MINIREVIEW
S-Layers as a basic building block in a molecular
construction kit
Uwe B. Sleytr, Eva M. Egelseer, Nicola Ilk, Dietmar Pum and Bernhard Schuster
Center for NanoBiotechnology, University of Natural Resources and Applied Life Sciences Vienna, Austria
Introduction
Methods for organizing materials at the nanometer
level are essential for the fabrication of supramolecular
structures and devices. Thus, molecular self-assembly
systems that exploit the molecular scale manufacturing
precision of biological systems are prime candidates in
nanobiotechnology.
Crystalline bacterial cell surface layer (S-layer) pro-
teins have been optimized during billions of years of
biological evolution as building blocks of one of the
simplest self-assembly systems (Fig. 1) [1–3]. S-Layers
are now recognized as the most common outermost
cell envelope components of prokaryotic organisms
[3,4]. Most S-layers are composed of a single protein
or glycoprotein species endowed with the ability to
assemble into monomolecular arrays on the supporting
envelope layer, representing the simplest biological
membrane developed during evolution. The wealth of
information accumulated on the structure, chemistry,
morphogenesis, genetics, and function of S-layers has
led to a broad spectrum of application in nanobiotech-
nology and biomimetics [2,5]. Most importantly,
S-layers represent very versatile self-assembly systems
with unique features as the structural basis for a com-
plete supramolecular construction kit, involving all
major types of biological molecules: proteins, lipids,

generating more complex supramolecular assemblies and structures, as
required for life and nonlife science applications.
Abbreviations
Bet v1, major birch pollen allergen; EGFP, enhanced green fluorescent protein; PSA, prostate-specific antigen; SbpA, S-layer protein of
Bacillus sphaericus CCM177; SbsB, S-layer protein of Geobacillus stearothermophilus PV72 ⁄ p2; SbsC, S-layer proteins of Geobacillus
stearothermophilus ATCC 12980; SCWP, secondary cell wall polymer; SgsE, S-layer protein of Geobacillus stearothermophilus NRS 2004 ⁄ 3a
variant 1; S-layer, crystalline bacterial cell surface layer; SLH, S-layer-homology; S-liposomes, S-layer coated liposomes; SPR, surface
plasmon resonance; SUM, S-layer ultrafiltration membrane; ZZ, two copies of the synthetic analog of the IgG-binding domain of protein A
from Staphylococcus aureus.
FEBS Journal 274 (2007) 323–334 ª 2006 The Authors Journal compilation ª 2006 FEBS 323
S-Layers, as periodic structures, exhibit identical
physicochemical properties on each molecular unit
down to the subnanometer level and possess pores of
identical size and morphology. Moreover, functional
groups are aligned on the surface and within the pores
of the lattice in well-defined positions and orientation.
The possibility to change the natural properties of
S-layer proteins by genetic engineering and incorporate
single-functional or multifunctional domains into
S-layer lattices has opened up new strategies for the fine-
tuning of their structural and functional features [6–8].
Major areas of application of S-layers include: (a)
production of isoporous ultrafiltration membranes; (b)
supporting structures for defined immobilization or
incorporation of functional molecules (e.g. antigens,
antibodies, ligands, enzymes); (c) matrix for the devel-
opment of biosensors including solid-phase immuno-
assays and label-free detection systems; (d) support
and stabilizing matrices for functional lipid mem-
branes, liposomes, and emulsomes; (e) adjuvants for

 70 nm thick. Transmission electron microscopic
studies on the mass distribution of S-layers (Fig. 1A)
and subsequent 2D and 3D analysis, including compu-
ter image enhancement, have produced structural
information down to 0.35–1.5 nm [2]. High-resolution
images of the surface topography of S-layers under
biological conditions have been obtained by scanning
force microscopy [2,12]. A common feature of S-layers
is, with respect to the orientation on the cell, their
smooth outer surface and more corrugated inner
A B
Fig. 1. (A) Electron micrograph of a freeze-etched and Pt ⁄ C-shadowed preparation of a Gram-positive organism exhibiting a square (p4)
S-layer lattice. The bar corresponds to 100 nm. (B) Schematic drawing illustrating the various S-layer lattice types. In the oblique lattice, one
morphological unit (red) consists of one (p1) or two (p2) identical subunits. Four subunits constitute one morphological unit in the square
(p4) lattice type, whereas the hexagonal lattice type is either composed of three (p3) or six (p6) subunits. Modified from Sleytr et al [2] with
permission from Wiley-VCH.
Molecular construction kit based on S-layers U. B. Sleytr et al.
324 FEBS Journal 274 (2007) 323–334 ª 2006 The Authors Journal compilation ª 2006 FEBS
surface. The proteinaceous subunits of S-layers are
aligned either in lattices with oblique (p1, p2), square
(p4), or hexagonal (p3, p6) symmetry (Fig. 1B) with a
center-to-center spacing of the morphological units of
 3–35 nm [2,8]. Hexagonal lattice symmetry is pre-
dominant among archaea [3]. S-Layers are highly por-
ous protein lattices with a surface porosity of 30–70%.
As S-layers are mostly composed of identical species of
subunits, they exhibit pores of identical size and mor-
phology [2,3,5]. However, in many protein lattices, two
or more distinct classes of pores with diameters in the
range 2–8 nm have been identified.

The reassembly occurs after removal of the disrupting
agent used in the dissolution and isolation procedure.
In general, complete disintegration of S-layer lattices
into their constituent protein subunits on bacterial
cells can be achieved using high concentrations of cha-
otropic agents (e.g. guanidine hydrochloride, urea), by
lowering or raising the pH, or by applying metal-
chelating agents or cation substitution [5]. The forma-
tion of self-assembled arrays is only determined by
the amino-acid sequence of the polypeptide chains
and consequently the tertiary structure of the S-layer
protein species. In various S-layer proteins from Bacil-
lacaea it has been shown that significant portions of
the C-terminal or N-terminal part can be deleted with-
out loss of the capability of the subunits for lattice
formation [15]. Further, for the S-layer protein of
Bacillus sphaericus CCM 2177 (SbpA), truncation of
the amino-acid sequence led to a change in the S-layer
lattice type from square (p4) to oblique (p1) lattice
symmetry [16].
Fig. 2. Schematic drawing of the isolation of
native S-layer proteins from bacterial cells
and the reassembly of native and recombin-
ant S-layer proteins into crystalline arrays in
suspension, on a solid support, at the air ⁄
water interface and on a planar lipid film,
and on liposomes or nanocapsules. An
example of S-layer proteins reassembling
with hexagonal (p6) lattice symmetry is
shown here. Modified from Pum et al. [8],

assembly is important.
As S-layer proteins represent an important class of
secreted proteins, numerous S-layer genes from bac-
teria and archaea have been sequenced and cloned
[4,13]. For S-layer proteins of Gram-positive bacteria
at least, common structural organization principles
have been identified. A cell-wall-targeting domain was
found at the N-terminal region of many S-layer pro-
teins, which mediates binding to a specific hetero-
polysaccharide, termed secondary cell wall polymer
(SCWP), by a lectin-type binding. For Gram-positive
bacteria, at least two types of binding mechanism
between the N-terminal region of S-layer proteins and
SCWPs have been described [16–18]. With respect to
the first binding mechanism, so-called S-layer-homol-
ogy (SLH) motifs, each comprising about 55 amino
acids, recognize a distinct type of pyruvylated SCWP
as the correct anchoring structure. The second binding
mechanism has been described for Geobacillus stearo-
thermophilus wild-type strains and is characterized by
the interaction of a nonpyruvylated SCWP containing
the negatively charged 2,3-dideoxydiacetamidomanno-
samine uronic acid with a highly conserved N-terminal
region lacking an SLH domain. However, the cell-
wall-targeting domain is not necessarily located in the
N-terminal region of the S-layer protein. Well-docu-
mented examples of C-terminal anchoring are the
S-layer proteins of Lactobacillus acidophilus ATCC
4556 and Lactobacillus crispatus [7].
To elucidate the structure–function relationship of

phy is also being pursued. This can be explained by (a)
the molecular mass of the S-layer subunits being too
large for NMR analysis, (b) their high tendency to
form 2D lattices preventing the formation of isotropic
3D crystals required for X-ray crystallography, and (c)
the very low solubility of isolated subunits. First, 3D
crystallization studies were carried out with water-
soluble N-terminally or C-terminally truncated forms
of SbsC. For the C-terminally truncated form, recom-
binant SbsC
31-844
, crystals that diffracted to a resolu-
tion of 3 A
˚
using synchrotron radiation could be
obtained [22]. Native and heavy atom derivative data
confirmed the results of the secondary-structure predic-
tion, which indicated that the N-terminal region com-
prising the first 257 amino acids is mainly organized as
a-helices, whereas the middle and C-terminal parts of
SbsC consist of loops and b-sheets. Information on the
Molecular construction kit based on S-layers U. B. Sleytr et al.
326 FEBS Journal 274 (2007) 323–334 ª 2006 The Authors Journal compilation ª 2006 FEBS
3D structure of S-layer proteins would open up the
possibility of rationally designing S-layer fusion pro-
teins incorporating functional domains, for example
within the pore areas of the protein lattice.
As cell surface components can generally be consid-
ered to be nonconservative structures that determine
the interaction between the living cell and its environ-

rently used for the construction of functional S-layer
fusion proteins [6,7]. S-Layer fusion proteins based on
the S-layer proteins, SbsB, SbsC and SbpA, incorpor-
ate an accessible N-terminal SCWP-binding domain,
the self-assembly domain, as well as a biologically act-
ive sequence fused to the C-terminus. After heterolo-
gous expression of the genes encoding chimeric S-layer
proteins in Escherichia coli, it could be shown that the
self-assembling properties of the S-layer protein moiety
as well as the functionality of the fused sequences were
retained in all S-layer proteins (Fig. 3).
In order to build up functional monomolecular
S-layer protein lattices on artificial solid supports such
as gold, silicon, glass, indium tin oxide or polymers
(Fig. 3A,B), the surface has to be functionalized with
covalently attached chemically modified SCWP, to
which the S-layer fusion proteins bind with their N-ter-
minal part, leaving the C-terminal part with the fused
functional sequence exposed to the environment [6].
Such chimeric S-layers recrystallized on solid supports
in defined orientation (Fig. 3C) should find application
in diagnostics and biochip technology (laboratory-
on-a-chip), as well as for the development of specific
cell targeting and delivery systems [2,4,6–8,24].
In a first approach, S-layer fusion proteins compri-
sing the C-terminally truncated form, recombinant
(r)SbpA
31-1068
, and the hypervariable region of heavy
chain camel antibodies recognizing lysozyme or a pros-

ible PSA molecules were bound per morphological unit
of the square S-layer lattice [25]. To summarize,
S-layer fusion proteins incorporating camel antibody
sequences can be considered key elements for the
development of sensing layers for label-free detection
systems such as SPR, surface acoustic wave or quartz
crystal microbalance, in which the binding event can
be measured directly by mass increase without the
need for any labeled molecule.
The genes encoding the chimeric S-layer proteins,
rSbsC
31-920
⁄ Bet v1 and rSbpA
31-1068
⁄ Bet v1, carrying
the major birch pollen allergen Bet v1 at the C-terminus
maintained the ability to self-assemble as well as the
functionality of the fused allergen to bind the Bet
v1-specific monoclonal mouse antibody [26]. In a recent
study, rSbsC
31-920
⁄ Bet v1 was shown to contain all rele-
vant B and T cell epitopes of Bet v1. Compared with
free Bet v1, in cells of birch pollen-allergic individuals,
the histamine-releasing capacity caused by the fusion
protein was significantly reduced, and no Th2-like
immune response was observed like after stimulation
with free Bet v1 [27]. Owing to its immunomodulating
capacity, this fusion protein is generally considered to
be a novel approach to specific treatment of allergic

match followed the Langmuir isotherm. The detection
limit for hybridized oligonucleotides was found to be in
the picomolar range [24]. To conclude, hybridization
experiments with biotinylated and fluorescently labeled
oligonucleotides using SPR spectroscopy indicated that
a functional sensor surface could be generated by
recrystallization of heterotetramers on gold chips. Such
promising structures that combine self-assembly prop-
erties of an S-layer protein with the biotin-binding
properties of streptavidin should find numerous
applications in (nano)biotechnology (Fig. 3C).
The S-layer fusion protein, rSbpA
31-1068
⁄ ZZ, incor-
porates two copies of the Fc-binding domain (ZZ), a
synthetic analog of the IgG-binding domain of pro-
tein A from Staphylococcus aureus [28]. As demonstra-
ted by SPR, the amount of human IgG that could be
bound on the native rSbpA
31-1068
⁄ ZZ monolayer was
slightly higher than on the rSbpA
31-1068
⁄ ZZ monolayer
cross-linked with the bifunctional imidoester dimethylpi-
melinimidate. On average,  66% of the theoretical sat-
uration capacity of a planar surface was covered by IgG
with the Fab regions in the condensed state. Novel bio-
compatible microparticles for the microsphere-based
detoxification system used for extracorporeal blood

receptors, and membrane-bound enzymes. These pro-
teins are key factors in the cell’s metabolism and thus
are the preferred target for pharmaceuticals. Currently
more than 60% of drugs consumed act on membrane
proteins [10,29]. Therefore not only biological mem-
branes, but also the biomimetic approach to generate
stabilized lipid membranes with functional membrane
proteins has attracted much interest in recent years.
The latter poses a challenge to apply membrane pro-
teins as key elements in drug discovery, protein–ligand
screening, and biosensors.
A promising approach for the generation of biomi-
metic membrane systems includes stabilization of lipid
membranes with S-layer lattices (Fig. 5). These com-
posite structures mimic the supramolecular assembly
B
Teflon
aperture
A
water
Patch clamp
pipette
air
E
(a)
(b)
(c)
(d)
(e)
(f)

coated nanocapsules [8,10,11,30].
The interaction of S-layer proteins with lipid mole-
cules has been demonstrated to be noncovalent. Elec-
trostatic interaction between exposed carboxy groups
on the inner face of the S-layer lattice and the zwitter-
ionic lipid head groups is primarily responsible for the
binding and defined orientation of the S-layer subunits
to form a closed lattice structure. For such an align-
ment, it has been suggested that there are at least two
to three contact points between the lipid film and the
attached S-layer protein. Therefore, only a few lipid
molecules are anchored via their head groups to pro-
tein domains on the S-layer lattice, whereas the
remaining scores of lipid molecules diffuse freely in the
membrane between the pillars consisting of anchored
lipid molecules. Because of its widely retained fluid
characteristic, this nano-patterned type of lipid mem-
brane is also referred to as ‘semifluid membrane’ [31].
However, most importantly, the attached S-layer
lattices reveal no effect on the hydrophobic lipid
acyl chains. Thus, S-layer lattices constitute unique
scaffolding for lipid membranes [11,29,30]. This
observation has been confirmed by the functional
reconstitution of transmembrane proteins.
Supported lipid membranes can also be generated
on S-layer ultrafiltration membranes (SUMs; Fig. 5C),
with S-layer fragments deposited in microfiltration
membranes as an active filtration layer [32], and on
S-layer-coated electrodes or structured silicon chips
(Fig. 5D), with the S-layer as a stabilizing biomimetic

indicates the formation of dimeric gramicidin pores. Because of pore formation, a cascaded increase in electric current is observed. The
schematic drawing on the right indicates the dissociation of gramicidin dimers. Gramicidin monomers do not form pores, and thus the con-
ductance decreases cascaded for each dissociated gramicidin dimer. Conditions: 1
M KCl; pH ¼ 5.8; V
m
¼ +150 mV; T ¼ 22 °C.
Molecular construction kit based on S-layers U. B. Sleytr et al.
330 FEBS Journal 274 (2007) 323–334 ª 2006 The Authors Journal compilation ª 2006 FEBS
electronic or optical devices, and might even find appli-
cation in DNA sequencing [10,11,29,30].
Artificial lipid vesicles termed liposomes are widely
used as delivery systems for enhancing the efficiency of
various biological active molecules. S-Layer-coated
liposomes (S-liposomes) represent simple model sys-
tems resembling features of archaeal cell or virus enve-
lopes (Fig. 5E). S-Layer proteins, once crystallized on
liposomes, can be cross-linked and exploited as a mat-
rix for the covalent attachment of functional molecules
as required for drug-targeting or immunodiagnostic
assays ([5,34] and references therein).
In a recent study, the fusion protein, rSbpA
31)1068
⁄
EGFP, carrying the sequence of enhanced green
fluorescent protein (EGFP) at the C-terminus was
recrystallized on positively charged liposomes. Because
of the ability of EGFP to fluoresce, positively charged
liposomes coated with rSbpA
31-1068
⁄ EGFP represent a

nanoparticles were formed either by reduction of the
metal salt with H
2
S or under the electron beam in a
transmission electron microscope. The latter approach
is technologically important, as it allows the definition
of areas where nanoparticles are eventually formed. As
determined by electron diffraction, the gold nano-
particles were crystalline but their ensemble was not
crystallographically aligned. Later, the wet chemical
approach was used in the formation of Pd–, Ni–, Pt–,
Pb–, and Fe–nanoparticle arrays. Recently, small spot
X-ray photoelectron emission spectroscopy was used
to characterize the elemental composition of the nano-
clusters. This technique demonstrated that they consis-
ted primarily of elemental gold [37].
Binding of preformed nanoparticles
Although wet chemical methods lead to crystalline
arrays of nanoparticles with spacing in register with
the underlying S-layer lattice, they do not allow parti-
cle size to be precisely controlled and hence the con-
tact distances of neighboring particle surfaces, both of
which are important for studying and exploiting quan-
tum phenomena. Thus, the binding of preformed
nanoparticles into regular arrays on S-layers has
significant advantages over wet chemical approaches
for the development of nanoscale electronic devices.
Studies on binding of biomolecules, such as enzymes
and antibodies [4,5], to S-layers have shown that
metallic and semiconducting nanoparticles can be

biotinylated peptides and proteins, in particular fer-
ritin. Furthermore, metal-binding peptides can also be
used as fusion partners in the design and expression of
S-layer fusion proteins.
Conclusions and perspectives
The cross-fertilization of biology, genetics, chemistry,
and material sciences is opening up a great variety of
opportunities in nanobiotechnology and biomimetics.
S-Layer research has clearly demonstrated that nature
provides most elegant examples of nanometer-sized,
molecular self-assembly systems (Figs 1 and 2) as
required for generating bottom-up nanostructured
materials, which may be exploited at mesoscopic and
macroscopic levels. Of particular importance is the
possibility to change the natural properties of S-layer
proteins by genetic manipulation and to incorporate
single-functional or multifunctional domains in S-layer
lattices (Fig. 3). The spontaneous association of identi-
cal S-layer (glyco)protein subunits in suspension or on
surfaces or interfaces (Fig. 2) results in stable well-
defined isoporous matrices, which can be considerably
strengthened by introducing intermolecular and ⁄ or
intramolecular bonds. Moreover, even native S-layers
of some archaea can assemble and function in the
most extreme environmental conditions in which the
particular organisms are able to dwell (e.g. tempera-
tures up to 120 °C, pH ¼ 0, concentrated salt solution,
high hydrostatic pressure).
An important line of development is the combining
of S-layer and lipid membrane technologies (Fig. 5).

emerge (e.g. neoglycobiology) [14].
Acknowledgements
This work was supported by the Austrian Science
Fund (FWF, projects P16295-B10, P17170-B10, and
P18510-B12), by the Erwin Schro
¨
dinger Society for
Nanosciences, by the Austrian Federal Ministry of
Transport, Innovation and Technology (MNA-Net-
work), by the EU project NAS-SAP, and by the US
Air Force Office of Scientific Research (projects
F49620-03-1-0222 and FA9550-06-1-0208).
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