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Infection and Immunity, July 2008, p. 3273-3280, Vol. 76, No. 7
0019-9567/08/$08.00+0     doi:10.1128/IAI.00366-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Interaction of Porphyromonas gingivalis with Oral Streptococci Requires a Motif That Resembles the Eukaryotic Nuclear Receptor Box Protein-Protein Interaction Domain

Carlo Amorin Daep,¹ Richard J. Lamont,² and Donald R. Demuth^1*

Department of Periodontics, Endodontics and Dental Hygiene, University of Louisville, Louisville, Kentucky,¹ Department of Oral Biology, University of Florida, Gainesville, Florida²

Received 21 March 2008/ Returned for modification 29 April 2008/ Accepted 4 May 2008

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Porphyromonas gingivalis initially colonizes the oral cavity by interacting with organisms in supragingival plaque, such as the oralis group of oral streptococci. This interaction involves the association of the streptococcal antigen I/II with the minor fimbrial antigen (Mfa1) of P. gingivalis. Our previous studies showed that a peptide (BAR) derived from antigen I/II inhibits P. gingivalis adherence and subsequent biofilm formation on streptococcal substrates. In addition, screening a combinatorial peptide library identified select amino acid substitutions in the NITVK active region of BAR that increased the adherence of P. gingivalis to streptococci. Here we report that incorporating these residues in a synthetic peptide results in more-potent inhibition of P. gingivalis adherence and biofilm formation (I₅₀ [50% inhibition] at 0.52 µM versus I₅₀ at 1.25 µM for BAR). In addition, a second structural motif in BAR, comprised of the amino acids KKVQDLLKK, was shown to contribute to P. gingivalis adherence to streptococci. Consistent with this, the KKVQDLLKK and NITVK motifs are conserved only in antigen I/II proteins expressed by the oralis group of streptococci, which interact with P. gingivalis. Interestingly, the primary and secondary structures and the functional characteristics of the amphipathic VQDLL core -helix resemble the consensus nuclear receptor (NR) box protein-protein interacting domain sequence (LXXLL) of eukaryotes. BAR peptides containing amino acid substitutions with the potential to disrupt the secondary structure of VQDLL were less-effective inhibitors of P. gingivalis adherence and biofilm formation, suggesting that the -helical character of VQDLL is important. Furthermore, replacing the lysines that flank VQDLL with acidic amino acids also reduced inhibitory activity, suggesting that the association of VQDLL with Mfa1 may be stabilized by a charge clamp. These results indicate that the Mfa1-interacting interface of streptococcal antigen I/II encompasses both the KKVQDLLKK and NITVK motif and suggest that the adherence of P. gingivalis to streptococci is driven by a protein-protein interaction domain that resembles the eukaryotic NR box. Thus, both motifs must be taken into account in designing potential peptidomimetics that target P. gingivalis adherence and biofilm formation.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Adult periodontitis is associated with elevated levels of several gram-negative organisms in the subgingival oral biofilm, including the asaccharolytic, obligate anaerobe Porphyromonas gingivalis (17, 27). In this primary niche, P. gingivalis interacts with a variety of other gram-negative obligate and facultative anaerobes, such as Fusobacterium nucleatum (3, 22, 23), Treponema denticola (14, 19, 24), and Tannerella forsythus (18, 37), through specific receptor-ligand interactions. However, the initial colonization of the oral cavity by P. gingivalis likely occurs through adherence to organisms in the supragingival biofilm. The successful colonization of this niche by P. gingivalis is contingent upon a variety of factors, such as reduced oxygen tension and sufficient nutritional sources (28). Consistent with this, P. gingivalis has been shown to also adhere to organisms in supragingival plaque that may provide it with physiologic support, such as Streptococcus gordonii (4, 11, 26) and F. nucleatum (3).

The adherence of P. gingivalis to S. gordonii is multimodal. The long and short fimbriae of P. gingivalis have both been shown to be involved in this interaction (7). The structural subunit of the long fimbriae, FimA, interacts with cell surface glyceraldehyde-3-phosphate dehydrogenase of S. gordonii (29), whereas the minor fimbrial protein, Mfa1, interacts with streptococcal cell surface protein SspB (33), a member of the antigen I/II family of streptococcal proteins (18). Interestingly, virtually all of the oral streptococci express antigen I/II (20) but P. gingivalis selectively adheres to S. gordonii and the related oralis group of streptococci. Neither intact P. gingivalis cells nor purified Mfa1 interact with the antigen I/II protein of Streptococcus mutans, even though this protein is highly similar to SspB (4). Furthermore, Demuth et al. (11) and Cook et al. (8) showed that the Mfa1-SspB interaction is essential for the adherence of intact P. gingivalis to streptococcal cells and the subsequent development of P. gingivalis biofilms on streptococcal substrates. Consistent with this, P. gingivalis biofilm growth exhibits the same selectivity for streptococcal species (8, 11). Structure-function analyses on the mechanism of Mfa1-SspB interaction identified a discrete region of SspB, designated BAR (SspB Adherence Region) that was essential for the association of purified Mfa1 or intact P. gingivalis cells with S. gordonii (4, 11, 33). Furthermore, specific amino acid residues in the NITVK sequence contained within BAR are not conserved in the related antigen I/II protein of S. mutans and site-specific mutagenesis of these amino acids in SspB showed that they were essential for P. gingivalis adherence. This provides a mechanism for the selectivity of P. gingivalis adhesion (11).

The adherence of P. gingivalis to oral streptococci represents a potentially important target for the development of therapeutic agents, since interfering with the initial colonization of the supragingival biofilm by P. gingivalis may prevent it from colonizing and multiplying in its primary niche in subgingival plaque. Indeed, a synthetic peptide encompassing BAR functions as a potent competitive inhibitor (I₅₀ [50% inhibition] of approximately 1.3 µM) of P. gingivalis adherence to S. gordonii cells and blocks the formation of P. gingivalis biofilms (9). In addition, the results of screening a combinatorial peptide library in which amino acids in the NITVK sequence of BAR were replaced with all other common L-amino acids defined the physicochemical characteristics of the interacting interface of SspB and Mfa1 and suggested that peptides or peptidomimetics with specific inhibitory activities higher than that of BAR could be developed. Based on these previous findings, we report here that a synthetic BAR peptide containing amino acid substitutions in the NITVK sequence functions as a more-potent competitive inhibitor of P. gingivalis adherence and biofilm formation and has higher -helical content than BAR. We also show that a second structural motif in BAR (KKVQDLLKK) contributes to P. gingivalis adherence. The structural and functional characteristics of this motif, together with the NITVK region, suggest that P. gingivalis adherence to streptococci is driven by a domain in SspB that resembles the eukaryotic nuclear receptor (NR) box protein-protein interacting domain. These motifs are conserved in oral streptococci that interact with P. gingivalis but have diverged in the antigen I/II of S. mutans.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Growth of bacterial strains. P. gingivalis ATCC 33277 was grown in reduced Trypticase soy broth (Difco) supplemented with 0.5 percent yeast extract, 1 µg/ml (final concentration) menadione, and 5 µg/ml (final concentration) hemin. Twenty-five milliliters of medium were reduced for 24 h under anaerobic conditions by being equilibrated in an atmosphere consisting of 10% CO₂, 10% H₂, and 80% N₂. Following equilibration, P. gingivalis was inoculated into the medium and grown for 48 h at 37°C under anaerobic conditions. S. gordonii DL-1 (32) was cultured aerobically without shaking in brain-heart infusion (BHI) broth supplemented with 1% yeast extract for 16 h at 37°C.

P. gingivalis adherence and biofilm formation. The adherence of intact P. gingivalis to S. gordonii cells and the subsequent formation of biofilms was carried out by using an open-flow system essentially as previously described by Daep et al. (9) and Lamont et al. (25). The flow reactor comprised a Manostat Carter 4/8 cassette peristaltic pump (Fisher Scientific, Suwanee, GA) using 0.89-mm platinum-cured silicone tubing (Fisher Scientific, Suwanee, GA) and a BST FC 71 flow cell (Biosurface Technologies Corp., Bozeman, MT). A single surface of a 15- by 40-mm cover glass (Fisher Scientific, Suwanee, GA) was coated with 0.22-µm-filter-sterilized saliva and incubated at 37°C for 30 min. The saliva-coated cover glass was then washed with sterile 1x phosphate-buffered saline (PBS) at a flow rate of 6 ml per hour for 30 min to remove unbound salivary constituents.

S. gordonii DL-1 cells were harvested by centrifugation at 4,000 rpm at 4°C for 15 min and suspended in 25 ml of sterile 1x PBS. S. gordonii cells were labeled with 20 µl of hexidium iodide (1.6 mg/ml; Molecular Probes, Eugene, OR) at 25°C for 30 min in the dark and washed with PBS. The adherence of streptococci to the saliva-coated cover glass was carried out by delivering S. gordonii cells to the flow chamber at a flow rate of 6 ml per hour for approximately 2 h. Following inoculation with S. gordonii, the flow cell was washed with sterile 1x PBS for 30 min at 6 ml per hour to remove nonadherent bacteria from the cover glass.

P. gingivalis ATCC 33277 cells were harvested by centrifugation at 4,000 rpm at 4°C for 45 min, suspended in 25 ml of sterile 1x PBS, and introduced into the flow cell at a flow rate of 6 ml per hour for 2 h at 25°C to allow P. gingivalis to adhere and accumulate on the streptococcal substrate. The flow cells were subsequently washed with sterile 1x PBS to remove nonadherent P. gingivalis cells. To visualize adherent P. gingivalis cells, rabbit anti-P. gingivalis 33277 polyclonal antibody diluted 3:5,000 in 5 ml of sterile 1x PBS was flowed into the cell at a rate of 6 ml per hour. The flow cell was then washed with sterile 1x PBS for 1 h, reacted with anti-rabbit immunoglobulin G fluorescein isothiocyanate (FITC) conjugate (Sigma, St. Louis, MO) in sterile 1x PBS (1:5,000) for 1 h at 6 ml per hour, and washed a final time with sterile 1x PBS as described above. Under these conditions, the streptococci bound to the saliva-coated cover glass as a nonconfluent layer comprising single cells and small aggregates. P. gingivalis subsequently adhered to and formed distinct microcolonies on the immobilized streptococci. The microcolonies were visualized and quantified by confocal microscopy as described below.

Confocal microscopy and analysis of P. gingivalis biofilms. P. gingivalis-S. gordonii biofilms were visualized by using an Olympus FluoView 500 confocal laser scanning microscope (Olympus, Pittsburgh, PA) under x600 magnification, using an Argon laser for visualization of the FITC labeling and an HeNe-G laser to visualize hexidium iodide-labeled streptococci. The number and depth of FITC-labeled P. gingivalis microcolonies was determined from 30 to 60 randomly chosen frames by using the FluoView software package provided by Olympus. The microcolony depth was determined by performing Z-plane scans from 0 µm to 30 µm above the cover glass surface. P. gingivalis microcolonies that formed on S. gordonii in the absence of peptide inhibitor ranged from 7 to 16 µm in depth under the experimental conditions used.

Inhibition of P. gingivalis adherence and biofilm formation. For adherence/biofilm inhibition experiments, BAR peptide analogs were preincubated with P. gingivalis cells at concentrations of 0 to 3.4 µM at 25°C for 30 min. The P. gingivalis-peptide suspension was then introduced into the flow cell for 2 h at a flow rate of 6 ml per hour as described above, and P. gingivalis microcolonies were analyzed by confocal microscopy. The data were analyzed by using GraphPad InStat3 (GraphPad Software Co.). A paired t test was utilized to determine pairwise statistical differences in colony number and depth between experimental samples and the control reaction that did not contain inhibitor.

CD spectroscopy. Circular dichroism (CD) spectroscopy experiments were carried out at 25°C by using a Jasco J-810 spectropolarimeter (Jasco, Easton, MD). A 40-µl sample of peptide at a concentration of 1.4 mM in filter-sterilized 0.1 M phosphate buffer (pH 7.2) was analyzed in a 0.01-cm quartz cuvette (Starna Cells, Inc., Atascadero, CA) using the following parameters: sensitivity, 100 mdeg; start wavelength, 340 nm; end wavelength, 180 nm; continuous scanning mode at 200 nm/minute; response, 1 second; and bandwidth, 1 nm. Nitrogen was flushed into the system at a rate of 31.8 to 42.4 cubic feet/min during each experiment. A total of 10 scans were accumulated and averaged, and the appropriate blanks were subtracted from the spectra. The resulting spectra were expressed in molar ellipticity. The normalized CD data were analyzed by using K2D software (http://www.embl-heidelberg.de/andrade/k2d.html) to estimate the -helical, β-sheet, and random coil content (2).

Peptide synthesis. The synthetic peptides used in this study are listed in Table 1. All of the peptides were derived from the sequence of the BAR peptide, which comprises residues 1167 to 1193 of the SspB (antigen I/II) protein sequence of S. gordonii (4). Peptides were synthesized by BioSynthesis, Inc. (Lewisville, TX) and were obtained at more than 85% purity. Prior to use, peptides were suspended at the appropriate concentration in nuclease- and protease-free water (Fisher Scientific Co., Fairlawn, NJ).

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TABLE 1. Peptides used in this study

Secondary structure prediction and sequence comparison. The secondary structure of the synthetic peptides was predicted by using the Protein Structure Prediction Server (Psipred; http://bioinf.cs.ucl.ac.uk/psipred/) (1, 5, 21, 30). Comparison of the BAR region of SspB with sequences in the protein database was carried out by using the BlastP program (1, 35), available through NCBI (http://www.ncbi.nlm.nih.gov/BLAST/BLAST.cgi). For these searches, the entire SspB polypeptide sequence was used as the query to identify related sequences in the database. An "e" score of 1e-10 for the complete sequence was used as the threshold for significance. Subsequently, the regions corresponding to the BAR sequence in each of the related sequences that were identified were recorded and compiled.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Active-site amino acid substitutions in BAR increase its specific antibiofilm activity. A synthetic peptide (BAR) derived from the SspB protein sequence was previously shown to be sufficient for P. gingivalis adherence to S. gordonii, and in solution, this peptide functioned as a potent inhibitor of P. gingivalis adherence and biofilm formation on streptococcal substrates (4, 9). Subsequently, using a combinatorial approach in which mixtures of peptides were screened for their ability to promote P. gingivalis adherence, Daep et al. (9) showed that the substitution of Arg for Asn¹¹⁸² and Ile for Val¹¹⁸⁵ in the NITVK region of BAR resulted in increased in vitro adherence of P. gingivalis. To determine if these results could be exploited to produce a more potent inhibitor of P. gingivalis biofilm formation, a derivative of the BAR peptide containing the Asn¹¹⁸²Arg and Val¹¹⁸⁵Ile substitutions (BAR-II) was synthesized and analyzed. As shown by the results in Table 2, BAR-II inhibited the adherence and subsequent formation of P. gingivalis microcolonies on the streptococcal substrate by 25% and 82% at peptide concentrations of 0.34 µM and 0.85 µM, respectively. In contrast, the parent BAR peptide inhibited microcolony formation by only 3% and 7%, respectively, at these peptide concentrations. From the dose responses shown, the I₅₀ for BAR-II was calculated to be 0.52 µM and was approximately 2.5-fold greater than the I₅₀ calculated for the BAR peptide (1.25 µM, which was consistent with the I₅₀ of 1.3 µM previously reported for BAR (9). Thus, the BAR peptide that was altered in the NITVK region interacts with P. gingivalis at higher affinity and functions as a more potent competitive inhibitor of P. gingivalis adherence and biofilm formation.

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TABLE 2. Comparison of the specific inhibitory activities of BAR and R1182I1185-BAR

Data from the combinatorial screen and our previous site-specific mutagenesis experiments also indicated that the introduction into NITVK of amino acids with the potential to disrupt secondary structure (e.g., Pro and Gly) reduced P. gingivalis adherence, suggesting that the secondary structure of BAR may be important for binding (9, 11). As shown in Table 3, the predicted secondary structure of BAR-II does not differ from that of BAR, but the results of the analysis of CD spectrographs of BAR and BAR-II indicated that the latter contained significantly greater -helical content (28% versus 10%) and less β-sheet and random coil in solution, suggesting that the presence of Arg and Ile stabilized the secondary structure of BAR. To determine if the BAR-mediated inhibition of P. gingivalis adherence correlates with the -helical content of the peptide, CD spectra were also obtained for a cyclic, conformationally constrained BAR peptide (CR-BAR) that was previously shown to be a poor inhibitor of biofilm formation (9). This peptide (designated BAR-III for this study) exhibited a specific activity that was 13-fold less than that of BAR (9; data not shown). Unexpectedly, BAR-III also contained 28% -helix, similar to that of BAR-II (Table 3). This suggests that the inhibitory activity is independent of secondary structure or, alternatively, that the introduction of Cys to generate BAR-III disrupts a second structural motif that is important for the interaction of BAR with P. gingivalis. To address this, the sequence of BAR was reexamined, focusing on the regions that were altered to create BAR-III.

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TABLE 3. Secondary structural analysis of BAR peptides

The putative -helical region upstream of NITVK is also essential for BAR interaction with P. gingivalis. The disulfide-bridged peptide BAR-III was synthesized by substituting Cys residues for Leu¹¹⁷⁷ and Leu¹¹⁹¹ (9). Leu¹¹⁹¹ is within a region of BAR that is well conserved in the antigen I/II protein of S. mutans, which does not interact with P. gingivalis. In contrast, the sequence around Leu¹¹⁷⁷ has diverged from that of the S. mutans protein. Leu¹¹⁷⁷ is part of a predicted amphipathic -helix (VXXLL, actual sequence VQDLL) that resembles the consensus sequence of a known protein-protein interaction domain, the NR box (consensus LXXLL), which mediates the interaction of coactivator/repressor proteins with the NR family of transcriptional regulators in eukaryotes (31, 34). Furthermore, other structural characteristics of the VQDLL region in BAR are consistent with the known properties of the NR box. For example, VQDLL is flanked by charged amino acids in BAR (Lys¹¹⁷², Lys¹¹⁷³, Lys¹¹⁷⁹, and Lys¹¹⁸⁰), and charged residues in the corresponding positions in the NR box have been suggested to stabilize the coactivator-NR interaction (15). The specificity of NR-coactivator interactions is not dictated by the consensus region LXXLL but by sequences that reside either upstream or downstream of the LXXLL motif (10). In BAR, the NITVK motif resides immediately downstream of VXXLL, and we previously showed that NITVK is responsible for the selectivity of P. gingivalis adherence to oral streptococci (11).

Therefore, to determine if the VXXLL region contributes to the interaction of BAR with P. gingivalis, two decapeptides comprising either the VQDLL (BAR-IV) or NITVK (BAR-V) regions and a 20-mer containing both VQDLL and NITVK (BAR-VII) were synthesized and tested as inhibitors of P. gingivalis-S. gordonii adherence and biofilm formation. A decamer (BAR-VI) and a 20-mer (BAR-VIII) derived from the regions of S. mutans antigen I/II that correspond to NITVK and VQDLL/NITVK, respectively (see Table 1), were also synthesized and used as control peptides, since antigen I/II of S. mutans neither interacts with P. gingivalis nor promotes biofilm formation (4). As shown in Fig. 1A, BAR-VII inhibited P. gingivalis biofilm formation in a dose-dependent manner, whereas the negative-control peptide BAR-VIII was inactive. In contrast, BAR-IV and BAR-V, encoding the individual VXXLL or NITVK motifs, respectively, exhibited no inhibitory activity (Fig. 1B), suggesting that the interaction of BAR with P. gingivalis requires both the VXXLL and NITVK motifs. The control S. mutans peptide (BAR-VI) was also inactive.

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FIG. 1. Inhibition of P. gingivalis-streptococcus biofilm formation by BAR peptide analogs. (A) Peptides of 20 amino acids that contain the VQDLL and NITVK motifs of BAR (BAR-VII) or the corresponding regions from the antigen I/II protein of S. mutans (BAR-VIII) were incubated with P. gingivalis cells and inoculated onto adherent streptococci. The resulting biofilms were visualized by confocal laser scanning microscopy and analyzed as described in Materials and Methods. A statistically significant difference (*) in microcolony numbers (P < 0.001) was observed when a pairwise comparison of specific inhibitory activity was made for BAR-VII, BAR-VIII, and untreated cells. Light-gray and dark-gray bars represent 0.845 µM and 3.38 µM peptide, respectively. (B) Decapeptides containing either the VQDLL or NITVK motifs of BAR (BAR-IV and BAR-V, respectively) or the control peptide comprising the region corresponding to NITVK in the antigen I/II protein of S. mutans (BAR-VI) did not inhibit the formation of P. gingivalis biofilms, even when P. gingivalis cells were treated with the peptides at a concentration of 16.9 µM. Light-gray and dark-gray bars represent 3.38 µM and 16.9 µM peptide, respectively. (C) Amino acid substitutions in the VXXLL motif influence biofilm inhibitory activity of the BAR peptide. Analogs of the BAR peptide that were altered at the variable positions in the core VXXLL sequence (BAR-X), the lysine residues flanking VXXLL (BAR-XI), or at the hydrophobic Val and Leu residues of VXXLL (BAR-IX) were synthesized and analyzed for biofilm inhibition as described in Materials and Methods. A functional VXXLL motif requires the maintenance of -helicity and positive charge through the region. A single asterisk designates a significant difference where the P value is <0.001; a double asterisk indicates a P value of <0.01. For all experiments, percent inhibition was calculated as follows: average number of microcolonies per frame (control peptide) – average number of microcolonies per frame (experimental peptide)/average number of microcolonies per frame (control peptide). At least 30 independent frames from three separate biofilm cultures were analyzed for each peptide sample. Error bars show standard errors of the means.

Functional properties of VXXLL resemble those of the NR box. A short, hydrophobic, -helical motif in eukaryotic coactivator proteins (consensus LXXLL) is essential for their interaction with the cognate NR. This interaction is stabilized by hydrogen bonding and/or charge-charge interactions of amino acids flanking LXXLL (15) with the ligand binding domain (LBD) of the NR. To determine if VXXLL in BAR functions similarly to the NR box, we examined the contribution of each of these characteristics, i.e., the -helical content, the hydrophobic character of VXXLL, and the contribution of charged amino acids flanking VXXLL, to the interaction of BAR with P. gingivalis. As shown by the results presented in Fig. 1C, the BAR-X peptide in which the internal variable residues in VXXLL were altered to Pro and Gly (i.e., VPGLL) was a poor inhibitor of biofilm formation (15% inhibition at 3.38 µM) relative to inhibition by the control BAR peptide (75% inhibition at 3.38 µM). This suggests that amino acids with the potential to disrupt -helix strongly reduce the potency of the peptide inhibitor. A reduction in inhibitory activity was also obtained when the positively charged Lys residues flanking VXXLL were altered to Asp in BAR-XI (40% inhibition versus 75% for BAR). Secondary structural predictions of the modified peptide suggested that the LysAsp replacements did not alter the overall secondary structure of BAR. Therefore, the basic residues that reside upstream and/or downstream of VXXLL also appear to contribute to the interaction of BAR with P. gingivalis. However, reducing the hydrophobicity of VXXLL by substituting neutral or charged residues for the hydrophobic amino acids (e.g., VXXLL to DXXND) without changing the predicted -helical character of VXXLL had no effect on inhibitory activity, as observed for BAR-IX. Thus, the -helical content and the presence of basic residues play a greater role in the association of BAR with P. gingivalis than does the hydrophobic character of the putative VXXLL -helix.

Distribution of VXXLL and NITVK among streptococcal sequences. To determine if the NR box-like VXXLL motif and the NITVK region of BAR are conserved in other antigen I/II-related polypeptides, a BlastP search was conducted using the SspB sequence as query. Fifteen different deduced polypeptide sequences were identified from 10 different streptococcal species and 1 species of Lactococcus. The regions of these sequences that correspond to the BAR peptide are compared in Fig. 2. The NITVK sequence is conserved only in S. gordonii, Streptococcus oralis, and Streptococcus sanguinis. All of the other antigen I/II sequences contained substitutions of Gly for Asn¹¹⁸², Pro for Val¹¹⁸⁵ or both in the region corresponding to NITVK. These substitutions in NITVK were previously shown in our combinatorial screen and by site-specific mutagenesis of the sspB sequence to completely inhibit the adherence of P. gingivalis (9, 11). Consistent with this, each of the species containing NITVK in antigen I/II adhere to P. gingivalis and support biofilm formation similarly to S. gordonii (not shown). Thus, the presence and distribution of the NITVK motif is consistent with the ability of P. gingivalis to selectively adhere to streptococci and form biofilms.

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FIG. 2. Comparison of the BAR region sequences in antigen I/II-related polypeptides of streptococci and lactococci. The VXXLL and NITVK motifs are conserved only in S. gordonii, S. oralis, and S. sanguinis (blue and yellow boxes, respectively). Sequences corresponding to NITVK are also present in the antigen I/II of the other organisms but contain amino acid substitutions that are incompatible with P. gingivalis adherence (see text). NR box-like motifs (underlined) similar to VXXLL and containing hydrophobic residues at the +1, +4, and +5 positions occur in most of the other antigen I/II sequences. A VXXML motif is conserved in antigen I/II of S. downei, S. sobrinus, and S. criceti (red box), and VXXVL is present in S. intermedius and L. lactis (green boxes). Furthermore, these NR box-like motifs are flanked by charged amino acid residues. In all antigen I/II sequences, with the exception of that of Streptococcus pyogenes, proline and alanine residues occupy positions –3 and +8, respectively, where +1 represents the first hydrophobic residue in the NR box-like motif. A consensus sequence of the BAR region of antigen I/II is shown on the last line. The consensus residues shown were present in at least 8 of the 14 antigen I/II sequences analyzed. Residues that were not conserved at this level are indicated by dashes.

The VXXLL motif is also conserved only in S. gordonii, S. oralis, and S. sanguinis. However, NR box-like motifs (Fig. 2) (characterized by Hy-X-X-Hy-Hy, where Hy is a hydrophobic amino acid) are present in all of the other antigen I/II-related sequences as well. A VXXML motif is present in the antigen I/II proteins of Streptococcus downei, Streptococcus sobrinus, and Streptococcus criceti, and VXXVL is found in S. intermedius and Lactococcus lactis. In each case, the NR box-like sequence is flanked by 1 or more charged residues and resides 2 residues downstream from a conserved proline. This motif is also separated from the NITVK (or related motif) by a single conserved alanine. Thus, the overall structure and spacing of the BAR peptide region of antigen I/II is well conserved but variable residues that may influence the function of the proteins exist in both the VXXLL and NITVK regions.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
To colonize the oral cavity, P. gingivalis can adhere to select species of oral streptococci and/or Fusobacterium nucleatum in supragingival plaque before becoming established in its primary niche in the subgingival pocket. The adherence and the subsequent formation of P. gingivalis biofilms on streptococci is mediated by a protein-protein interaction between the minor fimbrial antigen Mfa1 and a discrete region of the streptococcal SspB (antigen I/II) polypeptide (9, 11, 33). Daep et al. previously showed that a peptide (BAR) comprising the interacting region of SspB was a potent inhibitor of P. gingivalis-S. gordonii adherence and subsequent formation of biofilms and that some combinatorial mixtures of peptides containing substitutions in the NITVK active-site region of BAR resulted in increased P. gingivalis adherence (9). We have now shown that a pure synthetic peptide that contains two of the putative adherence-promoting substitutions identified by Daep et al. (9) functions as an 2.5-fold more potent competitive inhibitor of P. gingivalis adherence and biofilm formation. This suggests that altering the NITVK sequence in BAR to RITIK (by substituting Arg for Asn¹¹⁸² and Ile for Val¹¹⁸⁵) increases the affinity of its interaction with Mfa1. Other potential adherence-promoting amino acid substitutions that were predicted by the combinatorial screening of Daep et al. (9) are currently being tested, e.g., Lys or Ser for Asn¹¹⁸² and Phe for Val¹¹⁸⁵. Interestingly, the Asn¹¹⁸⁵Lys substitution occurs naturally in several other antigen I/II-like proteins expressed by other nonoral streptococci and lactococci (Fig. 2). However, this substitution occurs in conjunction with a Val¹¹⁸⁵Pro alteration which we have previously shown to completely inhibit the SspB-Mfa1 and P. gingivalis-BAR peptide interaction (9, 11). Indeed, comparison of antigen I/II amino acid sequences deduced from the available streptococcal genome sequences indicates that Pro is the consensus residue at position 1185 (Fig. 2). Valine is present only in the antigen I/II proteins expressed by organisms in the oralis group of streptococci which interact with P. gingivalis.

In the absence of a three-dimensional structure of the SspB binding pocket of Mfa1, it is difficult to determine mechanistically how the Asn¹¹⁸²Arg and Val¹¹⁸⁵Ile substitutions affect the interaction of the proteins. In silico predictions suggest that the substitutions do not alter the secondary structure of the BAR peptide, but CD analysis indicated that BAR-II contained significantly more -helical content than BAR. One possible explanation is that the amino acid substitutions reduce flexibility and stabilize the structure of the peptide in an "active" conformation in solution. Consistent with this, a conformationally constrained disulfide-linked BAR-III in which Leu¹¹⁷⁷ and Leu¹¹⁹¹ were replaced with Cys exhibited an -helical content similar to that of BAR-II. However, BAR-III was a poor inhibitor of P. gingivalis biofilm formation (9), and our results now suggest that the poor inhibitory activity of BAR-III arises from the substitution of Cys for Leu¹¹⁷⁷, which disrupts a second important region of BAR that is required for P. gingivalis adhesion. Indeed, Leu¹¹⁷⁷ is part of a motif, VXXLL, that resembles a known protein-protein interacting domain, the NR box found in coactivators of NRs of eukaryotes. Thus, our results indicate that both the VXXLL and NITVK motifs of BAR participate in binding to Mfa1.

The NR box is a protein-protein interacting domain that is present in coactivator proteins that interact with NRs in eukaryotic organisms (16). It is comprised of a hydrophobic or amphipathic -helical motif with a consensus sequence of LXXLL or FXXFF, although Val is tolerated in place of the Leu in position +1 (16) and Met can functionally replace the Leu in position +4 (where the core motif is numbered +1 to +5 from left to right) (36). The VXXLL motif in BAR is a predicted amphipathic -helix, and its sequence and structural properties are consistent with the consensus NR box. Furthermore, the introduction of amino acids with the potential to disrupt the secondary structure of VXXLL reduced the specific inhibitory activity of BAR, suggesting that the putative -helical character of VXXLL is important for the interaction of BAR with Mfa1. In addition, the association of the coactivator with the NR box of the NR LBD is stabilized by a charge clamp that arises from electrostatic or hydrogen-bonding interactions that occur between charged residues flanking either or both sides of the NR box and amino acids in the LBD (15, 34). The VXXLL motif is also flanked by Lys residues at the –1, –2, +6 and +7 positions, and the substitution of acidic amino acids at these positions resulted in reduced inhibitory activity, suggesting that the BAR-Mfa1 interaction may also be stabilized by a charge clamp involving the flanking Lys residues. Finally, the specificity of the coactivator-NR interaction is driven by amino acids that reside either upstream or downstream from the core LXXLL motif (10). The VXXLL motif in BAR resides immediately upstream from NITVK, which we previously showed was responsible for the specificity of P. gingivalis adherence to the oralis group of streptococci (11). Together, these results suggest that P. gingivalis adherence to streptococci is mediated by a protein-protein interacting domain of SspB (antigen I/II) that resembles the eukaryotic NR box domain. To our knowledge, this is the first identification of a functional NR box-like protein-protein interaction motif in a prokaryotic system. Based on these results, a model depicting the interacting interface between SspB and Mfa1 is shown in Fig. 3.

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FIG. 3. Schematic representation of the interaction of SspB and Mfa1. The adherence of P. gingivalis to streptococci requires the VXXLL and NITVK motifs of SspB. The initial interaction between SspB (bottom) and Mfa1 (top) may occur via the amphipathic, -helical VXXLL motif. This interaction may be stabilized by a charge clamp that involves hydrogen bonding or electrostatic interactions between the lysine residues that flank VXXLL and adjacent amino acids in Mfa1. Our current and previous results suggest that the specificity of the Mfa interaction with antigen I/II proteins from oralis streptococci is dictated by the downstream NITVK motif and that the alteration of Asn1182 and Val1185 can either facilitate or inhibit the association of SspB and Mfa1 (20).

As discussed above, the NITVK motif is conserved only in the oralis group of streptococci. The remaining antigen I/II-like proteins each possess Asn¹¹⁸²Gly and/or Val¹¹⁸⁵Pro substitutions that are incompatible with P. gingivalis adherence (11). Interestingly, the VXXLL motif is also present only in the oralis streptococci, supporting our finding that both motifs are required for P. gingivalis adherence. However, most of the other antigen I/II-like proteins possess a motif related to VXXLL that also resembles the consensus NR box, and the amino acids that occupy the +1, +4, and +5 positions fall among those that are functionally tolerated in the NR box. The two exceptions are antigen I/II-like proteins of Streptococcus suis and Streptococcus agalactiae, which contain Lys at the +4 position (see Fig. 2). Given the presence and conservation of this motif in antigen I/II proteins, it is tempting to speculate that they may also mediate protein-protein or interspecies interactions in these organisms and that the specificity of these interactions may be dictated by sequence variability that is present in the downstream motif corresponding to NITVK.

In general, there exist few if any therapeutic compounds that are targeted toward specific pathogenic organisms in the oral cavity. However, the potent inhibitory activity exhibited by BAR raises the possibility that peptide or peptidomimetic analogs of BAR could be developed to reduce P. gingivalis colonization of supragingival plaque. Interestingly, the NR box has been targeted for drug intervention since biochemical studies suggested that the coactivator-NR interaction could be blocked by small peptides containing the LXXLL motif (6). In addition, Galande and colleagues (12, 13) showed that disulfide- and thioether-bridged side-chain cyclization induced -helicity in peptides comprising the LXXLL motif and that these peptides modulated estrogen receptor activity. Thus, modification of the VXXLL and NITVK motifs, either to constrain the flexibility of the peptide or to maintain its -helical character, might further improve the specific inhibitory activity of BAR peptide analogs. Another consideration in developing the BAR peptide as a potential therapeutic agent against P. gingivalis is its susceptibility to proteolytic degradation. P. gingivalis is a highly proteolytic organism, and cell extracts from P. gingivalis cultures degrade BAR. However, it is interesting that the addition of minor fimbrial antigen (Mfa1; the receptor for BAR) to these extracts protects the peptide from degradation (D. Demuth, unpublished data), suggesting that the affinity of BAR for Mfa1 is greater than for the proteases secreted by P. gingivalis. This provides an explanation as to why BAR competitively inhibits P. gingivalis adherence to streptococci in the presence of proteolytic activities.

In summary, our studies have expanded the interacting interface between the SspB and Mfa1 proteins to include the predicted VXXLL -helix. Together with the NITVK motif, this domain resembles the eukaryotic NR box protein-protein interaction domain, and this similarity may provide a foundation upon which to generate additional analogs of the BAR peptide that may be more-potent antibiofilm compounds by targeting the initial adherence of P. gingivalis to oral streptococci.

 
We thank J. B. Chaires and Nichola Garbett of the Biophysics Core Facility at the James Graham Brown Cancer Center at the University of Louisville for their assistance with circular dichroism spectroscopy.

This work was supported by PHS grant RO1DE12505 from the NIDCR.

 
* Corresponding author. Mailing address: Department of Periodontics, Endodontics, and Dental Hygiene, University of Louisville School of Dentistry, 501 South Preston Street, Room 209, Louisville, KY 40292. Phone: (502) 852-3807. Fax: (502) 852-4052. E-mail: drdemu01{at}louisville.edu

Published ahead of print on 12 May 2008.

Editor: A. Camilli

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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Infection and Immunity, July 2008, p. 3273-3280, Vol. 76, No. 7
0019-9567/08/$08.00+0     doi:10.1128/IAI.00366-08
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