Proteasome-independent Degradation of Canonical NFκB Complex Components by the NleC Protein of Pathogenic Escherichia coli

The NFκB transcription factor is a key component of immune and inflammatory signaling as its activation induces the expression of antimicrobial reagents, chemokines, cytokines, and anti-apoptotic factors. Many pathogens encode effector proteins that target factors regulating NFκB activity and can provide novel insights on regulatory mechanisms. Given the link of NFκB dysfunction with inflammatory diseases and some cancers, these effectors have therapeutic potential. Here, screening enteropathogenic Escherichia coli proteins for those implicated in suppressing NFκB function revealed that eGFP-NleC, unlike eGFP, strongly inhibited basal and TNFα-induced NFκB reporter activity to prevent secretion of the chemokine, IL-8. Work involving NleC variants, chemical inhibitors, and immunoprecipitation studies support NleC being a zinc metalloprotease that degrades NFκB-IκBα complexes. The findings are consistent with features between residues 33–65 recruiting NFκB for proteasomal-independent degradation by a mechanism inhibited by metalloprotease inhibitors or disruption of a consensus zinc metalloprotease motif spanning NleC residues 183–187. This raises the prospect that mammalian cells, or other pathogens, employ a similar mechanism to modulate NFκB activity. Moreover, NleC represents a novel tool for validating NFκB as a therapeutic target and, indeed, as a possible therapeutic reagent. The NFκB transcription factor is a key component of immune and inflammatory signaling as its activation induces the expression of antimicrobial reagents, chemokines, cytokines, and anti-apoptotic factors. Many pathogens encode effector proteins that target factors regulating NFκB activity and can provide novel insights on regulatory mechanisms. Given the link of NFκB dysfunction with inflammatory diseases and some cancers, these effectors have therapeutic potential. Here, screening enteropathogenic Escherichia coli proteins for those implicated in suppressing NFκB function revealed that eGFP-NleC, unlike eGFP, strongly inhibited basal and TNFα-induced NFκB reporter activity to prevent secretion of the chemokine, IL-8. Work involving NleC variants, chemical inhibitors, and immunoprecipitation studies support NleC being a zinc metalloprotease that degrades NFκB-IκBα complexes. The findings are consistent with features between residues 33–65 recruiting NFκB for proteasomal-independent degradation by a mechanism inhibited by metalloprotease inhibitors or disruption of a consensus zinc metalloprotease motif spanning NleC residues 183–187. This raises the prospect that mammalian cells, or other pathogens, employ a similar mechanism to modulate NFκB activity. Moreover, NleC represents a novel tool for validating NFκB as a therapeutic target and, indeed, as a possible therapeutic reagent. IntroductionInteraction of inflammatory stimuli, such as cytokines and microbial products, with their cognate receptors induces ubiquitination and phosphorylation cascades that alter the transcriptional profile of the cell to produce factors including antimicrobials, chemokines (e.g. IL-8), and cytokines (e.g. TNFα; see Refs. 1O'Dea E. Hoffmann A. Wiley Interdiscip Rev. Syst. Biol. Med. 2009; 1: 107-115Crossref PubMed Scopus (65) Google Scholar, 2Vallabhapurapu S. Karin M. Annu. Rev. Immunol. 2009; 27: 693-733Crossref PubMed Scopus (1998) Google Scholar, 3Hayden M.S. Ghosh S. Cell. 2008; 132: 344-362Abstract Full Text Full Text PDF PubMed Scopus (3440) Google Scholar, 4Hiscott J. Nguyen T.L. Arguello M. Nakhaei P. Paz S. Oncogene. 2006; 25: 6844-6867Crossref PubMed Scopus (200) Google Scholar). A critical player in this process is the transcription factor nuclear factor κB (NFκB), 3The abbreviations used are: NFκBnuclear factor κBIκBinhibitor of NFκBIKKIκB kinaseeGFPenhanced GFPEPECenteropathogenic E. coli. which forms homo- or heterodimers composed of p65 (RelA), RelB, c-Rel, p50, and p52 proteins, with the p65/p50 dimer being the most abundant and associated with the canonical NFκB pathway. The p65/p50 dimer is retained within the cytoplasm bound to the inhibitor of κB (IκB) until receptor-mediated activation of the inhibitor of κB kinase (IKK) phosphorylates IκB, thereby triggering its proteasomal degradation to release NFκB for import into the nucleus. Signaling by many receptors converges at the IKK complex, composed of IKKα, IKKβ, and the NFκB essential modulator (IKKγ), but often involves distinct or overlapping upstream adaptors including TRAF and MyD88 and serine/threonine kinases such as IRAK, TGF-β-activated kinase 1, and RIP1 (1O'Dea E. Hoffmann A. Wiley Interdiscip Rev. Syst. Biol. Med. 2009; 1: 107-115Crossref PubMed Scopus (65) Google Scholar, 2Vallabhapurapu S. Karin M. Annu. Rev. Immunol. 2009; 27: 693-733Crossref PubMed Scopus (1998) Google Scholar, 3Hayden M.S. Ghosh S. Cell. 2008; 132: 344-362Abstract Full Text Full Text PDF PubMed Scopus (3440) Google Scholar, 4Hiscott J. Nguyen T.L. Arguello M. Nakhaei P. Paz S. Oncogene. 2006; 25: 6844-6867Crossref PubMed Scopus (200) Google Scholar). The classic mechanism to terminate NFκB transcriptional activity involves the NFκB-dependent transactivation of IκB, which shuttles the transcription factor back into the cytoplasm (5Chiao P.J. Miyamoto S. Verma I.M. Proc. Natl. Acad. Sci. U.S.A. 1994; 91: 28-32Crossref PubMed Scopus (390) Google Scholar, 6Finco T.S. Baldwin A.S. Immunity. 1995; 3: 263-272Abstract Full Text PDF PubMed Scopus (385) Google Scholar). However, additional regulatory mechanisms have been described including proteasome-dependent degradation (7Saccani S. Marazzi I. Beg A.A. Natoli G. J. Exp. Med. 2004; 200: 107-113Crossref PubMed Scopus (213) Google Scholar) or processing of p65 by caspase and serine proteases to generate forms with inhibitory functions (8Ravi R. Bedi A. Fuchs E.J. Bedi A. Cancer Res. 1998; 58: 882-886PubMed Google Scholar, 9Franzoso G. Biswas P. Poli G. Carlson L.M. Brown K.D. Tomita-Yamaguchi M. Fauci A.S. Siebenlist U.K. J. Exp. Med. 1994; 180: 1445-1456Crossref PubMed Scopus (85) Google Scholar). The importance of NFκB in immune and inflammatory signaling is reflected by the fact that its dysregulation is linked to many diseases including cancer, diarrhea, arthritis, inflammatory bowel disease, and neurodegenerative diseases (10Sethi G. Sung B. Aggarwal B.B. Exp. Biol. Med. 2008; 233: 21-31Crossref PubMed Scopus (389) Google Scholar, 11Kumar A. Takada Y. Boriek A.M. Aggarwal B.B. J. Mol. Med. 2004; 82: 434-448Crossref PubMed Google Scholar).Given the co-evolution of micro-organisms with mammals, it is not surprising that many, mostly pathogens, inhibit NFκB signaling as part of their strategy to colonize normally privileged niches. Collectively, bacteria and viruses possess “effector” proteins that target most of the proteins known to play roles in transmitting signals that report the presence of foreign antigens (12Sansonetti P.J. Di Santo J.P. Immunity. 2007; 26: 149-161Abstract Full Text Full Text PDF PubMed Scopus (151) Google Scholar, 13Bhavsar A.P. Guttman J.A. Finlay B.B. Nature. 2007; 449: 827-834Crossref PubMed Scopus (400) Google Scholar, 14Schröder M. Bowie A.G. Biochem. Soc. Trans. 2007; 35: 1512-1514Crossref PubMed Scopus (26) Google Scholar). Examples of how pathogens inhibit NFκB signaling at diverse levels include the A52R protein of the vaccinia virus that acts as a dominant-negative homologue of MyD88 (15Bowie A. Kiss-Toth E. Symons J.A. Smith G.L. Dower S.K. O'Neill L.A. Proc. Natl. Acad. Sci. U.S.A. 2000; 97: 10162-10167Crossref PubMed Scopus (383) Google Scholar), the Yersinia YopP/J protein whose acetylation of IKKβ prevents activation (16Ruckdeschel K. Mannel O. Richter K. Jacobi C.A. Trülzsch K. Rouot B. Heesemann J. J. Immunol. 2001; 166: 1823-1831Crossref PubMed Scopus (128) Google Scholar, 17Mittal R. Peak-Chew S.Y. McMahon H.T. Proc. Natl. Acad. Sci. U.S.A. 2006; 103: 18574-18579Crossref PubMed Scopus (218) Google Scholar) and the Shigella OspG protein, which prevents IκB ubiquitination by targeting an E2 ubiquitin-conjugating enzyme (18Kim D.W. Lenzen G. Page A.L. Legrain P. Sansonetti P.J. Parsot C. Proc. Natl. Acad. Sci. U.S.A. 2005; 102: 14046-14051Crossref PubMed Scopus (268) Google Scholar). Another bacterium, enteropathogenic Escherichia coli (EPEC), has recently been reported to deliver at least three effectors into host cells to inhibit NFκB function. Although two of these effectors, NleB and NleE, are speculated to block signaling at the level of TGF-β-activated kinase 1 or IKKβ kinases (19Newton H.J. Pearson J.S. Badea L. Kelly M. Lucas M. Holloway G. Wagstaff K.M. Dunstone M.A. Sloan J. Whisstock J.C. Kaper J.B. Robins-Browne R.M. Jans D.A. Frankel G. Phillips A.D. Coulson B.S. Hartland E.L. PLoS Pathog. 2010; 6: e1000898Crossref PubMed Scopus (177) Google Scholar, 20Nadler C. Baruch K. Kobi S. Mills E. Haviv G. Farago M. Alkalay I. Bartfeld S. Meyer T.F. Ben-Neriah Y. Rosenshine I. PLoS Pathog. 2010; 6: e1000743Crossref PubMed Scopus (139) Google Scholar), the NleH protein binds the NFκB cofactor RPS3 (ribosomal protein S3) to inhibit the transcription of a subset of genes (21Gao X. Wan F. Mateo K. Callegari E. Wang D. Deng W. Puente J. Li F. Chaussee M.S. Finlay B.B. Lenardo M.J. Hardwidge P.R. PLoS Pathog. 2009; 5: e1000708Crossref PubMed Scopus (134) Google Scholar). In this study, we describe that the EPEC gene nleC encodes a protein which targets p65, p50, and IκBα proteins for degradation by a proteasome-independent mechanism. The findings suggest that NleC is a zinc metalloprotease that recruits NFκB complexes for degradation.DISCUSSIONHere, it is demonstrated that the nleC gene of enteropathogenic E. coli encodes a protein whose expression as an eGFP-fusion protein within HeLa cells potently inhibits the basal- and TNFα-stimulated activity of the NFκB transcription factor. This finding has applications for understanding and controlling the function of this critical component of mammalian inflammatory, immune modulation, and anti-apoptotic responses as its dysfunction is linked to inflammatory diseases and some cancers (10Sethi G. Sung B. Aggarwal B.B. Exp. Biol. Med. 2008; 233: 21-31Crossref PubMed Scopus (389) Google Scholar, 11Kumar A. Takada Y. Boriek A.M. Aggarwal B.B. J. Mol. Med. 2004; 82: 434-448Crossref PubMed Google Scholar). Although a variety of proteins delivered into host cells by bacterial pathogens have been described to inhibit NFκB activity (12Sansonetti P.J. Di Santo J.P. Immunity. 2007; 26: 149-161Abstract Full Text Full Text PDF PubMed Scopus (151) Google Scholar, 13Bhavsar A.P. Guttman J.A. Finlay B.B. Nature. 2007; 449: 827-834Crossref PubMed Scopus (400) Google Scholar), this study reveals that NleC represents a novel mechanism. Thus, ectopic expression of NleC is shown to lead to a dramatic decrease in the cellular levels of both p65 (RelA) and p50 that comprise the most abundant form of the dimeric transcription factor in the canonical NFκB signaling pathway (3Hayden M.S. Ghosh S. Cell. 2008; 132: 344-362Abstract Full Text Full Text PDF PubMed Scopus (3440) Google Scholar). Although most pathogen-encoded proteins target upstream kinases, ubiquitinases, and adaptor molecules (12Sansonetti P.J. Di Santo J.P. Immunity. 2007; 26: 149-161Abstract Full Text Full Text PDF PubMed Scopus (151) Google Scholar, 13Bhavsar A.P. Guttman J.A. Finlay B.B. Nature. 2007; 449: 827-834Crossref PubMed Scopus (400) Google Scholar), some target NFκB components as illustrated by chlamydia infection leading to p65 processing, whereas another EPEC effector, NleH, binds RPS3 to inhibit transcription of genes under the control of a NFκB-RPS3 complex (21Gao X. Wan F. Mateo K. Callegari E. Wang D. Deng W. Puente J. Li F. Chaussee M.S. Finlay B.B. Lenardo M.J. Hardwidge P.R. PLoS Pathog. 2009; 5: e1000708Crossref PubMed Scopus (134) Google Scholar, 31Lad S.P. Li J. da Silva Correia J. Pan Q. Gadwal S. Ulevitch R.J. Li E. Proc. Natl. Acad. Sci. U.S.A. 2007; 104: 2933-2938Crossref PubMed Scopus (80) Google Scholar). By contrast, ectopic NleC expression induced the rapid proteasomal-independent loss of p65 without evidence of processing intermediates. Interestingly, the proteasomal inhibitor augmented NleC-mediated inhibition of NFκB activity to suggest that the effector may be subjected to proteasomal degradation. Importantly, NleC expression also induced the cellular loss of p50 and IκBα, but not the upstream pathway components IKKα or IKKβ, thereby demonstrating degradation specificity. IκB acts to retain NFκB within the cytoplasm until IKKα/β-induced phosphorylation triggers its proteasomal-dependent degradation and release of NFκB for nuclear import (2Vallabhapurapu S. Karin M. Annu. Rev. Immunol. 2009; 27: 693-733Crossref PubMed Scopus (1998) Google Scholar). This suggests that NleC targets NFκB-IκBα complexes. Proteasome-independent, not dependent, degradation of NFκB components represents a novel inhibitory mechanism.Fluorescent microscopy studies revealed a pool of eGFP-NleC within the nucleus, unlike the similarly sized eGFP-NleH protein that accumulates at the cell periphery (21Gao X. Wan F. Mateo K. Callegari E. Wang D. Deng W. Puente J. Li F. Chaussee M.S. Finlay B.B. Lenardo M.J. Hardwidge P.R. PLoS Pathog. 2009; 5: e1000708Crossref PubMed Scopus (134) Google Scholar) (data not shown). As NleC is significantly larger than the cut-off size for free nuclear entry (<50 kDa), this suggests that it may carry a novel nuclear translocation signal (not evident by bioinformatic analysis; data not shown) or enters by associating with nuclear-targeted protein(s). Of interest, the EPEC EspF effector protein enters the nucleus by a process dependent on a domain with no recognizable signal sequence (32Dean P. Scott J.A. Knox A.A. Quitard S. Watkins N.J. Kenny B. PLoS Pathog. 2010; 6: e1000961Crossref PubMed Scopus (37) Google Scholar). Given that NleC is normally delivered into the cytoplasm and degrades p65, p50, and IκBα, it is likely that its target relates to the cytoplasmic NFκB-IκBα complex. Given the multifunctional nature of EPEC effectors (26Dean P. Kenny B. Curr. Opin. Microbiol. 2009; 12: 101-109Crossref PubMed Scopus (169) Google Scholar), it is possible that nuclear import represents a distinct function or perhaps relates to targeting NFκB-IκBα complexes being shuttled to the cytoplasm. Time course studies (epifluorescent microscopy and Western blot analysis) have failed to resolve whether NleC has a preference for cytoplasmic or nuclear pools of NFκB (data not shown). The mechanism and role of nuclear NleC import in the NFκB inhibitory process deserves further investigation.Interestingly, ectopic NleC expression inhibited the basal NFκB activity in immortalized HeLa cells to a greater extent than the NFκB activation inhibitor from This suggests that this basal activity involves additional of which some may be to is likely that such to other NFκB composed of RelB, c-Rel, p52 Moreover, as NleC expression cellular p50, this that it also alter the expression of genes by p50 J. PubMed Scopus Google Scholar). are to whether NleC degrades of NFκB RelB, c-Rel, IκB (e.g. (e.g. transcription factors (e.g. that NleC a novel mechanism for NFκB function by the finding that loss of transcription activity not involves proteasome-independent degradation of p65, p50, and in the of p65 processing but linked to NleC being a The metalloprotease by of the NFκB inhibitory activity of NleC by cells with metalloprotease inhibitors or unlike that with or Moreover, NleC a consensus zinc metalloprotease motif spanning residues whose disruption a similar as metalloprotease A inhibitory activity for NleC and the NleC metalloprotease relates to the of or with its transcription activity. p65 and p50 be with NleC it its function. Importantly, residues within the residues of NleC shown to be and essential to p65 and Interestingly, the residues for the inhibitory process thereby that features between residues or canonical NFκB studies are to the residues and mechanism or by which p65, p50 and IκBα are to NleC for degradation. The for residues of the zinc metalloprotease but not residues in the inhibitory process an on a NleC protein for its activity. Collectively, the data is consistent with a or with NFκB-IκBα complexes features between residues and for degradation its function as a zinc is known the role of NleC in the of as the nleC gene not on at least in and O. S. F. R.M. S. A. Phillips A.D. Hartland E.L. M.J. Frankel G. 2005; PubMed Scopus Google Scholar, M. E. R. O. S. Badea L. S. M. Frankel G. Robins-Browne R.M. Hartland E.L. 2006; PubMed Scopus Google Scholar). a suggest that NFκB cellular levels EPEC NFκB function has been inhibited M. Kenny B. Microbiol. 2007; PubMed Scopus Google Scholar). suggests that such inhibition is to the activity of the NleB and effectors to at the level of IKK or the upstream kinase TGF-β-activated kinase 1 (19Newton H.J. Pearson J.S. Badea L. Kelly M. Lucas M. Holloway G. Wagstaff K.M. Dunstone M.A. Sloan J. Whisstock J.C. Kaper J.B. Robins-Browne R.M. Jans D.A. Frankel G. Phillips A.D. Coulson B.S. Hartland E.L. PLoS Pathog. 2010; 6: e1000898Crossref PubMed Scopus (177) Google Scholar, 20Nadler C. Baruch K. Kobi S. Mills E. Haviv G. Farago M. Alkalay I. Bartfeld S. Meyer T.F. Ben-Neriah Y. Rosenshine I. PLoS Pathog. 2010; 6: e1000743Crossref PubMed Scopus (139) Google Scholar). these studies are consistent with for other effectors, a illustrated by the role for NleH (21Gao X. Wan F. Mateo K. Callegari E. Wang D. Deng W. Puente J. Li F. Chaussee M.S. Finlay B.B. Lenardo M.J. Hardwidge P.R. PLoS Pathog. 2009; 5: e1000708Crossref PubMed Scopus (134) Google Scholar). Although EPEC can deliver NleC into host cells, an EPEC evidence of nleC gene transcription or NleC under the L. Wang D. D. A. C. A. R. Smith 2007; PubMed Scopus Google Scholar). studies support EPEC of NleC in host cells it cellular levels of p65 and p50 (data not shown). EPEC have mechanisms to the level of NleC within host cells the of the effectors to NFκB to be the role for NleC in NFκB function within the of an the protein has a degradation activity that may be for the function of this transcriptional factor whose dysfunction is linked to inflammatory diseases and some cancers (10Sethi G. Sung B. Aggarwal B.B. Exp. Biol. Med. 2008; 233: 21-31Crossref PubMed Scopus (389) Google Scholar, 11Kumar A. Takada Y. Boriek A.M. Aggarwal B.B. J. Mol. Med. 2004; 82: 434-448Crossref PubMed Google Scholar). Thus, the novel of NleC has as proteins as the can be delivered into cells in and J.S. M.J. Mol. Sci. 2008; PubMed Scopus Google Scholar), NleC may be not as a NFκB tool but also in validating NFκB as a therapeutic target in indeed, as a therapeutic reagent. IntroductionInteraction of inflammatory stimuli, such as cytokines and microbial products, with their cognate receptors induces ubiquitination and phosphorylation cascades that alter the transcriptional profile of the cell to produce factors including antimicrobials, chemokines (e.g. IL-8), and cytokines (e.g. TNFα; see Refs. 1O'Dea E. Hoffmann A. Wiley Interdiscip Rev. Syst. Biol. Med. 2009; 1: 107-115Crossref PubMed Scopus (65) Google Scholar, 2Vallabhapurapu S. Karin M. Annu. Rev. Immunol. 2009; 27: 693-733Crossref PubMed Scopus (1998) Google Scholar, 3Hayden M.S. Ghosh S. Cell. 2008; 132: 344-362Abstract Full Text Full Text PDF PubMed Scopus (3440) Google Scholar, 4Hiscott J. Nguyen T.L. Arguello M. Nakhaei P. Paz S. Oncogene. 2006; 25: 6844-6867Crossref PubMed Scopus (200) Google Scholar). A critical player in this process is the transcription factor nuclear factor κB (NFκB), 3The abbreviations used are: NFκBnuclear factor κBIκBinhibitor of NFκBIKKIκB kinaseeGFPenhanced GFPEPECenteropathogenic E. coli. which forms homo- or heterodimers composed of p65 (RelA), RelB, c-Rel, p50, and p52 proteins, with the p65/p50 dimer being the most abundant and associated with the canonical NFκB pathway. The p65/p50 dimer is retained within the cytoplasm bound to the inhibitor of κB (IκB) until receptor-mediated activation of the inhibitor of κB kinase (IKK) phosphorylates IκB, thereby triggering its proteasomal degradation to release NFκB for import into the nucleus. Signaling by many receptors converges at the IKK complex, composed of IKKα, IKKβ, and the NFκB essential modulator (IKKγ), but often involves distinct or overlapping upstream adaptors including TRAF and MyD88 and serine/threonine kinases such as IRAK, TGF-β-activated kinase 1, and RIP1 (1O'Dea E. Hoffmann A. Wiley Interdiscip Rev. Syst. Biol. Med. 2009; 1: 107-115Crossref PubMed Scopus (65) Google Scholar, 2Vallabhapurapu S. Karin M. Annu. Rev. Immunol. 2009; 27: 693-733Crossref PubMed Scopus (1998) Google Scholar, 3Hayden M.S. Ghosh S. Cell. 2008; 132: 344-362Abstract Full Text Full Text PDF PubMed Scopus (3440) Google Scholar, 4Hiscott J. Nguyen T.L. Arguello M. Nakhaei P. Paz S. Oncogene. 2006; 25: 6844-6867Crossref PubMed Scopus (200) Google Scholar). The classic mechanism to terminate NFκB transcriptional activity involves the NFκB-dependent transactivation of IκB, which shuttles the transcription factor back into the cytoplasm (5Chiao P.J. Miyamoto S. Verma I.M. Proc. Natl. Acad. Sci. U.S.A. 1994; 91: 28-32Crossref PubMed Scopus (390) Google Scholar, 6Finco T.S. Baldwin A.S. Immunity. 1995; 3: 263-272Abstract Full Text PDF PubMed Scopus (385) Google Scholar). However, additional regulatory mechanisms have been described including proteasome-dependent degradation (7Saccani S. Marazzi I. Beg A.A. Natoli G. J. Exp. Med. 2004; 200: 107-113Crossref PubMed Scopus (213) Google Scholar) or processing of p65 by caspase and serine proteases to generate forms with inhibitory functions (8Ravi R. Bedi A. Fuchs E.J. Bedi A. Cancer Res. 1998; 58: 882-886PubMed Google Scholar, 9Franzoso G. Biswas P. Poli G. Carlson L.M. Brown K.D. Tomita-Yamaguchi M. Fauci A.S. Siebenlist U.K. J. Exp. Med. 1994; 180: 1445-1456Crossref PubMed Scopus (85) Google Scholar). The importance of NFκB in immune and inflammatory signaling is reflected by the fact that its dysregulation is linked to many diseases including cancer, diarrhea, arthritis, inflammatory bowel disease, and neurodegenerative diseases (10Sethi G. Sung B. Aggarwal B.B. Exp. Biol. Med. 2008; 233: 21-31Crossref PubMed Scopus (389) Google Scholar, 11Kumar A. Takada Y. Boriek A.M. Aggarwal B.B. J. Mol. Med. 2004; 82: 434-448Crossref PubMed Google Scholar).Given the co-evolution of micro-organisms with mammals, it is not surprising that many, mostly pathogens, inhibit NFκB signaling as part of their strategy to colonize normally privileged niches. Collectively, bacteria and viruses possess “effector” proteins that target most of the proteins known to play roles in transmitting signals that report the presence of foreign antigens (12Sansonetti P.J. Di Santo J.P. Immunity. 2007; 26: 149-161Abstract Full Text Full Text PDF PubMed Scopus (151) Google Scholar, 13Bhavsar A.P. Guttman J.A. Finlay B.B. Nature. 2007; 449: 827-834Crossref PubMed Scopus (400) Google Scholar, 14Schröder M. Bowie A.G. Biochem. Soc. Trans. 2007; 35: 1512-1514Crossref PubMed Scopus (26) Google Scholar). Examples of how pathogens inhibit NFκB signaling at diverse levels include the A52R protein of the vaccinia virus that acts as a dominant-negative homologue of MyD88 (15Bowie A. Kiss-Toth E. Symons J.A. Smith G.L. Dower S.K. O'Neill L.A. Proc. Natl. Acad. Sci. U.S.A. 2000; 97: 10162-10167Crossref PubMed Scopus (383) Google Scholar), the Yersinia YopP/J protein whose acetylation of IKKβ prevents activation (16Ruckdeschel K. Mannel O. Richter K. Jacobi C.A. Trülzsch K. Rouot B. Heesemann J. J. Immunol. 2001; 166: 1823-1831Crossref PubMed Scopus (128) Google Scholar, 17Mittal R. Peak-Chew S.Y. McMahon H.T. Proc. Natl. Acad. Sci. U.S.A. 2006; 103: 18574-18579Crossref PubMed Scopus (218) Google Scholar) and the Shigella OspG protein, which prevents IκB ubiquitination by targeting an E2 ubiquitin-conjugating enzyme (18Kim D.W. Lenzen G. Page A.L. Legrain P. Sansonetti P.J. Parsot C. Proc. Natl. Acad. Sci. U.S.A. 2005; 102: 14046-14051Crossref PubMed Scopus (268) Google Scholar). Another bacterium, enteropathogenic Escherichia coli (EPEC), has recently been reported to deliver at least three effectors into host cells to inhibit NFκB function. Although two of these effectors, NleB and NleE, are speculated to block signaling at the level of TGF-β-activated kinase 1 or IKKβ kinases (19Newton H.J. Pearson J.S. Badea L. Kelly M. Lucas M. Holloway G. Wagstaff K.M. Dunstone M.A. Sloan J. Whisstock J.C. Kaper J.B. Robins-Browne R.M. Jans D.A. Frankel G. Phillips A.D. Coulson B.S. Hartland E.L. PLoS Pathog. 2010; 6: e1000898Crossref PubMed Scopus (177) Google Scholar, 20Nadler C. Baruch K. Kobi S. Mills E. Haviv G. Farago M. Alkalay I. Bartfeld S. Meyer T.F. Ben-Neriah Y. Rosenshine I. PLoS Pathog. 2010; 6: e1000743Crossref PubMed Scopus (139) Google Scholar), the NleH protein binds the NFκB cofactor RPS3 (ribosomal protein S3) to inhibit the transcription of a subset of genes (21Gao X. Wan F. Mateo K. Callegari E. Wang D. Deng W. Puente J. Li F. Chaussee M.S. Finlay B.B. Lenardo M.J. Hardwidge P.R. PLoS Pathog. 2009; 5: e1000708Crossref PubMed Scopus (134) Google Scholar). In this study, we describe that the EPEC gene nleC encodes a protein which targets p65, p50, and IκBα proteins for degradation by a proteasome-independent mechanism. The findings suggest that NleC is a zinc metalloprotease that recruits NFκB complexes for degradation.