ==
While ubiquitous among Archaea, and common in bacteria, the S-layers of diverse organisms have unique structural properties, including symmetry and unit cell dimensions, due to fundamental differences in their constituent building blocks.<ref name="Buhlheller2024" /><ref name="Pavkov2011">{{cite book |vauthors=Pavkov-Keller T, Howorka S, Keller W |title=Molecular Assembly in Natural and Engineered Systems |date=2011 |chapter=The structure of bacterial S-layer proteins |series=Progress in Molecular Biology and Translational Science V. 103 |volume=103 |pages=73–130 |doi=10.1016/B978-0-12-415906-8.00004-2 |pmid=21999995 |isbn=978-0-12-415906-8 }}</ref> Sequence analyses of S-layer proteins have predicted that S-layer proteins have sizes of 40-200 kDa and may be composed of multiple domains some of which may be structurally related.<ref name="Buhlheller2024" /> Since the first evidence of a macromolecular array on a bacterial cell wall fragment in the 1950s,<ref name = "ref6">{{cite journal |author=Houwink, AL |title=A macromolecular mono-layer in the cell wall of Spirillum spec. |journal=Biochim Biophys Acta |volume=10 |issue=3 |pages=360–6|year=1953 |pmid=13058992 |doi=10.1016/0006-3002(53)90266-2}}</ref> S-layer structures have been investigated extensively by electron microscopy. These studies have provided useful information on overall S-layer morphology.

In general, S-layers exhibit either oblique (p1, p2), square (p4) or hexagonal (p3, p6) lattice symmetry. Depending on the lattice symmetry, each morphological unit of the S-layer is composed of one (p1), two (p2), three (p3), four (p4), or six (p6) identical protein subunits. The center-to-center spacing (or unit cell dimensions) between these subunits range from 4 to 35&nbsp;nm.<ref name="Sleytr2014"/><ref name="Sleytr2025" /><ref name="Buhlheller2024" />

For example, high-resolution structures of an archaeal S-layer protein (MA0829 from ''[[Methanosarcina acetivorans]]'' C2A) of the [[Methanosarcinales S-layer Tile Protein]] family and a bacterial S-layer protein (SbsB), from ''[[Geobacillus stearothermophilus]]'' PV72, have been determined by X-ray [[crystallography]].<ref name = "ref7">{{cite journal |vauthors=Arbing MA, Chan S, Shin A, Phan T, Ahn CJ, Rohlin L, Gunsalus RP |title=Structure of the surface layer of the methanogenic archaean Methanosarcina acetivorans. |journal=Proc Natl Acad Sci U S A |volume=109 |issue=29 |pages=11812–7 |year=2012 |pmid=22753492 |doi=10.1073/pnas.1120595109 |pmc=3406845|bibcode=2012PNAS..10911812A |doi-access=free }}</ref><ref name = "ref8">{{cite journal |vauthors=Baranova E, Fronzes R, Garcia-Pino A, Van Gerven N, Papapostolou D, Péhau-Arnaudet G, Pardon E, Steyaert J, Howorka S, Remaut H |title=SbsB structure and lattice reconstruction unveil Ca2+ triggered S-layer assembly |journal=Nature |volume=487 |issue=7405 |pages=119–22 |year=2012 |pmid=22722836 |doi=10.1038/nature11155|bibcode=2012Natur.487..119B |s2cid=4389187 |url=https://biblio.vub.ac.be/vubirfiles/5658510/nature11155.pdf }}</ref> In contrast with existing crystal structures, which have represented individual domains of S-layer proteins or minor proteinaceous components of the S-layer, the MA0829 and SbsB structures have allowed high resolution models of the ''M''. ''acetivorans'' and ''G''. ''stearothermophilus'' S-layers to be proposed. These models exhibit hexagonal (p6) and oblique (p2) symmetry, for ''M''. ''acetivorans'' and ''G''. ''stearothermophilus'' S-layers, respectively, and their molecular features, including dimensions and porosity, are in good agreement with data from electron microscopy studies of archaeal and bacterial S-layers.<ref name="Sleytr2025" /><ref name="Buhlheller2024" /><ref name="Pavkov2011" />

Finally, in connection with questions of structure-function investigations on S-layers, it should be mentioned that the recent introduction of SymProFold,<ref name="Buhlheller2024" /> which utlizes the high accuracy of AlphaFold-Multimer predictions to derive symmetrical assemblies from protein sequeces has proven to be a groundbreaking method for the accurate structural prediction of S-layer arrays. The predicted models could be vallidated using available experimental data at the cellular level, and additional crystal structures were obtained to confirm the symmetry and interfaces of numerous SymProFold assemblies. Thus, this  methodological approach to the structural elucidation of S-layers opens possibilities for exploring functionalities and designing targeted applications in diverse fields such as nanotechnology, biotechnology, nanomedicine, and environmental sciences.<ref name="Sleytr2014" /><ref name="Buhlheller2024" /><ref name="Ilk2011">{{cite journal |vauthors=Ilk N, Egelseer EM, Sleytr UB |date=2011 |title=S-layer fusion proteins - construction principles and applications |journal=Curr. Opin. Biotechnol. |volume=22 |issue=6 |pages=824–831 |doi=10.1016/j.copbio.2011.05.510 |pmc=3271365 |pmid=21696943 }}</ref>