4,6- O-Pyruvyl Ketal Modified N-Acetylmannosamine of the Secondary Cell Wall Polysaccharide of Bacillus anthracis Is the Anchoring Residue for Its Surface Layer Proteins

Article abstract of DOI:10.1021/jacs.8b08857

The secondary cell wall polysaccharide (SCWP) of Bacillus anthracis plays a key role in the organization of the cell envelope of vegetative cells and is intimately involved in host-guest interactions. Genetic studies have indicated that it anchors S-layer and S-layer-associated proteins, which are involved in multiple vital biological functions, to the cell surface of B. anthracis. Phenotypic observations indicate that specific functional groups of the terminal unit of SCWP, including 4,6-O-pyruvyl ketal and acetyl esters, are important for binding of these proteins. These observations are based on genetic manipulations and have not been corroborated by direct binding studies. To address this issue, a synthetic strategy was developed that could provide a range of pyruvylated oligosaccharides derived from B. anthracis SCWP bearing base-labile acetyl esters and free amino groups. The resulting oligosaccharides were used in binding studies with a panel of S-layer and S-layer-associated proteins, which identified structural features of SCWP important for binding. A single pyruvylated ManNAc monosaccharide exhibited strong binding to all proteins, making it a promising structure for S-layer protein manipulation. The acetyl esters and free amine of SCWP did not significantly impact binding, and this observation is contrary to a proposed model in which SCWP acetylation is a prerequisite for association of some but not all S-layer and S-layer-associated proteins.

Full text of DOI:10.1021/jacs.8b08857

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Article
4,6-O-Pyruvyl Ketal Modified N-Acetyl Mannosamine of
the Secondary Cell Polysaccharide of Bacillus anthracis
is the anchoring residue for its Surface Layer Proteins
Robert N. Chapman, Lin Liu, and Geert-Jan Boons
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08857 • Publication Date (Web): 19 Nov 2018
Downloaded from http://pubs.acs.org on November 21, 2018
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4,6-O-Pyruvyl Ketal Modified N-Acetyl Mannosamine of the Secondary
Cell Polysaccharide of Bacillus anthracis is the anchoring residue for its
Surface Layer Proteins
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Robert N. Chapman,†,‡,⊥ Lin Liu,† ,⊥ Geert-Jan Boons*,†,‡,§
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†Complex Carbohydrate Research Center, University of Georgia, Athens, GA 30602,
United States
‡Department of Chemistry, University of Georgia, Athens, GA 30602, United States
§Department of Chemical Biology and Drug Discovery, Utrecht Institute for
Pharmaceutical Sciences, and Bijvoet Center for Biomolecular Research, Utrecht
University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
⊥These authors contributed equally to this work.
*Corresponding author. Email: G.J.H.P.Boons@uu.nl or gjboons@ccrc.uga.edu
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ABSTRACT
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The secondary cell wall polysaccharide (SCWP) of Bacillus anthracis plays a key role in
the organization of the cell envelope of vegetative cells and is intimately involved in
host-guest interactions. Genetic studies have indicated that it anchors S-layer and S-layer
associated proteins, which are involved in multiple vital biological functions, to the cell
surface of B. anthracis. Phenotypic observations indicate that specific functional groups
of the terminal unit of SCWP, including 4,6-O-pyruvyl ketal and acetyl esters, are
important for binding of these proteins. These observations are based on genetic
manipulations and have not been corroborated by direct binding studies. To address this
issue, a synthetic strategy was developed that could provide a range of pyruvylated
oligosaccharides derived from B. anthracis SCWP bearing base labile acetyl esters and
free amino groups. The resulting oligosaccharides were used in binding studies with a
panel of S-layer and S-layer associated proteins, which identified structural features of
SCWP important for binding. A single pyruvylated ManNAc monosaccharide exhibited
strong binding to all proteins making it a promising structure for S-layer protein
manipulation. The acetyl esters and free amine of SCWP did not significantly impact
binding, and this observation is contrary to a proposed model in which SCWP acetylation
is a prerequisite for association of some but not all S-layer and S-layer associated
proteins.
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KEYWORDS
Oligosaccharides, synthesis, S-layer protein, Bacillus anthracis, secondary cell wall
polysaccharide
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INTRODUCTION
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S-layers are bi-dimensional para-crystalline protein surface layers that exist in nearly
every taxonomic group of bacteria and represent an almost universal feature of Archaea.1
The S-layer functions as a protective barrier with regularly spaced pores for selective
permeability.2 It is important for many biological functions such as maintenance of cell
integrity, enzyme display, protection to phagocytosis, and interactions with the host and
its immune system. Due to their critical functions and unique crystallization ability, S-
layer proteins are of great interest for drug delivery, biomaterial engineering and vaccine
development.3 Exploiting the biomedical potential of S-layer proteins will require a
detailed understanding how these proteins are tethered to the cell surface and assemble
into stable two-dimensional surface layer.
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The S-layer of B. anthracis, which is the causative agent of anthrax in humans and
other mammals,4 is mainly comprised of surface array protein (Sap) and extractable
antigen 1 (EA1).5 These proteins are composed of an N-terminal S-layer homology
(SLH) and a C-terminal crystallization domain.5b While the crystallization domain is
responsible for the formation of a stable two-dimensional surface layer, the conserved
SLH domain tethers the proteins to the cell surface.6 The S-layer can contain up to
twenty-two additional B. anthracis S-layer associated proteins (Bsl) as minor
constituents.2c These proteins also possess an SLH domain for binding to the cell surface
and an additional domain that exerts specific functions. For example, BslK is expressed
under conditions of iron limitation and acts to scavenge heme for iron utilization enabling
survival in the host. BslA facilitates infection by mediating adherence to host cells
thereby functioning as a virulence factor,2c,7 and BslO has peptidoglycan hydrolase
activity and plays a role in the separation of bacilli from elongating chains.8 BslR, BslS,
BslT, and BslU are predicted murein hydrolases.9
Earlier biochemical studies have shown that the SLH domains of the S-layer and S-
layer associated proteins of B. anthracis are anchored to a cell wall-associated polymer
through non-covalent interactions.10 Subsequent genetic studies demonstrated that csaB,
which is the gene responsible for pyruvylation, is critical for retention of SLH-containing
proteins.11 Structural examinations by Carlson and coworkers uncovered that the
secondary cell wall polysaccharide (SCWP) of B. anthracis is modified by a pyruvyl
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ketal,12 and it was postulated that this polysaccharide serves as the anchor for the SLH
containing proteins (Fig. 1).
SCWP of B. anthracis is composed of trisaccharide repeating units having a structure
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of
4)-β-D-ManNAc-(1,4)-β-D-GlcNAc-(1,6)-α-D-GlcNAc-(1→)
with
non-
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stoichiometric galactosylation creating considerable heterogeneity (Fig. 1). The distal
trisaccharide is modified by a (S)-4,6-O-pyruvyl ketal on the β-ManNAc residue, and
thus this moiety is the putative ligand for SLH-containing proteins. The C-3 hydroxyl of
the β-GlcNAc moiety of the distal repeating unit is non-stoichiometrically modified by an
acetyl ester and the α-GalNAc residue can be N-deacetylated (Fig. 1).13 A model for O-
acetylation of SCWP has been proposed in which the membrane bound O-
acetyltransferases patB1 performs the acetylation whereas patB2 is required for
translocation of an acetyl donor across the plasma membrane from a cytoplasmic
source.14
Genetic studies have indicated that fine structural details of SWCP are critical for
proper S-layer formation.11,15 In this respect, deletion of patA1 and patA2, which
significantly reduces O-acetylation of SWCP, leads to elongated chains of vegetative
cells. Although the patA1/patA2 mutant assembles an S-layer, it cannot retain the S-layer
protein EA1 and is mainly composed of Sap. Furthermore, the deposition of the BslO,
which is a murein hydrolase implicated in cell separation, was perturbed at the cell
division septa. Compared to wild type cells, the mutant displayed an aberrant deposition
pattern of BslO, only covering half of the cell division septa. It was also observed that
EA1, BslO, and BslA were not retained at the cell surface and released in the medium.
The mutant produced quantities of SCWP comparable to wild-type cells but having
substantial fewer acetyl esters. Based on these observations, it was proposed that patA1
and patA2 mediated O-acetylation of SCWP is important for retaining and depositing
EA1, BslO and BslA to specific regions of the cell envelope, which in turn is critical for
proper cell division.15
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PDGA capsule
S-layer protein
SLH domain
S-layer-associated protein
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Secondary cell wall
polysaccharide
OH
O
O
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OH
NHAc
O
O
O
O
HO
O
AcO
AcHN
O
Cell wall
HO
HO
H2N
O
Non-Reducing Terminal Unit of
Secondary Cell Wall Polysaccharide
Membrane
Figure 1. Schematic representation of the cell wall of B. anthracis. The cytoplasmic membrane
of B. anthracis is encased in a thick layer of cell wall peptidoglycan that serves as the anchoring
point for the secondary cell wall polysaccharide (SCWP) and the poly-γ-D-glutamate (PDGA)
capsule. It is postulated that the trimeric SLH domain of S-layer and S-layer associated proteins
are attached to SCWP through non-covalent binding.
The involvement of SCWP in anchoring S-layer and S-layer associated proteins to
cell surface is mainly based on genetic studies and has not been validated by detailed
binding assays. Furthermore, the mechanism by which O-acetylation regulates the
deposition of specific S-layer and S-layer associated proteins is not well understood. In
large measures, this is due to the structural heterogeneity of SCWP, which complicates
binding studies. Chemical synthesis is an attractive way to overcome this hurdle and in
principle can provide well-defined structures for probing the binding between SCWP and
proteins.
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Figure 2. Synthetic targets for binding studies and building blocks for synthesis of B. anthracis
SCWP derivatives.
Herein, we report the chemical synthesis of seven trisaccharides and four
monosaccharides derived from the distal unit of B. anthracis SCWP (Fig. 2) that have
perturbations in the 4,6-O-pyruvyl ketal and O- and N-acetylation patterns. The synthetic
compounds were employed in microarray and ELISA binding assays to establish the
importance of functional groups, such as the pyruvate ketal and the O- and N-acetyl
groups, for binding. These studies confirm that SCWP functions as the anchoring point of
the S-layer and S-layer associated proteins, and it was found that only the terminal
ManNAc moiety modified by a 4,6-ketal pyruvyl is important for binding. O-acetylation
of the GlcNAc moiety did, however, not substantially impact association with the various
proteins and this observation is contrary to a proposed model15 in which SCWP
acetylation is a prerequisite for the direct association of some but not all S-layer and S-
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layer associated proteins. The role of O-acetylation of SCWP in proper S-layer formation
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is more complex than previously thought.
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RESULTS AND DISCUSSION
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Chemical Synthesis. The trisaccharides 1-6, which all contain a 4,6-O-pyruvyl ketal, but
differ in the pattern of O-acetylation and N-deacetylation, were prepared to probe the
importance of these functionalities for S-layer protein binding (Fig. 2). Trisaccharide 7,
lacking the pyruvyl ketal group, was included as a negative control because genetic
studies had indicated that this functionality is critical for binding of S-layer associated
proteins.11,16 The monosaccharides 8-11 were prepared to establish the minimum epitope
and the involvement of the C-2 acetamido function of ManNAc for binding. The target
compounds are modified by an aminopentyl linker for controlled coupling to a surface or
carrier protein to facilitate binding studies.
The chemical synthesis of the trisaccharides represented a considerable challenge
because it required accommodation of an acetyl ester, a pyruvyl ketal and a free amine.
To preserve the acetyl ester, base cleavable protecting groups needed to be avoided
during the deprotection steps. In addition, the presence of a carboxylic acid and free
amine in the final products complicated the selection of a linker for conjugation purposes,
and commonly employed linkers bearing an amino or carboxylic acid group could not be
used. Therefore, a thiol-modified linker was selected, which was installed at a late stage
of synthesis to avoid problems with poisoning the Pd-catalyst required for the removal of
benzyl ethers, which were selected for permanent protection. Furthermore, suitable C-2
amino protecting groups needed to be identified for selective amino group modification
and for controlling stereoselective outcomes of glycosylations. The latter includes a β-
linked 2-amino-mannoside that is challenging to install due to unfavorable steric and
electronic effects.17 The poor reactivity of pyruvylated donors18 further complicated the
preparation of the target compounds.
A synthetic strategy was designed employing building blocks 12-17 that are modified
by a carefully selected set of protecting groups (Fig. 2). The pyruvyl ketal is protected as
a benzyl (Bn) ester, which can be removed during the final hydrogenation step. A
combination of azido (N3), trichloroacetyl (TCA), t-butoxycarbonyl (Boc), and
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benzyloxycarbonyl (Cbz) were utilized as amino protecting groups19 and their differential
reactivity made it possible to selectively modify the various amino groups of the target
compounds. Furthermore, TCA at C-2 of 14 can perform neighboring group participation
during glycosylations to give selectively 1,2-trans-glycosides. Subsequently, TCA can be
reduced to the desired acetamido function by treatment with Bu3SnH and AIBN.20 The
Cbz protecting groups can be removed by catalytic hydrogenation in the presence of Boc
allowing selective installation of a thiol spacer on the amine of the linker. Next, Boc can
be cleavage by using TFA/DCM revealing the free amine of the target compounds.
Building block 12 has a 3-O-p-methoxybenzyl (PMB) ether, which can be selectively
removed to install an acetyl ester, while the 3-O-benzyl ether of 13 can be carried through
the final stages of the synthesis to afford targets having a C3 hydroxyl.
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Installation of the β-mannosamine linkage of the target trisaccharides proved to be
challenging. It is well known that pre-activation of a mannosyl donor protected with a
4,6-O-benzylidene acetal provides mainly β-mannosides.21 In this glycosylation,
preactivation results in the formation of an α-triflate which in an SN2 fashion is
displaced by a sugar alcohol to give a β-mannoside. Glycosylation through direct
displacement is favored due to conformational constraints and dipoles imposed by the
benzylidene acetal.22 This methodology has been successfully extended to the preparation
of 2-azido-2-deoxy-β-mannosides.23 Therefore, we were compelled to investigate
whether the 4,6-O-pyruvyl ketal of glycosyl donors such as 12 and 13 can direct
glycosylations to provide the required β-mannosyl product (Scheme 1). Thus, treatment
of these glycosyl donors with diphenylsulfoxide (DPS), trifluromethanosulfonic
anhydride (Tf2O), and 2,4,6-tri-tert-butyl pyrimidine (TTBP) to form in-situ a triflate,
was followed by the addition of acceptor 14 resulting in the formation of disaccharides 18
(71%) and 21 (74%), respectively (Scheme 1) as separable mixtures of α/β anomers
(α/β=1/1.2 and 1/1.1, respectively). These results indicate that a 4,6-O-pyruvyl ketal does
not exert β-directing effect and probably its axial benzyl ester induces an unfavorable
dipole that hinders direct displace of the α-triflate. The ether protecting group at C-3 of
donors 12 and 13 was important for the anomeric outcome of the glycosylations because
the use of a similar donor having an acetyl ester at this position gave only α-anomeric
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product. Similar effects on stereochemical outcomes have been reported when using
standard benzylidene protected thioglycosides bearing a C-3 ester.24
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Scheme 1. General Synthetic Strategy: (a) i) DPS, TTBP, Tf2O, DCM, -78 to -55 °C; ii) Add
acceptor dropwise, warm to RT, (18: 71%, α/β = 1/1.2) (21: 74%, α/β = 1/1.1); (b) DDQ, DCM,
PBS buffer pH 7.4, 84%; (c) Ac2O/pyr, 100%; (d) HF-Py, THF (22: 90%; 23: 88%); (e)
CF3C(NPh)Cl, Cs2CO3, DCM (24: 78%; 25: 82%); (f) 15, TMSOTf, DCM, -20 to 0 °C (26: 67%;
27: 56%); (g) AIBN, Bu3SnH, toluene; (h) Ac2O/pyr (28: 96%, 29: 75%, 2-steps).
We also explored the installation of the 1,2-cis-mannoside using a pyruvylated
glucosyl donor having a participating ester at C-2. Such a donor makes it possible to
install a 1,2-trans glucoside by exploiting neighboring group participation during
glycosylation. Next, the C-2 ester can be removed and the resulting 2-hydroxyl converted
into a leaving group for SN2 displacement with azide resulting in the formation of a β-
linked 2-azido-mannoside.25 Several pyruvylated glucosyl donors and glycosylation
conditions were examined but in each case a low yield of disaccharide was obtained
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probably due to the low reactivity of the donors (see Scheme S2). Another strategy was
attempted that involved installation of pyruvyl moiety at the disaccharide stage.26
Unfortunately, this strategy resulted in an inseparable mixture of diastereomers (see
Scheme S5 for details).
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Next, disaccharide 18 was treated with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) to remove the PMB ether27 to give 19 that was acetylated with Ac2O in pyridine.
The resulting disaccharide 20 was converted into glycosyl donor 24 by first removing the
anomeric TDS ether using HF-pyridine complex followed by reaction of the lactol 22
with N-phenyltrifluoroacetimidoyl chloride in the presence of cesium carbonate. A
TMSOTf catalyzed glycosylation of 24 with 15 yielded trisaccharide 26 as only the β-
glucoside due to neighboring group participation of the TCA function. Trisaccharide 27,
bearing a benzyl ether at the 3-O position of ManNAc, was prepared by a similar route
starting from disaccharide 21.
The TCA and azido moieties of 26 and 27 needed conversion into acetamido
functions. The presence of the acid sensitive Boc group precluded the use of Zn/acetic
acid, which is commonly used for such transformations. Instead, a radical reduction of 26
and 27 using AIBN and Bu3SnH was performed followed by acetylation with Ac2O to
provide the key trisaccharide intermediates 28 and 29 in excellent yield.
Deprotection of trisaccharides 28 and 29 with specific reagents provided target
compounds 1-6 (Scheme 2). Thus, catalytic hydrogenation of 28 and 29 with Pd(OH)2
resulted in the removal of the benzyl and benzyloxycarbonyl protecting groups. The free
amine of the pentyl linker on the resulting compounds 30 and 31 was reacted with 3,3'-
dithiobis-(sulfosuccinimidyl propionate) (DTSSP) and then DTT to install a thiol moiety
for conjugation purposes.28 Removal of the Boc protecting group on 32 and 33 with TFA
o
in DCM at 0 C gave targets 1 and 6 respectively. Compound 4 was obtained by first
treating 31 with LiOH to remove the acetyl ester, followed by modification of the amine
of the linker of 34 with DTSSP and DTT. Finally, cleavage of Boc on 35 using
TFA/DCM at 0 oC gave target 4.
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Scheme 2. General Synthetic Strategy: (a) Pd(OH)2, t-BuOH, H2O, H2 (1 atm.) (30: 67%; 31:
57%); (b) LiOH, THF, H2O (34: 73% from 31); (c) i) DTSSP, H2O, pH~8; ii) DTT, PBS buffer
(pH 7.4), (32: 73% from 30; 33: 91% from 31; 34: 99% from 34; (d) i) 1/3 TFA/DCM,
triethylsilane, 0 °C; ii) DTT, PBS buffer (pH 7.4), (1: 71% from 32; 6: 83% from 33; 4: 83%
from 35).
Target compounds 2, 3, and 5 having an acetamido group were prepared by first
o
treating 28 and 29 with TFA/DCM at 0 C followed acetylation with acetic anhydride to
afford the acetamido modified trisaccharides. The compounds were carried forward in a
manner similar to that illustrated in Scheme 2 to afford compounds 2, 3, and 5 (Schemes
S11, S12). The synthesis of the truncated targets employed a similar methodology
(Schemes S13-S16). The non-pyruvylated trisaccharide (7) was synthesized as previously
described.14
Binding studies. The S-layer of B. anthracis is comprised of two S-layer proteins (Sap
and EA1),5 and can be further decorated with up to twenty-two S-layer associated
proteins.2c The S-layer and S-layer associated proteins contain an SLH domain that
tethers the proteins to the cell surface via non-covalent binding to SCWP.16,29 Genetic
studies have indicated that pyruvylation of SCWP is essential for SLH binding.11
Furthermore, O-acetylation of SCWP appears to be important for deposition of some but
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not all S-layer and S-layer associated proteins, and may play a role in positioning proteins
at specific regions of the cell envelope.15
To establish the importance of specific structural features of SCWP for interacting
with various S-layer and S-layer associated proteins, the binding of the synthetic
compounds 1-11 with recombinant SLH domains of Sap, EA1, BslA, BSlI, BslK, BslM,
BslO, BslP, BslR, BslS, BslT, and BslU fused to mCherry16 was investigated. To rapidly
provide qualitative binding information, a microarray screening assay was developed.30
Thus, compounds 1-10, having a thiol containing linker, were immobilized onto
maleimide functionalized glass slides by piezoelectric microarray printing. The twelve
mCherry fusion proteins were screened for binding using an anti-mCherry antibody
followed by addition of secondary antibody modified by a fluorescent dye for detection.
This approach was required because mCherry was not sufficiently fluorescent to allow
direct detection of binding.
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Figure 3A shows a representative microarray result for Sap. Trisaccharides 1-6,
bearing different patterns of acetyl esters and free amino groups, gave similar
responsiveness. Compound 7, lacking the 4,6-O-pyruvyl ketal, did not exhibit binding,
which is in agreement with genetic studies that had indicated that this functionality is
critical for S-layer formation.11,31 Monosaccharide 8, having 4,6-O-pyruvyl ketal on
ManNAc, showed binding to Sap, indicating that the terminal monosaccharide of CSWP
is the key epitope for binding. The latter was supported by a lack of binding of
monosaccharides 9-10 that have perturbations in the C-2 acetamido moiety. Microarray
screening for binding for the other proteins indicated subtle differences in ligand
requirements of the examined proteins (see Figure S1).
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B.
6 0 0 0 0 0
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2 0 0 0 0 0
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C o m p o u n d
Figure 3. Microarray and ELISA results for the binding of Sap with compounds 1-11. A.
Compounds 1-11 were printed onto maleimide functionalized glass slides by piezoelectric
microarray printing. A fusion protein of the SLH domain of Sap with mCherry (0.5 µg/mL) was
screened for binding using an anti-mCherry antibody followed by addition of a fluorescently
labeled secondary antibody for detection. B. Dose response curves for Sap determined by ELISA.
Errors displayed as 95% confidence intervals.
Next, binding affinities of the synthetic compounds with the various SLH domains
was determined by ELISA (Figures 3B and S2). Thus, the thiol containing
oligosaccharides were conjugated to BSA modified by maleimide and the resulting
neoglycoconjugates were used for coating microtiter plates. Different concentrations of
the various SLH domains were exposed to the coated micro-titer plates to determine EC50
values. As can be seen in Table 1, all trisaccharides bearing a 4,6-O-pyruvyl ketal
(compounds 1-6) were recognized by the panel of proteins. O-acetylation of β-GlcNAc
(1 vs. 4, 2 vs. 5) or ManNAc (3 vs. 5, 4 vs. 6) had no or only a small effect on binding. In
a number of cases, O-acetylation of β-GlcNAc led to a slight decrease in binding (1 vs.
4, Sap and BslP). Furthermore, the free amine bearing compounds 1, 4, and 6 did not
show a substantial difference in binding compared to their acetamido containing
counterparts 2, 3, and 5, respectively.
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Table 1. EC50 values (mg/mL) of compounds 1-11 binding with the SLH domains of the various
proteins.
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9
Compound Sap
EA1
BslP
BslA
BslI
BslS
BslT
BslU
BslK
BslO
0.029
0.039
0.023
0.022
0.037
0.023
NB
BslR
0.037
0.060
0.039
0.023
0.054
0.027
NB
BslM
0.033
0.048
0.043
0.026
0.040
0.024
NB
1
2
0.042 0.015 0.061 0.005 0.020 0.025 0.028 0.013 0.035
0.059 0.021 0.101 0.007 0.026 0.035 0.034 0.017 0.048
0.040 0.016 0.101 0.006 0.023 0.050 0.027 0.017 0.040
0.026 0.016 0.045 0.004 0.016 0.043 0.023 0.012 0.028
0.048 0.020 0.077 0.006 0.024 0.032 0.030 0.017 0.044
0.030 0.013 0.041 0.004 0.015 0.021 0.022 0.011 0.028
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7
NB
NB
NB
NB
NB
NB
NB
NB
NB
8
0.029 0.015 0.063 0.006 0.025 0.067 0.036 0.013 0.032
0.021
NB
0.031
0.035
9
NB
NB
NB
NB
NB
NB
0.116 0.022 0.076 NB
0.121 0.015 0.066 NB
0.036 0.006 0.027 NB
NB
NB
NB
0.063 0.077
>0.166 0.075
>0.192 0.066
10
11
0.112 >0.177 NB
0.077 >0.132 >0.111 0.073
0.052
NB = no binding
The binding study supports that the terminal ManNAc moiety of SCWP modified by
a 4,6-ketal pyruvyl ketal functions as the anchoring point of S-layer and S-layer
associated proteins.11 It indicates that the role of O-acetylation of SCWP in S-layer
formation is more complex than previously thought,15 and does not support a model in
which SCWP acetylation is a prerequisite for the direct association of some (ea. EA1,
BslO, and BslA) but not of other S-layer and S-layer associated proteins. Alternative
models are therefore more probable and O-acetylation of SCWP may for example be
important for the proper localization of other proteins involved in S-layer formation such
as the translocases SecA2 and SlaP that have been implicated in the transport of specific
S-layer proteins to the cell surface.32 The binding studies have been performed with
individual proteins and does not probe possible retention of proteins through protein-
protein complexes. In this respect, it may be possible that deposition of one protein that
requires O-acetylated SCWP mediates the retention of other proteins.
Another interesting finding is that the terminal ManNAc moiety modified by a
pyruvyl ketal is the minimal epitope for binding of all examined S-layer and S-layer
associated proteins and monosaccharide 8 exhibited similar bindings (EC50 values)
compared to the trisaccharide counterparts. However, we observed differences in the
requirement of the axial acetamido moiety (monosaccharides 8 vs. 9 and 10 vs. 11) which
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is critical for Sap, EA1, BslS and BslT, but not for BslP, BslA, BslL, BslU, BslK, BslR,
and BslM, indicating that the SLH domains of various proteins utilize different structural
features for binding, and further studies33 are required to confirm such a mode of binding.
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CONCLUSIONS
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While S-layer and S-layer-associated proteins of B. anthracis play key roles in many
important biological processes, mechanisms by which these proteins are deposited at the
cell envelope to perform their functions are not well understood. We have designed and
prepared a range of oligosaccharides derived from the terminal repeating unit of SCWP
to examine whether specific functional groups, including 4,6-O-pyruvyl ketal and acetyl
esters, can regulate the deposition of specific S-layer and S-layer associated proteins. We
developed a synthetic strategy that addresses difficulties related to the synthesis of
pyruvylated oligosaccharides derived from B. anthracis SCWP bearing base labile esters
and free amino groups. Binding studies of a wide range of S-layer and S-layer associated
proteins with the resulting synthetic compounds provided direct biophysical evidence that
SCWP functions as the anchoring point of these proteins and confirmed that the pyruvyl
group is the critical binding determinant. Importantly, a single pyruvylated ManNAc
monosaccharide can provide strong binding to the S-layer proteins making it a promising
structure for S-layer protein manipulation. The presence of acetyl esters and free amine
did not have substantial effects on the binding of the various SLH proteins. Therefore, it
is likely that the acetyl esters influence the composition of the S-layer in indirect manners
by for example modulating the localization of other proteins involved in S-layer
formation.
ASSOCIATED CONTENT
Supporting Information
Preparation of the starting materials, procedures for the assembly of the oligosaccharides,
and copies of NMR spectra. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
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*G.J.H.P.Boons@uu.nl or gjboons@ccrc.uga.edu
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ORCID
Geert-Jan Boons: 0000-0003-3111-5954
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Author Contributions
⊥R.N.C. and L.L. contributed equally.
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Notes
The authors declare no conflict of interest.
ACKNOWLEDGMENTS
Research reported in this publication was in part supported by the Centers for Disease
Control and Prevention (to G.-J.B.). The findings and conclusions in this report are those
of the author(s) and do not necessarily represent the official position of the Centers for
Disease Control and Prevention. Recombinant SLH domains of Sap, EA1, BslA, BSlI,
BslK, BslM, BslO, BslP, BslR, BslS, BslT, and BslU fused to mCherry were kindly
provided by Dr. Justin Lunderberg and Prof. Olaf Schneewind, Department of
Microbiology, University of Chicago.
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A series of oligosaccharides derived from the terminal repeating unit of the secondary
cell wall polysaccharide of Bacillus anthracis has been synthesized to probe molecular
mechanisms of depositing S-layer and S-layer associated protein in the cell envelope.
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