Communications
DOI: 10.1002/anie.201002050
Protein Identification
Orthogonal Alkynyl Amino Acid Reporter for Selective Labeling of
Bacterial Proteomes during Infection**
Markus Grammel, Mingzi M. Zhang, and Howard C. Hang*
Bacterial pathogens have evolved sophisticated mechanisms
to evade host defenses and cause disease.[1] The emergence of
new and antibiotic-resistant bacterial pathogens demands a
better understanding of virulence mechanisms for antibacte-
rial-drug discovery. Although the discovery of bacterial
toxins, quorum sensing, and protein-secretion pathways has
revealed some key virulence mechanisms, the precise mech-
anisms by which intracellular bacterial pathogens subvert host
immune responses are still unclear.[1] The analysis of individ-
ual virulence factors has demonstrated that bacterial patho-
gens alter their protein expression to infect and replicate in
host tissues.[2] However, the system-wide identification and
analysis of bacterial proteins that are uniquely expressed or
secreted during infection is paramount for understanding
mechanisms of bacterial pathogenesis.[2,3] Comparative
genomics and mutagenesis studies have revealed bacterial
genes that are important for infection, but their precise
biochemical mechanisms and temporal expression patterns
can be elusive as a result of posttranscriptional regulation.[2,3]
Direct biochemical analysis of bacterial proteomes during
infection is needed.[3] The large excess of host proteins in
mixed pathogen–host lysates presents a significant challenge
for proteomic analysis of bacterial proteins during infec-
tions,[3] and even after the physical isolation of intact bacteria,
significant amounts of host proteins still remain.[4,5] This
factor is particularly important since many bacterial virulence
factors are often expressed at low levels.[2] New strategies are
therefore required to selectively enrich bacterial proteins
from host proteomes for their analysis during infection.
The incorporation of nonnatural amino acids in bacteria
has provided new methods to differentiate bacterial proteins
from host proteomes. For example, the incorporation of
phenylalanine analogues in mycobacteria by amber-stop-
codon-suppression technology has enabled the selective
labeling of green fluorescent protein expressed in Mycobac-
terium tuberculosis during the intracellular infection of
macrophages.[6] Alternatively, alkyne- or azide-functionalized
methionine (Met) surrogates can be incorporated by the
endogenous methionyl-tRNA synthetase (MetRS) into bac-
terial proteomes.[7,8] These amino acid chemical reporters
enable the metabolic labeling and detection of newly
synthesized proteins using bioorthogonal ligation methods,
ssuch as the copper(I)-catalyzed azide–alkyne cycloaddi-
tion.[10] Additionally, MetRS mutants have been identified
that can incorporate azidonorleucine (ANL, Figure 1a),[11,12]
a Met surrogate that is not efficiently activated by the wild-
type (wt) MetRS or other endogenous aminoacyl-tRNA
synthetases.
ANL can therefore be used as an orthogonal amino acid
reporter to selectively label proteins in nonpathogenic
Escherichia coli by the bacterial expression of MetRS mutants
in the presence of mammalian cells.[13] Although these studies
have demonstrated the selective targeting of bacterial pro-
teomes in the presence of host cells,[6,13] the use of nonnatural
amino acid reporters for the analysis of endogenously
expressed bacterial proteins during infection has not been
reported. Herein, we report a new orthogonal alkynyl amino
acid reporter for the specific imaging and enrichment of
bacterial proteomes during the infection of mammalian cells
with the Gram-negative intracellular bacterial pathogen
Salmonella typhimurium (Figure 1b).
On the basis of the reported superior selectivity of azide-
over alkyne-functionalized secondary CuAAC reagents[14]
and our own experience with fatty-acid chemical reporters,
we evaluated whether an alkynyl isostere of ANL, 2-amino-
octynoic acid (AOA, Figure 1a), could be accepted by
previously reported MetRS mutants in S. typhimurium.[13,15,16]
MetRS mutants were generated from the E. coli metG gene
(the S. typhimurium metG gene is 95% identical to E. coli
metG) by site-directed mutagenesis and ligated into the low-
copy-number plasmid pWSK29 under the expression control
of the lac promoter, which provides constitutive expression in
S. typhimurium. All plasmids were transformed into the
S. typhimurium strain IR715. AOA was synthesized by the
alkylation of diethyl acetamidomalonate with hex-5-ynyl-4-
methylbenzenesulfonate and sequentially deprotected to
yield the racemic product (see Scheme 1 in the Supporting
Information). For the in vitro analysis of the S. typhimurium
strains expressing MetRS mutants, bacteria were grown in full
Luria–Bertani (LB) medium to the stationary phase and
diluted into minimal medium containing Met or AOA.
Protein lysates were reacted with the azido-rhodamine (az-
Rho)[15] detection tag by CuAAC and analyzed by SDS-
PAGE and in-gel fluorescence scanning. AOA selectively and
[*] M. Grammel, M. M. Zhang, Dr. H. C. Hang
Laboratory of Chemical Biology and Microbial Pathogenesis
The Rockefeller University
1230 York Avenue, New York, NY 10065 (USA)
Fax: (+1)212-327-7276
E-mail: hhang@rockefeller.edu
[**] We thank The Rockefeller University Proteomics Resource Center for
mass spectrometry analysis, James Flexner for help with the
azidonorleucine synthesis, and members of the Hang Laboratory for
providing secondary detection reagents. M.M.Z. is supported by
A*STAR, Singapore. H.C.H. acknowledges support from The Rock-
efeller University, the Irma T. Hirschl/Monique Weill-Caulier Trust,
the Lerner Trust, and the Northeastern Biodefense Center NIH/
NIAID (2 U54 AI057158-06).
Supporting information for this article is available on the WWW
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 5970 –5974