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{{Short description|Protein-based part of the cell envelope found in most archaea and some bacteria}}
An '''S-layer''' (surface layer) is a part of the [[cell envelope]] found in almost all [[archaea]], as well as in many types of [[bacteria]].<ref name="Albers2011">{{cite journal |vauthors=Albers SV, Meyer BH |date=2011 |title=The archaeal cell envelope |journal=Nature Reviews Microbiology |volume=9 |issue= 6|pages=414–426 |doi=10.1038/nrmicro2576 |pmid=21572458 |s2cid=10297797 }}</ref><ref name= "Sleytr2014" >{{cite journal |vauthors = Sleytr UB, Schuster B, Egelseer EM, Pum D |date=2014 |title=S-layers: Principles and Applications |journal=FEMS Microbiology Reviews |volume=38 |issue=5 |pages=823–864 |doi=10.1111/1574-6976.12063 |pmc=4232325 |pmid=24483139 }}</ref><ref name= "Sleytr2025" >{{cite journal | vauthors = Sleytr UB, Pum D | date = 2025 | title = S-layers: from a serendipitous discovery to a toolkit for nanobiotechnology | journal = Quarterly Reviews of Biophysics | volume = 58 | issue = e4 | article-number = e4 | doi=10.1017/S0033583524000106 | pmid = 39819733 | doi-access = free }}</ref>
The S-layers of both archaea and bacteria consists of a [[Monolayer|monomolecular layer]] composed of only one (or, in a few cases, two) identical [[proteins]] or [[glycoproteins]].<ref name="Sleytr2014" /><ref name="Sleytr2025" /><ref>{{Cite journal|last1=Rodrigues-Oliveira|first1=Thiago|last2=Belmok|first2=Aline|last3=Vasconcellos|first3=Deborah|last4=Schuster|first4=Bernhard|last5=Kyaw|first5=Cynthia M.|date=2017-12-22|title=Archaeal S-Layers: Overview and Current State of the Art|journal=Frontiers in Microbiology|volume=8|page=2597|doi=10.3389/fmicb.2017.02597|issn=1664-302X|pmc=5744192|pmid=29312266|doi-access=free}}</ref> This structure is built via [[self-assembly]] and encloses the whole cell surface. Thus, the S-layer protein can represent up to 15% of the whole protein content of a cell.<ref name = "Sleytr1993">{{cite journal |vauthors=Sleytr U, Messner P, Pum D, Sára M |title=Crystalline bacterial cell surface layers |journal=Mol. Microbiol. |volume=10 |issue=5 |pages=911–6 |year=1993 |pmid=7934867 |doi=10.1111/j.1365-2958.1993.tb00962.x|s2cid=86119414 }}</ref> S-layer proteins are poorly conserved or not conserved at all, and can differ markedly even between related species.  Depending on species, the S-layers have a thickness between 5 and 25&nbsp;nm and possess identical pores 2–8&nbsp;nm in diameter.<ref name="Sleytr2014" /><ref name="Sleytr2025" /><ref name="Sleytr2016" /><ref name= " Buhlheller2024" >{{cite journal | vauthors=Buhlheller C, Sagmeister T, Grininger C, Gubensak N, Sleytr UB, Uson I, Pavkov-Keller T| date = 2024 | title = SymProFold: Structural prediction of symmetrical biological assemblies | journal = Nat Commun | volume = 15 | issue = 1| article-number = 8152 | doi=10.1038/s41467-024-52138-3| pmid = 39294115 | bibcode = 2024NatCo..15.8152B | hdl = 10261/372119 | hdl-access = free | pmc = 11410804 }}</ref>

The terminology "S-layer" was used the first time in 1976.<ref name="Sleytr1976">{{cite journal |author=Sleytr UB |date=1976 |title=Self-assembly of the hexagonally and tetragonally arranged subunits of bacterial surface layers and their reattachment to cell walls |journal=J. Ultrastruct. Res. |volume=55 |issue=3 |pages=360–367 |doi=10.1016/S0022-5320(76)80093-7 |pmid=6800 }}</ref> The general use was accepted at the "First International Workshop on Crystalline Bacterial Cell Surface Layers, Vienna (Austria)" in 1984, and in the year 1987 S-layers were defined at the [[European Molecular Biology Organization]] Workshop on "Crystalline Bacterial Cell Surface Layers", Vienna as "Two-dimensional arrays of proteinaceous subunits forming surface layers on prokaryotic cells" (see "Preface", page VI in Sleytr "et al. 1988"<ref name="Sleytr1988">{{cite book |vauthors=Sleytr UB, Messner P, Pum D, Sára M |editor1-first=Uwe B |editor1-last=Sleytr |editor2-first=Paul |editor2-last=Messner |editor3-first=Dietmar |editor3-last=Pum |editor4-first=Margit |editor4-last=Sára |date=1988 |title=Crystalline Bacterial Cell Surface Layers |url=https://www.springer.com/de/book/9783642735394 |location=Berlin |publisher=Springer |doi=10.1007/978-3-642-73537-0 |isbn=978-3-540-19082-0 |s2cid=20244135 }}</ref>). For a brief summary on the history of S-layer research see "References"''.''<ref name="Sleytr2014" /><ref name="Sleytr2016">{{cite book |author=Sleytr UB |date=2016 |title=Curiosity and Passion for Science and Art |volume=7 |location=Singapore |publisher=World Scientific Publishing |isbn=978-981-3141-81-0 |doi=10.1142/10084 |series=Series in Structural Biology }}</ref>A comprehensive historical account of the development of fundamental and applied S-layer research is given in the following current review.<ref name="Sleytr2025" />

== Location of S-layers ==
[[File:TEM FE Sinense.png|thumb|upright=1.25|TEM micrograph of a freeze-etched and metal shadowed preparation of the archaeum ''Methanocorpusculum sinense''. The S-layer exhibits hexagonal (p6) lattice symmetry with a center-to-center spacing of approx. 16 nm. The rope like structure is a flagellum that has collapsed on the cell surface during cell centrifugation <ref name= "Pum1991" >{{cite journal | vauthors = Pum D, Messner P, Sleytr UB | date = 1991 | title = Role of the S layer in morphogenesis and cell division of the Archaebacterium ''Methanocorpusculum sinense'' | journal = J. Bacteriol. | volume = 173 | issue = 21 | pages = 6865–6873 | doi=10.1128/jb.173.21.6865-6873.1991| pmid = 1938891 | pmc = 209039 }}</ref> [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International (CC BY 4.0) licence] [[File:CC-BY icon.svg|50px]].]]
[[File:CW-Architecture_2.png|thumb|upright=1.25|Schematic illustration of the supramolecular architecture of the major classes of prokaryotic cell envelopes containing surface (S) layers. S-layers in archaea with glycoprotein lattices as exclusive wall component are composed either of mushroom-like subunits with pillar-like, hydrophobic trans-membrane domains (a), or lipid-modified glycoprotein subunits (b). Individual S-layers can be composed of glycoproteins possessing both types of membrane anchoring mechanisms. Few archaea possess a rigid wall layer (e.g. pseudomurein in methanogenic organisms) as intermediate layer between the plasma membrane and the S-layer (c). In Gram-positive bacteria (d) the S-layer (glyco)proteins are bound to the rigid peptidoglycan-containing layer via secondary cell wall polymers. In Gram-negative bacteria (e) the S-layer is closely associated with the lipopolysaccharide of the outer membrane. Figure and figure legend were copied from Sleytr ''et al.'' 2025,<ref name="Sleytr2025" /> which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International (CC BY 4.0) licence] [[File:CC-BY icon.svg|50px]].]]
[[File: LatticeTypes natural 2.png|thumb| upright=1.25|Schematic drawing of the different S-layer lattice types. The proteins of one morphological unit are shown in red. For a more detailed description of the symmetry properties of S-layers see.<ref name= "Pum2021" >{{cite journal | vauthors = Pum D, Breitwieser A, Sleytr UB | date = 2021 | title = Patterns in Nature - S-Layer Lattices of Bacterial and Archaeal Cells | journal = Crystals | volume = 11 | issue = 8 | page = 869 | doi=10.3390/cryst11080869 |doi-access=free| bibcode = 2021Cryst..11..869P }}</ref> [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International (CC BY 4.0) licence] [[File:CC-BY icon.svg|50px]].]]
* In [[Gram-negative]] [[bacteria]], S-layers are associated to the [[lipopolysaccharides]] via protein–carbohydrate interactions.<ref name="Sleytr2014" /><ref name="Sleytr2025" />
* In [[Gram-positive]] [[bacteria]] whose S-layers often contain surface layer homology (SLH) domains, the binding occurs to the [[peptidoglycan]] and to a secondary [[cell wall]] polymer (e.g., teichoic acids). In the absence of SLH domains, the binding occurs via electrostatic interactions between the positively charged [[N-terminus]] of the S-layer protein and a negatively charged secondary [[cell wall]] polymer. In Lactobacilli the binding domain may be located at the [[C-terminus]].<ref name="Sleytr2014" />
* In [[Gram-negative]] [[archaea]], S-layer proteins possess a hydrophobic anchor that is associated with the underlying lipid membrane.<ref name="Albers2011" /><ref name="Sleytr2014" />
* In [[Gram-positive]] [[archaea]], the S-layer proteins bind to pseudomurein or to methanochondroitin.<ref name="Albers2011" /><ref name="Sleytr2014" />

== Biological functions of the S-layer ==
For many bacteria, the S-layer represents the outermost interaction zone with their respective environment.<ref name="Sleytr2014" /><ref name="Sleytr2025" /><ref name="Sleytr1999">{{cite journal|last1=Sleytr|first1=UB|last2=Beveridge|first2=TJ|title=Bacterial S-layers|journal=Trends Microbiol.|volume=7|issue=6|pages=253–260|year=1999|pmid=10366863|doi=10.1016/s0966-842x(99)01513-9}}</ref> Its functions are very diverse and vary from species to species. In many archaeal species the S-layer is the only cell wall component and, therefore, is important for mechanical and osmotic stabilization.<ref name="Sleytr2014" /><ref name="Sleytr2025" /><ref>{{Cite journal |last1=Pfeifer |first1=Kevin |last2=Ehmoser |first2=Eva-Kathrin |last3=Rittmann |first3=Simon K.-M. R. |last4=Schleper |first4=Christa |last5=Pum |first5=Dietmar |last6=Sleytr |first6=Uwe B. |last7=Schuster |first7=Bernhard |date=2022-07-21 |title=Isolation and Characterization of Cell Envelope Fragments Comprising Archaeal S-Layer Proteins |journal=Nanomaterials |language=en |volume=12 |issue=14 |page=2502 |doi=10.3390/nano12142502 |pmid=35889727 |pmc=9320373 |issn=2079-4991 |doi-access=free }}</ref> The S-layer is considered to be porous, which contributes to many of its functions.<ref name="Sleytr2014" /><ref name="Sleytr2025" /><ref name="Buhlheller2024" /> A most relevant general function of S-layers of both, bacteria and archaea, seems to be their excellent anti-fouling properties.<ref name="Sleytr2014" /><ref name="Sleytr2025" /><ref name="Rothbauer2013">{{cite journal |vauthors=Rothbauer M, Küpcü S, Sticker D, Sleytr UB, Ertl P |date=2013 |title=Exploitation of S-layer Anisotropy: pH-dependent Nanolayer Orientation for Cellular Micropatterning |journal=ACS Nano |volume=7 |issue=9 |pages=8020–8030 |doi=10.1021/nn403198a |pmid=24004386 }}</ref> In Archaea that possess S-Layers as the exclusive cell wall component, a general function of S-layer lattices is that of a cell shape-determining/maintaining scaffold.<ref name="Pum1991" /><ref name= "Messner1986" >{{cite journal | vauthors = Messner P, Pum D, Sára M, Stetter KO, Sleytr UB | date = 1986 | title = Ultrastructure of the cell envelope of the archaebacteria ''Thermoproteus tenax'' and ''Thermoproteus neutrophilus'' | journal = J. Bacteriol. | volume = 166 | issue = 3 | pages = 1046–1054| doi=10.1128/jb.166.3.1046-1054.1986| pmid = 3086286 | pmc = 215230 }}</ref><ref name= "Zink2019" >{{cite journal | vauthors = Zink IA, Pfeifer K, Wimmer E, Sleytr UB, Schuster B, Schleper C | date = 2019 | title = CRISPR-mediated gene silencing reveals involvement of the archaeal S-layer in cell division and virus infection | journal = Nat. Commun. | volume = 10 | article-number = 4797| doi=10.1038/s41467-019-12745-x | pmid = 31641111 | pmc = 6805947 | bibcode = 2019NatCo..10.4797Z }}</ref> For an overview of functions of S-layers, see.<ref name="Sleytr2014" /><ref name="Sleytr2025" /> The spectrum  of functions associated with S-layers include:
* protection against [[bacteriophages]], [[Bdellovibrio]]s, and [[phagocytosis]]
* resistance against low [[pH]]
* barrier against high-molecular-weight substances (e.g., lytic [[enzymes]])
* adhesion (for [[glycosylated]] S-layers)
* stabilization of the membrane (e.g. the SDBC in ''[[Deinococcus radiodurans]]'') <ref name="Farci2016">{{cite journal |vauthors=Farci D, Slavov C, Tramontano E, Piano D |date=2016 |title=The S-layer Protein DR_2577 Binds Deinoxanthin and under Desiccation Conditions Protects against UV-Radiation in Deinococcus radiodurans |journal=Frontiers in Microbiology |volume=7 |page=155 |doi=10.3389/fmicb.2016.00155 |pmid=26909071 |pmc=4754619 |doi-access=free }}</ref><ref name="Farci2018">{{cite journal |vauthors=Farci D, Slavov C, Piano D |date=2018 |title=Coexisting properties of thermostability and ultraviolet radiation resistance in the main S-layer complex of Deinococcus radiodurans |journal=Photochem Photobiol Sci |volume=17 |issue=1 |pages=81–88 |doi=10.1039/c7pp00240h |pmid=29218340 |bibcode=2018PcPbS..17...81F |doi-access=free }}</ref>
* resistance against electromagnetic stress (e.g. ionizing radiations and high temperatures) <ref name="Farci2016">{{cite journal |vauthors=Farci D, Slavov C, Tramontano E, Piano D |date=2016 |title=The S-layer Protein DR_2577 Binds Deinoxanthin and under Desiccation Conditions Protects against UV-Radiation in Deinococcus radiodurans |journal=Frontiers in Microbiology |volume=7 |page=155 |doi=10.3389/fmicb.2016.00155 |pmid=26909071 |pmc=4754619 |doi-access=free }}</ref><ref name="Farci2018">{{cite journal |vauthors=Farci D, Slavov C, Piano D |date=2018 |title=Coexisting properties of thermostability and ultraviolet radiation resistance in the main S-layer complex of Deinococcus radiodurans |journal=Photochem Photobiol Sci |volume=17 |issue=1 |pages=81–88 |doi=10.1039/c7pp00240h |pmid=29218340 |bibcode=2018PcPbS..17...81F |doi-access=free }}</ref>
* provision of adhesion sites for exoproteins <ref name= "Egelseer1995" >{{cite journal |vauthors = Egelseer E, Schocher I, Sára M, Sleytr UB |date = 1995 | title= The S-layer from Bacillus stearothermophilus DSM 2358 functions as adhesion site for a high-molecular-weight amylase | journal = J Bacteriol | volume = 177 | issue = 6 | pages = 1444–1451 | doi=10.1128/jb.177.6.1444-1451.1995 |pmid = 7533757 | pmc = 176758 }}</ref>
* provision of a periplasmic compartment in Gram-positive prokaryotes together with the peptidoglycan and the cytoplasmic membranes <ref name="Sleytr1999" />
* biomineralization<ref name="Schultzelam1992">{{cite journal |vauthors=Schultze-Lam S, Harauz G, Beveridge TJ |date=1992 |title=Participation of a cyanobacterial S layer in fine-grain mineral formation|journal=J. Bacteriol. |volume=174 |issue=24 |pages=7971–7981 |doi=10.1128/jb.174.24.7971-7981.1992 |pmid=1459945 |pmc=207533 }}</ref><ref name="Shenton1997">{{cite journal |vauthors=Shenton W, Pum D, Sleytr UB, Mann S |date=1997 |title=Synthesis of CdS superlattices using self-assembled bacterial S-layers |journal=Nature |volume=389 |issue= 6651|pages=585–587 |doi=10.1038/39287 |s2cid=4317884 }}</ref><ref name="Mertig1999">{{cite journal |vauthors=Mertig M, Kirsch R, Pompe W, Engelhardt H |date=1999 |title=Fabrication of highly oriented nanocluster arrays by biomolecular templating |journal=Eur. Phys. J. D |volume=9 |issue=1 |pages=45–48 |doi=10.1007/s100530050397 |bibcode=1999EPJD....9...45M |s2cid=120507258 }}</ref>
* molecular sieve and barrier function<ref name="Sleytr2014"/><ref name="Sara1987">{{cite journal |vauthors=Sára M, Sleytr, UB |date=1987 |title=  Production and characteristics of ultrafiltration membranes with uniform pores from two-dimensional arrays of proteins |journal=J. Membr. Sci. |volume=33 |issue=1  |pages=27–49 |doi=10.1016/S0376-7388(00)80050-2 }}</ref><ref>{{cite journal |last1=von Kügelgen |first1=Andriko |last2=Cassidy |first2=C. Keith |last3=van Dorst |first3=Sofie |last4=Pagani |first4=Lennart L. |last5=Batters |first5=Christopher |last6=Ford |first6=Zephyr |last7=Löwe |first7=Jan |last8=Alva |first8=Vikram |last9=Stansfeld |first9=Phillip J. |last10=Bharat |first10=Tanmay A. M. |title=Membraneless channels sieve cations in ammonia-oxidizing marine archaea |journal=Nature |date=6 June 2024 |volume=630 |issue=8015 |pages=230–236 |doi=10.1038/s41586-024-07462-5 |doi-access=free|pmid=38811725 |pmc=11153153 |bibcode=2024Natur.630..230V }}</ref><ref name="Schuster2021">{{cite journal |vauthors=Schuster B, Sleytr UB |date=2021 |title=S-Layer Ultrafiltration Membranes |journal=Membranes |volume=11 |issue=4 |page=275 |doi=10.3390/membranes11040275 |pmid=33918014 |pmc=8068369 |doi-access=free }}</ref>
A great example of a bacterium which utilizes the biological functions of the S-layer is ''Clostridioides difficile.'' In ''C. difficile'', the S-layer has helped with [[biofilm]] formation, host [[cell adhesion]], and immunomodulation through [[cell signaling]] of the host response.<ref>{{Cite journal |last1=Ormsby |first1=Michael J. |last2=Vaz |first2=Filipa |last3=Kirk |first3=Joseph A. |last4=Barwinska-Sendra |first4=Anna |last5=Hallam |first5=Jennifer C. |last6=Lanzoni-Mangutchi |first6=Paola |last7=Cole |first7=John |last8=Chaudhuri |first8=Roy R. |last9=Salgado |first9=Paula S. |last10=Fagan |first10=Robert P. |last11=Douce |first11=Gillian R. |date=2023-06-29 |title=An intact S-layer is advantageous to Clostridioides difficile within the host |journal=PLOS Pathogens |language=en |volume=19 |issue=6 |article-number=e1011015 |doi=10.1371/journal.ppat.1011015 |issn=1553-7374 |pmc=10310040 |pmid=37384772 |doi-access=free }}</ref>

==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>

== Self-assembly ==

=== In vivo assembly ===
Assembly of a highly ordered coherent monomolecular S-layer array on a growing cell surface requires a continuous synthesis of a surplus of S-layer proteins and their translocation to sites of lattice growth.<ref name="Sleytr2014" /><ref name="Sleytr2025" /><ref name="Sleytr1999" /><ref name="Fagan2014">{{cite journal |vauthors=Fagan RP, Fairweather NF |date=2014 |title=Biogenesis and functions of bacterial S-layers |url= http://eprints.whiterose.ac.uk/97080/1/Fagan%20and%20Fairweather_for%20deposit.pdf|journal=Nature Reviews. Microbiology |volume=12 |issue=3 |pages=211–222 |doi=10.1038/nrmicro3213 |pmid=24509785 |s2cid=24112697 }}</ref> Moreover, information concerning this dynamic process were obtained from reconstitution experiments with isolated S-layer subunits on cell surfaces from which they had been removed (homologous reattachment) or on those of other organisms (heterologous reattachment).<ref name="Sleytr2025"/><ref name="Sleytr1976"/><ref name="Sleytr1975">{{cite journal |author=Sleytr UB |date=1975 |title=Heterologous reattachment of regular arrays of glycoproteins on bacterial surfaces |journal=Nature |volume=257 |issue= 5525|pages=400–402 |doi=10.1038/257400a0 |pmid= 241021|bibcode=1975Natur.257..400S |s2cid=4298430 }}</ref>

=== In vitro assembly ===
S-layer proteins have the natural capability to self-assemble into regular monomolecular arrays in solution and at interfaces, such as solid supports, the air-water interface, lipid films, liposomes, emulsomes, nanocapsules, nanoparticles or micro beads.<ref name="Sleytr2014" /><ref name="Sleytr2025" /><ref name="Pum2014">
{{cite journal |vauthors=Pum D, Sleytr UB |date=2014 |title=Reassembly of S-layer proteins |journal=Nanotechnology |volume=25 |issue= 31|article-number=312001 |doi=10.1088/0957-4484/25/31/312001 |pmid= 25030207|bibcode=2014Nanot..25E2001P |s2cid=39889746 }}</ref><ref name="Schuster2014">{{cite journal |vauthors=Schuster B, Sleytr UB |date=2014 |title=Biomimetic interfaces based on S-layer proteins, lipid membranes and functional biomolecules |url=http://rsif.royalsocietypublishing.org/node/7191.full |journal=J. R. Soc. Interface |volume=11 |issue=96 |article-number=20140232 |doi=10.1098/rsif.2014.0232 |pmid=24812051 |pmc=4032536 }}</ref> S-layer crystal growth follows a non-classical pathway in which a final refolding step of the S-layer protein is part of the lattice formation.<ref name="Chung2010">{{cite journal |vauthors=Chung S, Shin SH, Bertozzi CR, De Yoreo JJ  |date=2010 |title=Self-catalyzed growth of S layers via an amorphous-to-crystalline transition limited by folding kinetics |journal=Proc. Natl. Acad. Sci. USA |volume=107 |issue=38 |pages=16536–16541 |doi=10.1073/pnas.1008280107 |pmid=20823255 |pmc=2944705 |bibcode=2010PNAS..10716536C |doi-access=free }}</ref><ref name="Shin2012">{{cite journal |vauthors=Shin SH, Chung S, Sanii B, Comolli LR, Bertozzi CR, De Yoreo JJ |date=2012 |title=Direct observation of kinetic traps associated with structural transformations leading to multiple pathways of S-layer assembly |journal=Proc. Natl. Acad. Sci. USA |volume=109 |issue= 32|pages=12968–12973 |doi=10.1073/pnas.1201504109 |pmid=22822216 |pmc=3420203  |bibcode=2012PNAS..10912968S |doi-access=free }}</ref>

== Application ==
Native S-layer proteins have already been used three decades ago in the development of biosensors and ultrafiltration membranes.<ref name="Sleytr2014" /><ref name="Sleytr2025" /> Subsequently, S-layer fusion proteins with specific functional  domains (e.g. enzymes, ligands, mimotopes, antibodies or antigens) allowed to investigate completely new strategies for functionalizing surfaces in the life sciences, such as in the development of novel affinity matrices, mucosal vaccines, biocompatible surfaces, micro carriers and encapsulation systems, or in the material sciences as templates for biomineralization.<ref name="Sleytr2014" /><ref name="Sleytr2025" /><ref name="Ilk2011" /><ref name ="FEMS1997">{{cite journal |doi=10.1016/S0168-6445(97)00044-2 |vauthors=Sleytr U, Bayley H, Sára M, Breitwieser A, Küpcü S, Mader C, Weigert S, Unger F, Messner P, Jahn-Schmid B, Schuster B, Pum D, Douglas K, Clark N, Moore J, Winningham T, Levy S, Frithsen I, Pankovc J, Beale P, Gillis H, Choutov D, Martin K |title=Applications of S-layers |journal=FEMS Microbiol. Rev. |volume=20 |issue=1–2 |pages=151–75 |year=1997 |pmid=9276930}}</ref><ref name= " Qing2023" >{{cite journal | vauthors = Qing R, Xue MT, Zhao JY, Wu LD, Breitwieser A, Smorodina E, Schubert T, Azzellino G, Jin D, Kong J, Palacios T, Sleytr UB, Zhang SG |date=2023 | title= Scalable biomimetic sensing system with membrane receptor dual-monolayer probe and graphene transistor arrays | journal = Science Advances | volume = 9 | issue = 29 | article-number = eadf1402 | doi=10.1126/sciadv.adf1402|pmid=37478177 |pmc=10361598 |bibcode=2023SciA....9F1402Q | hdl = 10852/108278 | hdl-access = free }}</ref>

==References==
{{reflist}}

{{DEFAULTSORT:S-Layer}}
[[Category:Cell anatomy]]
[[Category:Membrane biology]]