9.B.324.  The Pore-forming S-layer protein (S-layer) Family 

Crystalline bacterial and archaeal cell surface layers (S-layers) represent the outermost cell envelope component in a broad range of bacteria and archaea (Pum and Sleytr 2014). They are monomolecular arrays composed of a single protein or glycoprotein species and represent the simplest proteinaceous biological membranes. They are highly porous protein mesh works with unit cell sizes in the range of 3 to 30 nm, and pore sizes of 2 to 8 nm. S-layers are usually 5 to 20 nm thick in bacteria but up to 70 nmthick in archaea. S-layer proteins are one of the most abundant biopolymers on earth. One of their key features,  is the intrinsic capability of isolated native and recombinant S-layer proteins to form self-assembled mono- or double layers in suspension, at solid supports, the air-water interface, planar lipid films, liposomes, nanocapsules, and nanoparticles. Reassembly is entropy-driven as an example of matrix assembly following a multistage pathway in which the process of S-layer protein folding is directly linked with assembly into extended clusters. Basic research on the structure, synthesis, genetics, assembly, and function of S-layer proteins laid the foundation for their application in novel approaches in biotechnology, biomimetics, synthetic biology, and nanotechnology (Pum and Sleytr 2014).



Konrad, Z. and J. Eichler. (2002). Lipid modification of proteins in Archaea: attachment of a mevalonic acid-based lipid moiety to the surface-layer glycoprotein of Haloferax volcanii follows protein translocation. Biochem. J. 366: 959-964.

Kontro, I., S.K. Wiedmer, U. Hynönen, P.A. Penttilä, A. Palva, and R. Serimaa. (2014). The structure of Lactobacillus brevis surface layer reassembled on liposomes differs from native structure as revealed by SAXS. Biochim. Biophys. Acta. 1838: 2099-2104.

Pum, D. and U.B. Sleytr. (2014). Reassembly of S-layer proteins. Nanotechnology 25: 312001.

Shalev, Y., I. Turgeman-Grott, A. Tamir, J. Eichler, and U. Gophna. (2017). Cell Surface Glycosylation Is Required for Efficient Mating of. Front Microbiol 8: 1253.

Trachtenberg, S., B. Pinnick, and M. Kessel. (2000). The cell surface glycoprotein layer of the extreme halophile Halobacterium salinarum and its relation to Haloferax volcanii: cryo-electron tomography of freeze-substituted cells and projection studies of negatively stained envelopes. J Struct Biol 130: 10-26.


TC#NameOrganismal TypeExample

S-layer protein of 469 aas and 1 N-terminal TMS, SlpA.  SlpA can be reassembled on unilamellar liposomes (Kontro et al. 2014).

SlpA of Lactobacillus brevis


TC#NameOrganismal TypeExample

Surface (S)-layer protein of 393 aas and 9 TMSs, Slp

Slp of Bacillus thuringiensis


TC#NameOrganismal TypeExample

Surface (S)-layer protein of 1099 aas and one N-terminal TMS, SbsC.

L-layer protein of Geobacillus stearothermophilus (Bacillus stearothermophilus)


TC#NameOrganismal TypeExample

Surface (S)-layer glycoprotein of 827 aas and 2 TMSs, one N-terminal and one C-terminal, Csg or Cwd.  It forms a paracrystalline mono-layered assembly of proteins which coat the surface of the archaeal cell (Trachtenberg et al. 2000). It can be modified by lipid attachment (Konrad and Eichler 2002). Cell surface glycosylation is required for efficient mating (Shalev et al. 2017).

Csg of Haloferax volcanii (Halobacterium volcanii)