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REVIEW ARTICLE
Chemical approaches to mapping the function
of post-translational modifications
David P. Gamblin, Sander I. van Kasteren, Justin M. Chalker and Benjamin G. Davis
Chemistry Research Laboratory, Department of Chemistry, University of Oxford, UK
Introduction
Post-translational modifications (PTMs) of proteins
modulate protein activity and greatly expand the diver-
sity and complexity of their biological function. The
ubiquity of PTMs is reflected in their widespread roles
in signaling, protein folding, localization, enzyme acti-
vation, and protein stability [1–3]. Indeed, the preva-
lence of such modifications in higher organisms, such
as humans, is a leading candidate for the origin of
such complex biological functions [4], which may arise
from a comparatively restricted genetic code [5–7]. As
a consequence of the lack of direct genetic control of
their biosynthesis, natural PTMs vary in site and level
of incorporation, leading to mixtures of modified pro-
teins that may differ in function. In order to fully dis-
sect the biological role of PTMs and determine precise
structure–activity relationships, access to pure protein
derivatives is essential. One approach is to exploit the
fine control that may be offered by chemistry [4]. A
combination of chemical, enzymatic and biological
augmentation strategies can provide a modification
process that occurs with the chemoselectivity and regio-
selectivity that is often lacking in the natural produc-
tion of post-translationally modified proteins [8]. This
allows the construction not only of post-translationally
Keywords
chemoselective ligation; post-translational
modification; protein glycosylation; protein
modification; synthetic proteins
Correspondence
B. G. Davis, Chemistry Research
Laboratory, 12 Mansfield Road,
Oxford OX1 3TA, UK
Fax: 44 (0) 1865 285 002
Tel: 44 (0) 1865 275652
E-mail: ben.davis@chem.ox.ac.uk
Website: http://www.chem.ox.ac.uk/
researchguide/bgdavis.html
Note
Taken in part from Young Investigator
Award lecture delivered to the MPSA 2006
meeting in Lille
(Received 18 July 2007, revised 10 February
2008, accepted 21 February 2008)
doi:10.1111/j.1742-4658.2008.06347.x
Strategies for the chemical construction of synthetic proteins with precisely
positioned post-translational modifications or their mimics offer a powerful
method for dissecting the complexity of functional protein alteration and
the associated complexity of proteomes.
Abbreviations
EPL, expressed protein ligation; glycoMTS, glycosyl methanethiosulfonates; glycoSeS, selenenylsulfide-mediated glycosylation;
MTS, methanethiosulfonates; NCL, native chemical ligation; PTM, post-translational modification; SBL, subtilisin Bacillus lentus.
FEBS Journal 275 (2008) 1949–1959 ª 2008 The Authors Journal compilation ª 2008 FEBS 1949
modified proteins but also of their mimics [4,9,10]. The
chemical motif introduced should thus be sufficiently
similar to the natural modification to mimic its func-
tion; varying this chemical appendage presents the
opportunity for imparting different or enhanced bio-
logical activity.
Among PTMs, protein glycosylation is the most pre-
valent and diverse [11,12]. The glycans on proteins
play key roles in expression and folding [13], thermal
and proteolytic stability [14], and cellular differentia-
tion [15]. Carbohydrate-bearing proteins also serve as
cell surface markers in communication events such as
microbial invasion [16], inflammation [17], and
immune response [11,12]. The study of these events is
taxing, as the biosynthesis of glycoproteins is not tem-
plate driven. This results in the formation of so-called
‘glycoforms’ [11,12], proteins with the same peptide
backbone that differ in the nature and site of glycan
incorporation. Ready access to homogeneous glyco-
forms is hampered by inadequate separation technol-
ogy that has afforded homogeneous glycoproteins only
in rare instances [18]. The limited availability of singu-
lar glycoforms has prompted a concerted effort to
develop new methods for their synthesis [8].
Biological methods to obtain glyco-
proteins
The natural expression of glycoproteins is highly
dependent on the host cell glycosylation machinery.
However, the re-engineering of the glycosylation path-
way in the yeast Pichia pastoris has resulted in near-
homogeneous expression [19–23], although, at present,
this method lacks flexibility and non-natural variants
are not tolerated. The examples of pure glycans dis-
played on recombinant proteins are therefore limited,
thus far, to only a few structures such as the bianten-
nary structure GlcNAc
2
Man
5
GlcNAc
2
[20] and its
extended variants Gal
2
GlcNAc
2
Man
3
GlcNAc
2
[19] and
Sia
2
Gal
2
GlcNAc
2
Man
3
GlcNAc
3
[21].
An alternative approach exploits ‘misacylated’
tRNAs in codon suppression read-through techniques
to produce homogeneous glycoproteins [24]. In vivo
evolution of a tRNA synthetase–tRNA pair from
Methanococcus jannaschii capable of accepting and
loading glycosylated amino acids has allowed the
introduction of O-b-d-GlcNAc-l-Ser [25] and
O-a-d-GalNAc-l-Thr [26] into proteins with efficien-
cies of 96% and  40% respectively.
In addition to expression-based approaches, biocata-
lytic methods can allow the so-called remodeling of
modifications such as glycosylation. Endoglycosidases
and glycosyltransferases have been used to modify
existing glycoforms, e.g. in the creation of a single
unnatural glycoform of enzyme RNaseB [27] catalyzed
by the glycoprotein endoglycosidase enzyme endo A
using novel synthetic oxazoline oligosaccharide
reagents [28,29].
The above solely biological methods offer great
potential. However, despite the impressive results listed
above, these strategies may be limited by the often
stringent specificity of natural catalytic machinery in a
way that can limit their versatility and general applica-
tion to modified protein (glycoprotein) synthesis.
Chemical strategies in glycoprotein
synthesis
The chemical attachment of glycans offers an alterna-
tive, pragmatic route to homogeneous glycoproteins.
Chemical methods can be divided into two complemen-
tary strategies [4] (Fig. 1): linear assembly, such as the
introduction of a well-defined modified peptide (glyco-
peptide) into a larger peptide backbone; and convergent
assembly, such as chemoselective ligation of a modifica-
tion (glycoside) to a side chain in an intact protein scaf-
fold. These terms reflect not only the linearity or
convergence of the chemical steps that may lead to a
given synthetic protein, but also the structural strategy
that links the (linear) segments of the protein backbone
or (convergently) attachs components ⁄ modifications to
this backbone (typically to residue side chains) with
little or no alteration of the backbone itself.
In linear assembly, small modified peptides (glyco-
peptides and glycoamino acids) can be ligated to other
peptide fragments. Linear assembly methods include
the use of native chemical ligation (NCL) [30], which
has been applied to form, for example, unmodified
protein barnase [31] and a poly(ethylene glycol)-modi-
fied variant of erythropoeitin (EPO) [32]. More
recently, the use of expressed protein ligation (EPL)
has provided access to larger peptide fragments. Mac-
millan et al. have used EPL to construct three well-
defined model GlyCAM-1 glycoproteins [33], the first
reported modular total synthesis of a biologically rele-
vant glycoprotein. The immediate compatibility of
NCL and EPL methods has led to their widespread
adoption. Other methods, however, also provide
emerging alternatives, such as traceless Staudinger pep-
tide [34] ligation and protease-mediated peptide liga-
tion [35,36].
Not withstanding these clear demonstrations of the
utility of linear ligation assembly, a convergent chemo-
selective approach can offer the key advantages of
more ready and flexible modification of a well-defined
protein structure. While also developing novel methods
Exploring post-translational modification D. P. Gamblin et al.
1950 FEBS Journal 275 (2008) 1949–1959 ª 2008 The Authors Journal compilation ª 2008 FEBS
for linear assembly [36], it is this convergent strategy
that we have typically adopted in our own efforts in
the synthesis and study of precisely modified proteins.
The central strategic concept behind this convergent
chemical protein modification (glycosylation) is one of
‘tag and modify’ (Fig. 2): the introduction of a tag into
the protein backbone followed by chemoselective mod-
ification of that tag. This allows for greater flexibility
in choice of protein, carbohydrate and modification
(glycosylation) site.
With the relatively low abundance and unique reac-
tivity profile of cysteine, S-linked chemical modifica-
tions are attractive targets for selective, well-defined
PTM mimicry. In protein glycosylation, surface-
exposed cysteine residues can be alkylated [37–39] or
converted to the corresponding disulfide [40]. Further-
more, when it is used in combination with site-directed
mutagenesis [41,42], glycans of choice can be intro-
duced at any predetermined site. First-generation
disulfide-forming reagents such as glycosyl methane-
thiosulfonates (glycoMTS) or phenylthiosulfonates
provided reliable access to homogeneous glycoproteins
with high efficiency [41,43]. These allowed the first
examples of the systematic modulation of enzyme
activity [amidase and esterase activity of the serine
protease subtilisin Bacillus lentus (SBL)] and demon-
strated not only precise glycosylation but also the
dependence of activity on the exact site and identity of
the disulfide-linked glycan [44].
Interestingly, judicious site selection for incorpora-
tion of a desired PTM revealed the dramatic effects of
‘polar patch’ modifications [45,46]. Precisely intro-
duced charged modifications converted the protease
SBL into an improved biocatalyst in peptide ligation.
Particularly striking was the broad substrate tolerance
that could be engineered (e.g. towards non-natural
amino acids) by appropriate incorporation of the polar
domain [47]. In an example that combines the explora-
tion of two modes of modification, ‘polar patch’-modi-
fied enzymes have also been applied to the catalysis of
glycan-modified glycopeptide ligation [36].
Our early success using glycoMTS-mediated protein
glycosylation along with a rich history of modifications
using MTS reagents [48] highlighted the method as a
general tool in protein modification, and we have since
used this chemistry in a variety of site-selective ‘tag
and modify’ reactions, reliably incorporating desired
functionality or PTM. For instance, a library of ‘cata-
lytic antagonists’ was engineered for affinity proteolysis
by incorporation of a variety of ligands onto protease
SBL, including examples of natural PTMs such as
biotinylation and d-mannosylation (Fig. 3) [49]. The
pendant ligands allowed SBL to selectively bind a
protein target or partner and, by virtue of proximity,
Fig. 1. Two complementary chemical strat-
egies for mimicking PTM. Taken from [4].
D. P. Gamblin et al. Exploring post-translational modification
FEBS Journal 275 (2008) 1949–1959 ª 2008 The Authors Journal compilation ª 2008 FEBS 1951
catalyze enhanced hydrolytic degradation of the target
protein.
More recently, the glycoMTS method has allowed
the synthesis of the first examples of a homogeneous
protein bearing symmetrically branched multivalent
glycans [50,51]. This new class of glycoconjugate, the
‘glycodendriprotein’, exists in two-arm, three-arm or
four-arm variants tipped with sugars. These are
designed to mimic the branching levels in complex
N-glycans, which come in bi-antennary, tri-antennary
and tetra-antennary form. For example, the synthe-
sized divalent, trivalent and tetravalent d-galacto-
syl-tipped glycodendriproteins effectively mimicked
glycoproteins with branched sugar displays, as indi-
cated by a high level of competitive inhibition of the
coaggregation between the pathogen Actinomyces naes-
lundii and its copathogen Streptococcus oralis. This
inhibition, when coupled with targeted pathogen
degradation, offers therapeutic potential for the treat-
ment of opportunistic pathogens [50,51].
This ‘tag and modify’ two-step approach has proved
a widely successful strategy for site-selective glycosyla-
tion, used by several groups. For example, Flitsch
et al. have employed glycosyliodoacetimides to site-
selectively modify erythropoietin [52]. A similar
strategy has been reported by Withers et al. where
glycosyliodoacetimides were used in conjunction with
site-selective modification of the protein endoxylanase
from Bacillus circulans (Bcx) [53]. A protected thiol-
containing sugar was conjugated and then chemically
exposed before enzymatic extension. Boons et al. have
used aerial oxidation and disulfide exchange to form
homogeneous disulfide-linked glycoproteins via a
cysteine mutation in the Fc region of IgG
1
[42,54].
More recently, second-generation thiol-selective pro-
tein glycosylation reagents that rely upon selenenyl-
Fig. 2. The ‘tag and modify’ strategy behind convergent modification, illustrated here for dual tag and dual modify. Taken from [10].
Exploring post-translational modification D. P. Gamblin et al.
1952 FEBS Journal 275 (2008) 1949–1959 ª 2008 The Authors Journal compilation ª 2008 FEBS
sulfide-mediated glycosylation (glycoSeS) have greatly
improved the efficiency of ‘tag and modify’ methods
[55]. In this approach, cysteine-containing proteins and
glycosyl thiols combine through phenyl selenenylsulfide
intermediates (Fig. 4). Preactivation of either the cyste-
ine mutant protein or thiosugar is possible following
exposure to PhSeBr.
GlycoSeS was initially demonstrated on simple
cysteine-containing peptides, and then shown to be
successful on a variety of different proteins, highlight-
ing its versatility for glycosylation in a variety of pro-
tein environments. This high-yielding procedure also
provided the first example of multisite-selective glyco-
sylation with the same glycan and the coupling of a
AB
Fig. 3. (A) The use of a thiol ‘tag and modify’ strategy allowed site-selective attachment of natural PTMs such as biotin (1) and D-mannose
(2) that, in turn, acted as ‘homing’ ligands for affinity proteolysis of target PTM-binding proteins. (B) A ring of modification sites (blue) around
the active site (red) of the modified protease was explored. Taken from [49].
Fig. 4. Two complementary routes in glyco-SeS: protein activation and glycosyl thiol activation. The disulfide-linked glycoproteins were
then readily processed in on-protein transformations catalyzed by glycosyltransferases, leading to, for example, a sialyl Lewis
X
-tetrasaccha-
ride glycan.
D. P. Gamblin et al. Exploring post-translational modification
FEBS Journal 275 (2008) 1949–1959 ª 2008 The Authors Journal compilation ª 2008 FEBS 1953
heptasaccharide. Importantly, the reaction proceeds to
completion using, in some cases, as little as one equiv-
alent of glycosylating reagent. This is a great improve-
ment on the sometimes greater than 1000 molar
equivalents used in standard protein modification
chemistry [8]. Furthermore, the disulfide-linked glyco-
protein was readily processed by glycosyltransferases,
as demonstrated by the enzymatic b-1,4-galactosylation
of an N-acetylglucosaminyl-modified SBL protein.
Recently, we have managed to further extend this
disaccharide using additional glycosyltransferases to
create, for example, sialyl Lewis
X
-tetrasaccharide on
the surface of the protein. Quantitative conversions
can be obtained for the chemical glycosylation and
each of these subsequent enzymatic glycosylations,
leading ultimately to one pure glycoform being
detected after chemical modification and each of three
successive enzymatic extensions. This maintenance of
purity compares favorably with enzymatic extensions
performed on other natural and unnaturally linked
glycoproteins [35,56]. We have also demonstrated
enzymatic extensions on complex-type and branched
oligosaccharides in synthetic glycoproteins.
Many of the above methods depend on a ready
source of glycosyl thiol. To aid their preparation from
natural sources, we have recently developed a novel
direct thionation reaction for both protected and
unprotected reducing sugars [57]. This allows the direct
synthesis of glycosyl thiols from naturally sourced,
unprotected glycans, which can then can be attached
using glycoSeS to proteins in a one-pot protein glyco-
sylation method [55]. Thus, natural sugars can be
stripped from a natural protein and reinstalled site-
selectively into an alternative protein scaffold of
choice.
To further explore the potential of selenenylsulfide-
mediated ligation in creating post-translationally modi-
fied proteins, we have mimicked protein prenylation
(Fig. 5). The attachment of prenyl moieties to protein
scaffolds is required for the correct function of the
modified protein [58], either as a mediator of mem-
brane association or as a determinant for specific
protein–protein interactions [59,60]. Furthermore, such
prenylated proteins have been shown to play crucial
roles in many cellular processes, such as signal trans-
duction [61], intracellular trafficking [62,63], and cyto-
skeletal structure alterations [64]. In order to fully
probe and access well-defined prenylated proteins, we
have recently developed a novel thionation reaction for
the direct conversion of prenyl alcohols to the corre-
sponding thiol, thereby allowing direct compatibility
with selenenylsulfide protein conjugation (D. P. Gam-
blin, S. I. van Kasteren, G. J. L. Bernardes, N. J. Old-
ham, A. J. Fairbanks & B. G. Davis, manuscript in
preparation). These preliminary results not only repre-
sent the first examples of site-selective protein lipida-
tion, but also demonstrate the dramatic effect of
prenylation upon the physical properties of the pro-
tein.
The construction of disulfide-linked post-transla-
tionally modified protein mimics has also been used
to explore dynamic regulatory PTMs such as tyrosine
phosphorylation [65,66] and glutathionation [67,68].
In all cases, the post-translationally modified protein
mimics displayed native biological responses in, for
example, antibody screening, highlighting the use of
chemistry to further adapt and enhance protein func-
tion.
Dual differential modification
In nature, modified proteins such as glycoproteins
often carry more than one distinct glycan on their sur-
face. In order to access dual, differentially modified
Fig. 5. A novel thionation reaction allows for the first examples of site-selective chemical protein prenylation.
Exploring post-translational modification D. P. Gamblin et al.
1954 FEBS Journal 275 (2008) 1949–1959 ª 2008 The Authors Journal compilation ª 2008 FEBS
proteins, orthogonal methodologies are required. A
strategy based on a combination of site-directed muta-
genesis, unnatural amino acid incorporation, a cop-
per(I)-catalyzed Huisgen cycloaddition [69,70] and
MTS reagents has successfully been used in the first
syntheses of doubly modified glycoproteins (Figs 2 and
6) [10].
The chemical protein tags were introduced through
site-directed mutagenesis and incorporation of either
azido- or alkyne-containing residues through methio-
nine replacement in an auxotrophic Escherichia coli
strain [71,72]. Treatment of these unnatural residues
with either propargylic or azido glycosides, respec-
tively, provided triazole-linked glycoproteins. This
double modification strategy was used to mimic a
putative glycoprotein domain of human Tamm–Hors-
fall protein, which carries two glycans, and the intro-
duction of two glycans onto a galactosidase (lacZ)
reporter protein. In all cases, the proteins maintained
native function as well as being endowed with
additional lectin-binding properties. The two methods
of modification, although employing different chemis-
tries, may be used in a complementary manner. They
are also mutually compatible (orthogonal), allowing
the chemistry to be performed in either order. The
disulfide formation method is more rapid than
the cycloaddition method, but under optimized
conditions, both allow complete conversion in a matter
of hours.
As a demonstration of the biological relevance, this
methodology was used to model the P-selectin-binding
domain of the mucin-like glycoprotein PSGL-1 [73,74].
This ligand is involved in the initial homing of leuko-
cytes to sites of inflammation [73,74]. The binding of
PSGL-1 to P-selectin is largely due to two PTMs,
namely an O-glycan that contains tetrasaccharide
sialyl-Lewis
X
, and a sulfated tyrosine [74]. By careful
selection of the amino acid residue accessibility and
Fig. 6. The use of orthogonal chemoselective strategies allows for multisite-selective differential protein glycosylation. Taken from [10].
D. P. Gamblin et al. Exploring post-translational modification
FEBS Journal 275 (2008) 1949–1959 ª 2008 The Authors Journal compilation ª 2008 FEBS 1955
inter-residue distance on the lacZ-reporter protein, the
PSGL-1 binding domain was imitated after modifica-
tion with a copper(I)-catalyzed Huisgen cycloaddition-
reactive sialyl Lewis
X
sugar and an MTS sulfonate as
a mimic of the tyrosine sulfate. Binding of this
PSGL-1 mimic to human P-selectin was shown by
ELISA. This PSGL mimic also retained its inherent
galactosidase activity. This dual-function, synthetic
protein is therefore an effective P-selectin ligand, while
simultaneously serving as a lacZ-like reporter. This
mimic, named PSGL-lacZ, was subsequently used for
the monitoring of acute and chronic inflammation in
mammalian brain tissue both in vitro and in vivo,
including in the detection of cerebral malaria.
Retooling of this reporter system also allowed sys-
tematic investigation of the role of GlcNAc-ylation as
a potentially important and emerging protein PTM
process [75]. Using a synthetic glycoprotein reporter
GlcNAc–lacZ, specific binding was detected with the
mouse innate immunity protein DC-SIGN-R2. This
synthetic protein probe also selectively bound to the
nuclei of a neuron subpopulation, with no binding to
the nuclei of glial cells. This result suggests that neu-
rons display selective GlcNAc-binding proteins, an
intriguing result in the light of previous work on the
proposed role of GlcNAc regarding both the nuclear
localization of Alzheimer’s-associated protein Tau [76]
and nuclear shuttling in Aplysia neurons [77]. This
work also illustrates that synthetic protein probes can
be highly effective in a manner that is complementary
to other protein probes such as monoclonal antibodies.
Future directions
Over the last few years, chemical protein glycosylation
has become a powerful tool for accessing and studying
the roles of single glycoforms [8]. In order to mimic
nature’s full arsenal of PTMs, the development of
additional mutually orthogonal strategies is needed.
This may require targeting traditionally ignored resi-
dues and application of transformations common to
organic synthesis but not yet amenable to protein
modification. As new methodologies emerge, the study
of other PTMs and that of regulatory PTMs will hope-
fully provide a powerful tool for shedding light on cer-
tain key processes in vivo and perhaps on one of the
origins of biological complexity itself.
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