Ligand-induced dimerization of Drosophila peptidoglycan recognition proteins in vitro Peter Mellroth*, Jenny Karlsson*, Janet Håkansson*, Niklas Schultz*, William E. Goldman†, and Håkan Steiner*‡ *Department of Genetics, Microbiology, and Toxicology, University of Stockholm, S-106 91 Stockholm, Sweden; and †Department of Molecular Microbiology, Washington University School of Medicine, Campus Box 8230, 660 South Euclid Avenue, St. Louis, MO 63110-1093 Edited by N. Avrion Mitchison, University College London, London, United Kingdom, and approved March 1, 2005 (received for review October 12, 2004)
Drosophila knockout mutants have placed peptidoglycan recognition proteins (PGRPs) in the two major pathways controlling immune gene expression. We now examine PGRP affinities for peptidoglycan. PGRP-SA and PGRP-LCx are bona fide pattern recognition receptors, and PGRP-SA, the peptidoglycan receptor of the Toll兾Dif pathway, has selective affinity for different peptidoglycans. PGRP-LCx, the default peptidoglycan receptor of the Imd兾Relish pathway, has strong affinity for all polymeric peptidoglycans tested and for monomeric peptidoglycan. PGRP-LCa does not have affinity for polymeric or monomeric peptidoglycan. Instead, PGRP-LCa can form heterodimers with LCx when the latter is bound to monomeric peptidoglycan. Hence, PGRP-LCa can be said to function as an adaptor, thus adding a new function to a member of the PGRP family.
subunits as shown in studies using RNA interference (4). The role of the third splice form, LCy, is still elusive because RNA interference knockdown of its expression does not affect the Imd兾Relish signaling upon challenge with peptidoglycan (4, 14). In this study we have used a biochemical approach to examine the relative affinities of these receptors for different types of cell wall preparations. We show that PGRP-SA is selective in its affinity to different polymeric peptidoglycans. PGRP-LCx binds strongly to all peptidoglycans tested, both polymeric and monomeric. Binding of monomeric peptidoglycan to PGRP-LCx confers affinity of LCx to PGRP-LCa. These results suggest that dimerization of the PGRP-LCx receptor can be accomplished by two different mechanisms.
Imd兾Relish 兩 innate immunity 兩 pattern recognition 兩 Toll兾Dif
Construction of Expression Vectors. The PGRP domains from cDNA clones encoding PGRP-LCx and PGRP-LCa (GenBank accession nos. AF500096 and AF207539, respectively) were PCR-amplified and subcloned into pMT-BiP兾V5-His (Invitrogen) under the metallothionein promoter. Genes to be expressed were cloned with a signal peptide for secretion. The BglII restriction site AGATCT, used for in-frame fusion with the signal sequence, adds an Arg and a Ser residue to the N terminus of the exported protein. The PGRP domain of PGRP-LCa (designated sLCa) and PGRP-LCx (sLCx) includes amino acid residues 343–520 and 336–500, respectively. The domains were amplified by using 5⬘ PCR primers that contain a BglII restriction sequence. The 3⬘ primers contain six His codons, a stop codon in frame with the last codon, and an XhoI or an XbaI site. The PCR products were subcloned into pMT-Bip兾V5-His by using the BglII and XhoI restriction sites for sLCa constructs and BglII and XbaI sites for sLCx constructs. Full-length PGRP-LCa(His)6 and PGRP-LCx(His)6 with Cterminal hexa-His tags were PCR-amplified and subcloned into pMT兾V5-His (Invitrogen). All constructs were confirmed by DNA sequencing (MGW Biotech, Ebersberg, Germany). The PGRP-SA(His)6 in pMT兾V5-His was produced as described in ref. 16.
acterial recognition by the innate immune system relies on binding of specific receptors to conserved structures of the bacterial cell. In mammals, Toll-like receptors recognize a wide range of microbial patterns, including LPS, lipotechoic acids, flagellin, and bacterial DNA (1, 2). In insects, which rely only on innate immune responses, two major pathways control the expression of inducible immune genes coding for effector molecules, such as antibacterial peptides. The Imd兾Relish pathway responds to most bacteria and directs the expression of cationic peptides, like diptericin and attacin. The Toll兾Dif pathway is homologous to the mammalian Toll-like receptor and IL-1R pathways and responds to fungi and some Gram-positive bacteria by expressing the antifungal peptide drosomycin (3). In contrast to the diversified microbial recognition in mammals, the insect recognition repertoire seems to focus mainly on the detection of the bacterial cell wall polymer peptidoglycan (4, 5). In a number of genetic screens, different peptidoglycan recognition proteins (PGRPs) have been shown to be part of the signaling pathways (6–11). PGRP-SA is a humoral Drosophila protein, and knockout flies are defective in the response to some Gram-positive bacteria, such as Micrococcus luteus, via the Toll兾Dif pathway (8). It has been shown that PGRP-SA forms a complex with another putative pattern recognition receptor, Gram-negative binding protein-1 (12, 13). A recent report also places the soluble PGRP-SD upstream of the Toll receptor. This putative receptor is involved in modulating the Toll-mediated response to Gram-positive bacteria (10). The PGRP-LC gene cluster encodes putative receptors of the Imd兾Relish pathway that are predicted to be membrane-bound. By alternative splicing, three different PGRP domains, LCa, LCx, and LCy, are fused to a common invariant non-PGRP domain. The 21- to 24-kDa PGRP domains are predicted to be extracellular, and the 30-kDa non-PGRP domain is expected to have an intracellular localization (14). The latter domain has no significant homology to any protein, but it was recently shown to mediate contact with the Imd protein (15). Only the PGRP-LCx variant is absolutely required for activation of the Imd兾Relish pathway (4, 6, 14). The PGRP-LCa form is, in addition to LCx, involved in the responses to small monomeric peptidoglycan www.pnas.org兾cgi兾doi兾10.1073兾pnas.0407559102
Cell Lines and Purification of Recombinant Proteins. Stable Schnei-
der 2 (S2) cell lines expressing PGRP-SA(His)6, PGRPLCx(His)6, PGRP-LCa(His)6, sLCx, sLCx(His)6, sLCa, and sLCa(His)6 were produced by using calcium phosphate transfection and hygromycin B selection. Protein expression and purification of His-tagged proteins were as described in ref. 16. However, after ammonium sulfate precipitation, the sLCa and sLCx-containing pellets were dissolved in a 50 mM Hepes buffer, pH 7.0. After dialysis against the same Hepes buffer, the proteins were applied to a Resource S cation exchange column (Amersham Biosciences). Elution was with a linear gradient to 500 mM This paper was submitted directly (Track II) to the PNAS office. Abbreviations: DAP, diaminopimelic acid; PGRP, peptidoglycan recognition protein; TCT, tracheal cytotoxin; S2, Schneider 2. ‡To
whom correspondence should be addressed. E-mail:
[email protected].
© 2005 by The National Academy of Sciences of the USA
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Materials and Methods
NaCl. The main peak eluted at 35% and contained a major 20-kDa band. High-molecular-weight contaminants were removed by gel filtration on a Sephacryl S-300 HR column. The chromatogram contains one major peak and three minor peaks. The major peak represents the ⬇20-kDa protein and was essentially free from other protein bands as judged from a Coomassie-stained SDS兾polyacrylamide gel (data not shown). The identity of the protein was confirmed by N-terminal protein sequencing. Cell Wall Components. Insoluble peptidoglycans from M. luteus
(strain Ml 11), Staphylococcus aureus (strain Cowan 1), Bacillus megaterium (strain Bm 11), Lactobacillus plantarum (American Type Culture Collection 4008), Lactobacillus fermentum (American Type Culture Collection 14931), and Lactobacillus casei C (American Type Culture Collection 25302) were prepared as described in ref. 16. Peptidoglycan from Escherichia coli was prepared as in ref. 4, and tracheal cytotoxin (TCT) was prepared as in ref. 17. Curdlan [-(1,3)-glucan] was from Sigma. CD. CD measurements were done by using a J-720 spectropolarimeter (Jasco, Easton, MD). All spectra were recorded in 10 mM sodium phosphate buffer, pH 7.2, with 100 mM sodium sulfate at 20°C in 0.02–0.05 cm of cells. Protein concentrations were estimated spectrophotometrically at 280 nm. The helical content was estimated from the mean residue elipticity at 222 nm according to ref. 18. Binding Studies with Intact Peptidoglycan and -(1,3)-Glucan. PGRPSA(His)6, sLCx, and sLCa, (at 100 g兾ml) in 20 mM Hepes, pH 7.2兾150 mM NaCl were incubated with 1 mg兾ml insoluble peptidoglycan or with 1 mg兾ml curdlan for 45 min at 22°C. Samples were centrifuged at 13,000 ⫻ g for 5 min. The pellet was washed three times in incubation buffer and then resuspended in the same volume as used in the incubation. Samples from the supernatant and pellet fractions were analyzed on a 14% SDS兾polyacrylamide gel followed by Coomassie brilliant blue staining.
gradient from 0% to 15% acetonitrile in 0.01% TFA at a flow rate of 0.3 ml兾min and with monitoring at 215 nm. N-Terminal Protein Sequencing. PGRP-sLCa and -sLCx(His)6 were
run on a 14% SDS兾polyacrylamide gel and blotted onto a poly(vinylidene difluoride) membrane. Protein bands were visualized with Coomassie staining, excised, and analyzed by using five cycles of Edman degradation to determine the N-terminal sequence. Immunohistochemistry. Stable-transformed S2 cell lines expressing full-length PGRP-LCx(His)6 or PGRP-LCa(His)6 under a metallothionein promotor were induced with CuSO4 and grown for 24 h. Control S2 cells were grown and treated similarly. Cells attached to coverslips were washed twice with PBS and then fixed with 4% paraformaldehyde in PBS for 5 min. After three successive washes with PBS, the coverslips were preincubated with 10% FCS in PBS for 20 min. The coverslips were incubated with a monoclonal mouse anti-penta-His IgG antibody (Qiagen) in 10% FCS兾PBS for 1 h in a humidified chamber. After two successive washings with PBS and another two washes with 10% FCS兾PBS, the coverslips were incubated for 1 h with Alexa Fluor 488-conjugated goat anti-mouse IgG antibody (Molecular Probes). After five washes with PBS, anti-fade solution (25 mg兾ml diazabicyclo[2.2.2]octane兾50% glycerol in PBS, pH 8.6) was applied. The cover glasses were mounted on object glasses and analyzed with a laser scanning confocal microscope (LSM 510, Zeiss) by using a planapochromat 63⫻ (numerical aperture 1.4) oil immersion objective. Optical sections of 0.5 m were collected through the thickness of the sample and merged to a maximum through-focus projection.
Results Characterization of Recombinant PGRPs. Recombinant PGRP pro-
teins were expressed in stable-transformed S2 cell lines. PGRP-SA was expressed with a hexa-His C-terminal tag and purified on a metal chelate affinity column. The extracellular PGRP part of the membrane-bound PGRP-LC receptor variants
Dimerization Assay. An affinity matrix was prepared by adding
His-bind resin (Novagen) charged with NiSO4 to a 384-well AcroPrep filter plate (Pall). Resin (50 l per well of a 50% slurry) was added, and the plate was centrifuged at 650 ⫻ g for 2 min. The resin was washed three times with 60 l of 20 mM Hepes, pH 7.5兾150 mM NaCl buffer. Samples (65 l) containing His-tagged and untagged proteins (150 g兾ml) were incubated with and without 100 ng兾ml TCT. Samples were preincubated for 45 min at 22°C, applied to the His-resin matrix, and centrifuged at 650 ⫻ g for 2 min. The flow-through was collected in a collector plate. After three washes with Hepes兾NaCl buffer, strip-buffer (65 l of 100 mM EDTA兾500 mM NaCl兾20 mM Tris, pH 7.9) was added, and the samples were eluted by centrifugation as above. Protein content of flow-through and eluted fractions were analyzed on 14% SDS兾polyacrylamide gel followed by Coomassie staining. Binding of PGRPs to TCT. The affinity matrix was prepared in a
96-well AcroPrep filter plate (Pall) as above by using 100 l of resin. Stoichometric amounts of PGRP proteins (180 g) and TCT (9 g) were preincubated for 1 h at 22°C and added to the matrix. After two successive washings with 200 l of 20 mM Hepes, pH 7.5兾150 mM NaCl buffer, TCT was eluted with 100 l of 0.1% trifluoroacetic acid (TFA) per well. A 25-l sample was diluted in 0.01% TFA to a final volume of 1 ml. After centrifugation, 950 l of the supernatant was injected into a 5-m Brownlee Lab 4.6- ⫻ 30-mm RP-18 column fitted to a Varian 5000 HPLC instrument. TCT was eluted with a linear 6456 兩 www.pnas.org兾cgi兾doi兾10.1073兾pnas.0407559102
Fig. 1. CD spectra of PGRP-LCa and PGRP-LCx with and without His tags in 10 mM phosphate buffer, pH 7.2兾100 mM Na2SO4. The color key is as follows: light blue, PGRP-LCx; dark blue, PGRP-LCx(His)6; red, PGRP-LCa; yellow, PGRP-LCa(His)6.
Mellroth et al.
LCa and LCx were expressed with and without a His tag. After chromatography, all proteins were essentially pure as judged from Coomassie-stained polyacrylamide gels (data not shown). Proteins were subjected to five cycles of N-terminal sequencing and shown to have N termini in consonance with a proper signal peptidase cleavage. Because the model to be presented is built partly on negative results regarding peptidoglycan binding to PGRP-LCa, it is crucial that the recombinant protein adopts a native conformation. The CD spectra for His-tagged and untagged sLCa are identical (Fig. 1). The spectrum for sLCx has a slightly different form compared with that of sLCa. The molar elipticity at 222 nm indicates that the sLCx variant contains 34% helix compared with 27% for the sLCa form, which is equivalent to approximately seven residues less being in helical conformation in sLCa compared with sLCx. Pairwise sequence alignments of the PGRP domains of the proteins used in this study show that the homologies by identity are LCa–SA, 31%; LCa–LCx, 37%; and LCx–SA, 40%. These findings indicate that PGRP-LCa is not very similar to LCx, which could allow for the slightly different CD spectra we recorded for these two splice variants. The overall conclusion from the CD measurements is that all four protein variants are ordered globular structures and that the His tag does not introduce any perturbation.
Fig. 3. Binding of PGRPs to isolated polymeric peptidoglycan. PGRP-SA, sLCa, and sLCx were incubated with polymeric peptidoglycan from M. luteus (M.l.), S. aureus (S.a.), B. megaterium (B.m.), E. coli (E.c.), L. casei C (L.c.), L. plantarum (L.p.), or L. fermentum (L.f.). Bound protein (B) was separated from free protein (F) by using centrifugation, and proteins were visualized by using Coomassie staining.
PGRP-SA Discriminates Between Different Peptidoglycans. PGRP-SA
is a receptor of the Toll兾Dif pathway, which shows elicitor specificity for bacteria with a peptidoglycan structure containing a Lys in the third position of the crosslinking tetrapeptide (5, 19). (Schematic structures of peptidoglycans from different bacteria are shown in Fig. 2.) Our results also show a peptidoglycanselective affinity for this protein. PGRP-SA binds strongly to Lys-type peptidoglycan from M. luteus, S. aureus, and L. casei C. PGRP-SA has poor affinity for diaminopimelic acid (DAP)containing peptidoglycan from B. megaterium but binds strongly to DAP-type peptidoglycan from E. coli and L. plantarum. Furthermore, PGRP-SA binds weakly to ornithine-containing peptidoglycan from L. fermentum (Fig. 3). analysis and immunoprecipitation demonstrated that the PGRP-LC epitope-tagged proteins are transmembrane proteins with the PGRP domain located on the outside of the cell and the non-PGRP domain on the inside of the cell (4, 15). We now show immunostaining of PGRP-LCx(His)6 using an antibody against the C-terminal His tag fused to the PGRP part of the full-length receptor. The cells were fixed but not permeabilized. Confocal microscopy clearly shows that the His-tagged PGRP part is outside the cytoplasmic membrane (Fig. 4). The spotted surface distribution of the LCx protein might be significant but should be interpreted with caution because it reflects PGRP-LCx localization in overexpressing cells. We also performed staining of cells overexpressing PGRP-LCa(His)6, and these cells stained similarly to PGRP-LCx(His)6, suggesting that both PGRP-LC
Fig. 2. Peptidoglycan structures. Peptidoglycan is composed of alternating N-acetylglucoseamine (GlcNAc) and N-acetylmuramic acid (MurNAc) units forming unbranched glycan strands. The strands are crosslinked by two tetrapeptides connected from the third residue of one tetrapeptide to the fourth residue of another tetrapeptide. Often, the connection is by means of an interpeptide bridge of variable but strain-specific length and composition. The general structure is shown for the E. coli peptidoglycan type, and the variable part (shown in boxes) is shown for other species. The structure of TCT is shown in a gray box. In this monomeric subunit, the N-acetylmuramic acid residue is in its 1,6-anhydro form, the result of enzymatic cleavage from intact peptidoglycan.
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Fig. 4. PGRP-LCx is localized in the cytoplasmic membrane with the PGRP domain facing outwards. Shown are the results of the immunostaining of S2 cells (Left) and cells overexpressing PGRP-LCx(His)6 (Right) with a primary antibody against the His epitope and a secondary antibody with an Alexa Fluor 488 conjugate. Cells were fixed but not permeabilized. The images are maximum through-focus projections of 0.5-m-thick optical sections. Lateral sections are shown as insets. PNAS 兩 May 3, 2005 兩 vol. 102 兩 no. 18 兩 6457
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The PGRP Domain of PGRP-LC Is Localized on the Cell Surface. FACS
PGRP-LCa Does Not Bind Peptidoglycan. The PGRP-LCa splice form has been reported to be involved in the response to monomeric peptidoglycan subunits (4). However, our results show that the PGRP domain of PGRP-LCa does not bind to the monomeric TCT (Fig. 5) or any intact peptidoglycan tested (Fig. 3). Because PGRP-LCa has been reported to be involved in the recognition of LPS (14) we also tested whether PGRP-LCa binds to different live E. coli LPS mutants (data not shown). We did not observe binding to any of these bacteria, further demonstrating the lack of ligand for PGRP-LCa (4, 5). Also, when considering the residues that constitute the linings of the binding cleft (20), it is evident that PGRP-LCa is an unusual PGRP. It has only four of eight residues conserved in the group comprising the⬎60% conserved residues that make up the floor of the binding groove (see figures 4 and 5 of ref. 20). Moreover, three of these four residues have an opposite charge compared with that of the conserved residue. Two of these residues are involved in hydrogen bonds to the peptidoglycan ligand (H208 and H231 in human PGRP-I␣). In contrast, all eight of the residues in the well conserved group are retained in PGRP-LCx. These observations are compatible with the notion that PGRP-LCa has lost the peptidoglycan affinity.
Fig. 5. PGRP-LCx binds monomeric peptidoglycan. Monomeric peptidoglycan (TCT) was incubated with His-tagged PGRP proteins, which subsequently were immobilized on a metal chelate matrix. After washing, TCT was eluted from the matrix-bound proteins with 0.1% trifluoroacetic acid. RP-HPLC chromatograms from the eluate after incubating TCT with sLCx(His)6 (line a), sLCa(His)6 (line b), SA(His)6 (line c), and Hepes buffer (line d). The absorbance at 215 nm (solid lines) refers to the left y axis, and the acetonitrile gradient (dashed line) refers to the right y axis.
forms have the PGRP domain oriented extracellularly (data not shown). PGRP-LCx Binds Strongly to All Types of Polymeric Peptidoglycan and to Monomeric TCT. We found that the extracellular PGRP domain
of the PGRP-LCx receptor had surprisingly strong affinity for all types of intact peptidoglycan, especially considering their differential induction of the Imd兾Relish pathway reported for mbn-2 cells in culture (5, 19). Hardly any sLCx protein could be seen in the unbound state under the conditions used (Fig. 3). It bound equally strongly to Lys-containing peptidoglycan as to peptidoglycans having ornithine or DAP residues in their tetrapeptides. The binding to peptidoglycan was specific, and no binding was observed to fungal -(1,3)-glucan. PGRP-sLCx also bound monomeric peptidoglycan, as shown in Fig. 5. This peptidoglycan is a fragment released by Bordetella pertussis and is known as TCT; it corresponds to a DAP-type peptidoglycan monomer with an anhydrous form of the N-acetylmuramic acid residue (disaccharide-tetrapeptide) (Fig. 2).
Specific Heterodimerization of sLCx and sLCa Is Induced by TCT But Not by Polymeric Peptidoglycan. In cell culture systems, TCT is a most
potent inducer of the Imd兾Relish pathway (4, 19). The finding that sLCx but not sLCa binds TCT and the fact that PGRP-LCa and LCx are required for Imd兾Relish-dependent activation by monomeric peptidoglycan (4) suggest that the activating complex is a heterodimer of these receptors. To test whether the TCT兾 sLCx complex shows affinity for sLCa, we developed a dimerization assay using the His tag for immobilization. After incubating sLCx(His)6 together with untagged sLCa in the presence or absence of TCT, samples were applied to the matrix and bound sLCa was separated from unbound sLCa by centrifugation. As expected, the tagged sLCx(His)6 bound to the matrix irrespective of the presence of TCT (Fig. 6A). The untagged sLCa did not bind to the matrix when incubated with TCT alone or with sLCx(His)6 alone. However, if TCT was included in the incubation mixture with sLCa and sLCx(His)6, we observed the sLCa variant in the bound fraction. Such a ligand-induced complex was not formed between sLCa and PGRP-SA(His)6. In gel filtration experiments, we have noted that the PGRP-LC proteins form dimers and not higher multimers (data not shown). However, the dimer formation was sensitive to temperature and buffer composition in such one-phase systems. Polymeric peptidoglycan could not be used in the dimerization assay because of its solid nature. Instead, we profited on this property and used insoluble B. megaterium peptidoglycan as affinity matrix to assay dimerization (Fig. 6b). In contrast to monomeric peptidoglycan, this polymeric peptidoglycan did not promote heterodimerization of PGRP-LCx and PGRP-LCa.
Fig. 6. PGRP-LCx兾PGRP–LCa heterodimers are induced by monomeric peptidoglycan. (A) Dimerization was assayed on a solid-phase metal-chelate affinity matrix. Proteins were preincubated in the presence or absence of monomeric peptidoglycan (TCT) and then applied to the matrix. Proteins that did not adhere to the matrix [free (F)] were collected, and bound (B) proteins were eluted from the matrix with a strip buffer. Fractions were applied to SDS兾PAGE followed by Coomassie staining. (B) Dimerization in the presence of insoluble, polymeric, peptidoglycan (PGN) was assayed by incubating proteins and peptidoglycan for 45 min followed by separation of free protein from bound by centrifugation and analysis of the fractions on SDS兾PAGE兾Coomassie staining. 6458 兩 www.pnas.org兾cgi兾doi兾10.1073兾pnas.0407559102
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Fig. 7. A model for elicitor-induced PGRP-LC dimerization and signal transduction. The model rationalizes the findings in the present paper and in earlier RNA interference studies (4, 14) and incorporates findings from Stenbak et al. (19). (A) PRGP-LCx (x) is a membrane-bound receptor with the PGRP domain on the outside of the cell. The cross indicates that LCa (a) cannot bind peptidoglycan ligands. (B) Polymeric peptidoglycan (PGN), consisting of alternating units of N-acetylmuramic acid (blue) and N-acetylglucosamine (purple), binds to LCx. Only when LCx is bound to an ultimate anhydrous form (open circle) of N-acetylmuramic acid is a conformational change (in red) induced. This change leads to LCx dimer formation and the activation of the Imd pathway. However, gross PGRP-LCx binding can be to all muramic acid units. Activation by polymeric peptidoglycan relies on the homophilic interaction of PGRP-LCx and by binding to peptidoglycan. LCa is excluded from this type of dimerization because it cannot bind peptidoglycan. (C) TCT (or another peptidoglycan monomer in anhydrous form) binds to LCx, causing the conformational change. (D) The conformational change leads to the heterodimer formation between LCx and LCa needed for peptidoglycan monomer-induced signal transduction.
now find that this activation is not achieved by PGRP-LCa recognition of monomeric peptidoglycan but rather by PGRPLCa serving as an adaptor protein for transduction of a signal from a small peptidoglycan subunit. This heterodimer formation is not observed after binding of PGRP-LCx to intact peptidoglycan and can therefore be considered specific for monomeric peptidoglycan. For polymeric peptidoglycan, the activating dimer formation would involve two PGRP-LCx receptors binding to two peptidoglycan epitopes. The two mechanisms suggested for monomeric and polymeric peptidoglycan elicitors are shown in Fig. 7. The model states that a critical step for activation of the Imd兾Relish pathway is the juxtaposition of two PGRP-LC molecules. This contact can be accomplished by two distinct mechanisms. Binding of PGRP-LCx to polymeric peptidoglycan will bring the receptors in close vicinity because of the repetitive nature of the elicitor. The bulk of PGRP-LCx bound to intact polymeric peptidoglycan most likely is to monomer units lacking the anhydro arrangement. Activation might only take place when one of the PGRP-LCx molecules in an LCx pair is bound to an anhydrous N-acetylmuramic acid residue. For activation with monomeric peptidoglycan, an activating dimer complex would consist of the receptor PGRP-LCx and the adaptor protein PGRP-LCa. We suggest that peptidoglycan binding will induce a conformational change in PGRP-LCx that exposes a binding site for LCa, allowing the heterodimer to form. Our results indicate that heterodimerization will only occur with monomeric peptidoglycan. Possibly, the conformational change needed for PGRP-LCa-binding is induced upon PGRP-LCx binding to monomeric peptidoglycan with the N-acetylmuramic acid in its anhydro form. Such a mechanism would be fully in line with the activation data presented by Stenbak et al. (19). The model postulates that, in the presence of TCT, the interaction between LCa and LCx is stronger than that between two LCx molecules. When starting this study, our expectation was to ascribe different elicitor affinities to the different PGRP variants and correlate binding data to earlier data on immunogenicity and RNA interference. A PGRP family story evocative of that of the Toll-like receptor family in mammals was foreseen. Instead, we find that one form, PGRP-SA, has selective affinity PNAS 兩 May 3, 2005 兩 vol. 102 兩 no. 18 兩 6459
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Discussion Several studies have used gene knockout techniques to place the PGRP proteins in the two signal transduction pathways controlling immune gene expression in Drosophila, Toll兾Dif and Imd兾 Relish (6–10). Purified bacterial components have been used to show that the pathways discriminate between bacteria with differences in the composition of their peptidoglycans. The identity of the third amino acid residue of the crosslinking tetrapeptide has been decisive for activation of Toll or Imd pathways. This residue connects two tetrapeptides either directly or by means of an interpeptide bridge (Fig. 2). Peptidoglycan with an L-lysine in this position has been shown to induce the Toll兾Dif pathway to a higher extent than the Imd兾Relish pathway, whereas peptidoglycans with a meso-DAP residue instead strongly induce the Imd兾Relish pathway (5, 19). In consonance with these activation data, we find that PGRP-SA indeed has good affinity for all Lys-type peptidoglycans, regardless of the amino acid composition of the interpeptide bridge (Fig. 3). Our data also demonstrate that the binding of PGRP–SA to DAPtype peptidoglycan from B. megaterium is weak. This finding is also in line with elicitor data from Lemaitre and coworkers (5, 19), showing that Bacillus species are poor activators of the Toll兾Dif pathway. However, we find that PGRP-SA binds to DAP-type peptidoglycans from E. coli and L. plantarum. PGRP-SA has modest, if any, affinity for peptidoglycan from L. fermentum, which has an ornithine residue in the third position of the tetrapeptide (21, 22) (see Fig. 2). The difference in affinities of PGRP-SA for the two DAP-type peptidoglycans from B. megaterium and L. plantarum points to other features also being important for affinity. Such features could be differences in the three-dimensional structure of the peptidoglycan network, the degree of glycan crosslinking, glycan modifications, or amidation of carboxyl groups, e.g., of D-Glu or DAP residues (23). Additional humoral factors influencing Toll pathway activation could be Gram-negative binding protein-1, PGRP-SD, and PGRP-LE that are involved in peptidoglycan recognition (10–13). PGRP-LCx has been shown to be required for activation of the Imd兾Relish pathway (6). We now show that the extracellular PGRP domain has strong affinity for all tested types of polymeric peptidoglycans. This general affinity would make PGRP-LCx a default peptidoglycan receptor in the Imd兾Rel pathway. The fact that there is not a perfect correlation between binding and activation data are not fully understood. A possible explanation was offered by Stenbak et al. (19) who found that a fixed -conformation of the muramic acid residue was probably needed for activation. This conformation might not be required for the overall binding we observe. There would then exist productive and nonproductive peptidoglycan binding. In addition to intact peptidoglycan, PGRP-LCx binds monomeric peptidoglycan subunits in the form of TCT. TCT is a peptidoglycan disaccharide-tetrapeptide that is actively released by growing B. pertussis and is a virulence factor that contributes to the symptoms of whooping cough (17). Although growing bacteria typically do not release peptidoglycan fragments, the transglycosylase that generates TCT (24) is a common feature of Gram-negative bacteria and is involved in cell wall processing and recycling. Therefore, TCT and similar peptidoglycan fragments would be expected to be present whenever bacterial lysis or autolysis is occurring, and these can serve as elicitors of the Drosophila immune system for recognition of Gram-negative bacteria. The finding that LPS is not an immune stimulant of the Imd兾Relish pathway (4, 5) also points to peptidoglycan fragments as being instrumental for recognition of Gram-negative bacteria. The importance of monomeric peptidoglycan recognition is corroborated by the involvement of a dedicated splice form, PGRP-LCa, for this signal transduction to take place. We
for different peptidoglycans, another form, PGRP-LCx, has a general affinity for all tested peptidoglycans, and a third form, PGRP-LCa, has no affinity for peptidoglycans. From our data, we have to conclude that LCa instead can bind to the bona fide peptidoglycan receptor, PGRP-LCx. A new role is thus ascribed to a member of a family already containing receptors and scavenger enzymes, namely that of an adaptor or signal transducer.
that showed PGRP-SA to be an enzyme in addition to being a receptor (25). Binding data of PGRP-SA to intact peptidoglycans partly overlap with data in the present paper and are in full agreement.
Note. During the preparation of the manuscript, an article was published
We thank Paavo Toivanen (University of Turku, Turku, Finland) and Jelka Tomasic (Institute of Immunology, Zagreb, Croatia) for supplying bacterial strains, Torbjo ¨rn Astlind for running CD spectra, and Camilla Matsson for technical assistance. The project was supported by Swedish Research Council Grant 621-2002-3978, Commission of the European Communities Grant QLKZ-2000-336, and Wallenberg Foundation Grant 2002.0147.
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