Cell Signaling and Function Organized by PB1 Domain Interactions
2006; Elsevier BV; Volume: 23; Issue: 5 Linguagem: Inglês
10.1016/j.molcel.2006.08.002
ISSN1097-4164
AutoresJorge Moscat, Marı́a T. Diaz-Meco, Armando Albert, Sonsoles Campuzano,
Tópico(s)Bone Metabolism and Diseases
ResumoThe PB1-domain-containing proteins p62, aPKC, MEKK2/MEKK3, MEK5, and Par-6 play roles in critical cell processes like osteoclastogenesis, angiogenesis, and early cardiovascular development or cell polarity. PB1 domains are scaffold modules that adopt the topology of ubiquitin-like β-grasp folds that interact with each other in a front-to-back mode to arrange heterodimers or homo-oligomers. The different PB1 domain adaptors provide specificity for PB1 kinases to ensure the effective transmission of cellular signals. Also, recent data suggest that PB1 domains may serve to orchestrate signaling cascades not involving other PB1 domains, such as the MEK5-ERK5 and p62-ERK1 interactions. The PB1-domain-containing proteins p62, aPKC, MEKK2/MEKK3, MEK5, and Par-6 play roles in critical cell processes like osteoclastogenesis, angiogenesis, and early cardiovascular development or cell polarity. PB1 domains are scaffold modules that adopt the topology of ubiquitin-like β-grasp folds that interact with each other in a front-to-back mode to arrange heterodimers or homo-oligomers. The different PB1 domain adaptors provide specificity for PB1 kinases to ensure the effective transmission of cellular signals. Also, recent data suggest that PB1 domains may serve to orchestrate signaling cascades not involving other PB1 domains, such as the MEK5-ERK5 and p62-ERK1 interactions. The PB1s are dimerization/oligomerization domains present in adaptor and scaffold proteins as well as kinases and serve to organize platforms that ensure specificity and fidelity during cellular signaling (Figure 1; http://smart.embl-heidelberg.de/smart/show_many_proteins.pl). A series of very recent studies has provided valuable information on the structural details that govern binding between the different PB1 modules (Hirano et al., 2004Hirano Y. Yoshinaga S. Ogura K. Yokochi M. Noda Y. Sumimoto H. Inagaki F. Solution structure of atypical protein kinase C PB1 domain and its mode of interaction with ZIP/p62 and MEK5.J. Biol. Chem. 2004; 279: 31883-31890Crossref PubMed Scopus (56) Google Scholar, Ito et al., 2001Ito T. Matsui Y. Ago T. Ota K. Sumimoto H. Novel modular domain PB1 recognizes PC motif to mediate functional protein-protein interactions.EMBO J. 2001; 20: 3938-3946Crossref PubMed Scopus (132) Google Scholar, Leitner et al., 2005Leitner D. Wahl M. Labudde D. Krause G. Diehl A. Schmieder P. Pires J.R. Fossi M. Wiedemann U. Leidert M. Oschkinat H. The solution structure of an N-terminally truncated version of the yeast CDC24p PB1 domain shows a different beta-sheet topology.FEBS Lett. 2005; 579: 3534-3538Abstract Full Text Full Text PDF PubMed Scopus (3) Google Scholar, Noda et al., 2003Noda Y. Kohjima M. Izaki T. Ota K. Yoshinaga S. Inagaki F. Ito T. Sumimoto H. Molecular recognition in dimerization between PB1 domains.J. Biol. Chem. 2003; 278: 43516-43524Crossref PubMed Scopus (69) Google Scholar, Terasawa et al., 2001Terasawa H. Noda Y. Ito T. Hatanaka H. Ichikawa S. Ogura K. Sumimoto H. Inagaki F. Structure and ligand recognition of the PB1 domain: a novel protein module binding to the PC motif.EMBO J. 2001; 20: 3947-3956Crossref PubMed Scopus (58) Google Scholar, Wilson et al., 2003Wilson M.I. Gill D.J. Perisic O. Quinn M.T. Williams R.L. PB1 domain-mediated heterodimerization in NADPH oxidase and signaling complexes of atypical protein kinase C with Par6 and p62.Mol. Cell. 2003; 12: 39-50Abstract Full Text Full Text PDF PubMed Scopus (154) Google Scholar, Yoshinaga et al., 2003Yoshinaga S. Kohjima M. Ogura K. Yokochi M. Takeya R. Ito T. Sumimoto H. Inagaki F. The PB1 domain and the PC motif-containing region are structurally similar protein binding modules.EMBO J. 2003; 22: 4888-4897Crossref PubMed Scopus (32) Google Scholar) and explains how they direct the formation of different macromolecular signaling complexes. Crystallographic and NMR experiments have established the 3D structure of a number of PB1 domains (Hirano et al., 2004Hirano Y. Yoshinaga S. Ogura K. Yokochi M. Noda Y. Sumimoto H. Inagaki F. Solution structure of atypical protein kinase C PB1 domain and its mode of interaction with ZIP/p62 and MEK5.J. Biol. Chem. 2004; 279: 31883-31890Crossref PubMed Scopus (56) Google Scholar, Hirano et al., 2005Hirano Y. Yoshinaga S. Takeya R. Suzuki N.N. Horiuchi M. Kohjima M. Sumimoto H. Inagaki F. Structure of a cell polarity regulator, a complex between atypical PKC and Par6 PB1 domains.J. Biol. Chem. 2005; 280: 9653-9661Crossref PubMed Scopus (55) Google Scholar, Muller et al., 2006Muller S. Kursula I. Zou P. Wilmanns M. Crystal structure of the PB1 domain of NBR1.FEBS Lett. 2006; 580: 341-344Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar, Terasawa et al., 2001Terasawa H. Noda Y. Ito T. Hatanaka H. Ichikawa S. Ogura K. Sumimoto H. Inagaki F. Structure and ligand recognition of the PB1 domain: a novel protein module binding to the PC motif.EMBO J. 2001; 20: 3947-3956Crossref PubMed Scopus (58) Google Scholar, Wilson et al., 2003Wilson M.I. Gill D.J. Perisic O. Quinn M.T. Williams R.L. PB1 domain-mediated heterodimerization in NADPH oxidase and signaling complexes of atypical protein kinase C with Par6 and p62.Mol. Cell. 2003; 12: 39-50Abstract Full Text Full Text PDF PubMed Scopus (154) Google Scholar, Yoshinaga et al., 2003Yoshinaga S. Kohjima M. Ogura K. Yokochi M. Takeya R. Ito T. Sumimoto H. Inagaki F. The PB1 domain and the PC motif-containing region are structurally similar protein binding modules.EMBO J. 2003; 22: 4888-4897Crossref PubMed Scopus (32) Google Scholar). All of them display the topology of a ubiquitin-like β-grasp fold, including six-stranded β-sheets and two α helices (Figure 2A). These structural studies also show that some of them include the OPCA motif, a short sequence containing a cluster of acidic amino acids initially termed AID or octicosapeptide (Ponting et al., 2002Ponting C.P. Ito T. Moscat J. Diaz-Meco M.T. Inagaki F. Sumimoto H. OPR, PC and AID: all in the PB1 family.Trends Biochem. Sci. 2002; 27: 10Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar), which folds along the hairpin formed by the β strands B4 and B5 and the α helix H2. Other PB1 domains cluster several positively charged residues on the topologically opposite side of the location of the OPCA motif. Finally, another group displays both the OPCA and the positively charged residues.Figure 2Molecular Structure of the PB1 DomainsShow full caption(A) Ribbon representation of the structure of the PB1 domain of PKCι (PDB code 1WMH). The conserved acidic residues of the OPCA motif are shown in stick representation.(B) Schematic representation of the PKCι/Par-6 PB1 domain heterodimer (PDB code 1WMH). The PKCι and Par-6 PB1 domains are depicted in blue and yellow, respectively. The residues responsible for the formation of the acidic and basic clusters are shown in stick representation. A schematic diagram of the front-to-back interaction between PB1 domains is also depicted.(C) The electrostatic potential on the interaction surface of the PKCι (left) and Par-6 (right) PB1 domains (red, negative; blue, positive) are shown. Each domain is rotated approximately 90 degrees with respect to the view in B in an open book style. The molecular surface is representative of the A-type (PKCι) and B-type (Par-6) domains. The conserved residues involved in the formation of the acidic and basic clusters (Ac and Bc) are labeled and encircled. Figures were produced according to Kraulis, 1991Kraulis P. MOLSCRIPT: a program to produce both detailed and schematic plots of protein structures.J. Appl. Crystallogr. 1991; 24: 946-950Crossref Google Scholar and Merritt and Murphy, 1994Merritt E.A. Murphy M.E. Raster3D version 2.0. A program for photorealistic molecular graphics.Acta Crystallogr. D Biol. Crystallogr. 1994; 50: 869-873Crossref PubMed Scopus (2786) Google Scholar.View Large Image Figure ViewerDownload Hi-res image Download (PPT) (A) Ribbon representation of the structure of the PB1 domain of PKCι (PDB code 1WMH). The conserved acidic residues of the OPCA motif are shown in stick representation. (B) Schematic representation of the PKCι/Par-6 PB1 domain heterodimer (PDB code 1WMH). The PKCι and Par-6 PB1 domains are depicted in blue and yellow, respectively. The residues responsible for the formation of the acidic and basic clusters are shown in stick representation. A schematic diagram of the front-to-back interaction between PB1 domains is also depicted. (C) The electrostatic potential on the interaction surface of the PKCι (left) and Par-6 (right) PB1 domains (red, negative; blue, positive) are shown. Each domain is rotated approximately 90 degrees with respect to the view in B in an open book style. The molecular surface is representative of the A-type (PKCι) and B-type (Par-6) domains. The conserved residues involved in the formation of the acidic and basic clusters (Ac and Bc) are labeled and encircled. Figures were produced according to Kraulis, 1991Kraulis P. MOLSCRIPT: a program to produce both detailed and schematic plots of protein structures.J. Appl. Crystallogr. 1991; 24: 946-950Crossref Google Scholar and Merritt and Murphy, 1994Merritt E.A. Murphy M.E. Raster3D version 2.0. A program for photorealistic molecular graphics.Acta Crystallogr. D Biol. Crystallogr. 1994; 50: 869-873Crossref PubMed Scopus (2786) Google Scholar. The interaction between two PB1s will differ depending on whether they have one OPCA, the basic cluster, or both, giving rise to different combinations of homo- and heterodimers. For example, the molecular architecture of two important PB1-PB1 heterodimers has recently been addressed by solving the X-ray structures of the P40phox-p67phox and the PKCλ/ι-Par-6 PB1 domain complexes (Hirano et al., 2005Hirano Y. Yoshinaga S. Takeya R. Suzuki N.N. Horiuchi M. Kohjima M. Sumimoto H. Inagaki F. Structure of a cell polarity regulator, a complex between atypical PKC and Par6 PB1 domains.J. Biol. Chem. 2005; 280: 9653-9661Crossref PubMed Scopus (55) Google Scholar, Wilson et al., 2003Wilson M.I. Gill D.J. Perisic O. Quinn M.T. Williams R.L. PB1 domain-mediated heterodimerization in NADPH oxidase and signaling complexes of atypical protein kinase C with Par6 and p62.Mol. Cell. 2003; 12: 39-50Abstract Full Text Full Text PDF PubMed Scopus (154) Google Scholar). The data show that these interactions involve specific electrostatic contacts between the acidic residues of the OPCA motif located in one side of a given PB1 and the few conserved basic residues of the other PB1 in a front-to-back fashion (Figure 2B). This leads to the classification of the PB1 domains according to their ability to interact with other PB1 domains through the acidic side (A-type), through the basic side (B-type), or through both sides (A-B type). Two clusters of acidic residues of the OPCA motif are buried on the interaction surface of the A-type PB1 domain: the acidic cluster 1 (Ac1), consisting of residues of the loop between β3 and β4; and the acidic cluster 2 (Ac2) that includes residues at the N terminus of the A2 α helix. The B-type PB1 interaction surface involves residues forming the two basic clusters Bc1 and Bc2 that belong to distant secondary structural elements (Figure 2C). The comparative description of the P40phox-p67phox and PKCλ/ι-Par-6 PB1 complexes shows that the formation of salt bridges between Ac1-Bc1 and Ac2-Bc2 residues is the only general structural property governing PB1-PB1 interactions (Hirano et al., 2005Hirano Y. Yoshinaga S. Takeya R. Suzuki N.N. Horiuchi M. Kohjima M. Sumimoto H. Inagaki F. Structure of a cell polarity regulator, a complex between atypical PKC and Par6 PB1 domains.J. Biol. Chem. 2005; 280: 9653-9661Crossref PubMed Scopus (55) Google Scholar). Biochemical and mutagenesis studies have shown that binding between the conserved lysine residue on the B1 β strand included in Bc1, and the acidic residues of the OPCA loop is essential for the oligomerization of PB1 domains (Noda et al., 2003Noda Y. Kohjima M. Izaki T. Ota K. Yoshinaga S. Inagaki F. Ito T. Sumimoto H. Molecular recognition in dimerization between PB1 domains.J. Biol. Chem. 2003; 278: 43516-43524Crossref PubMed Scopus (69) Google Scholar, Wilson et al., 2003Wilson M.I. Gill D.J. Perisic O. Quinn M.T. Williams R.L. PB1 domain-mediated heterodimerization in NADPH oxidase and signaling complexes of atypical protein kinase C with Par6 and p62.Mol. Cell. 2003; 12: 39-50Abstract Full Text Full Text PDF PubMed Scopus (154) Google Scholar, Yoshinaga et al., 2003Yoshinaga S. Kohjima M. Ogura K. Yokochi M. Takeya R. Ito T. Sumimoto H. Inagaki F. The PB1 domain and the PC motif-containing region are structurally similar protein binding modules.EMBO J. 2003; 22: 4888-4897Crossref PubMed Scopus (32) Google Scholar). The atomic superimposition of the PB1 structures (Sutcliffe et al., 1987Sutcliffe M.J. Haneef I. Carney D. Blundell T.L. Knowledge based modelling of homologous proteins, part I: three-dimensional frameworks derived from the simultaneous superposition of multiple structures.Protein Eng. 1987; 1: 377-384Crossref PubMed Scopus (375) Google Scholar) shows that their backbones overlap more than 70 Cα atoms and that the main differences are located in residues that are not involved in the oligomerization, particularly in the area covered by the β strands B2 and B3, and the α helix H2 of the B-type PB1domains. On the other hand, the 3D structure-based alignment of key PB1 domains shows that the residues involved in the interaction between the PB1 domains of aPKC and Par-6 are conserved in the A-type or B-type PB1 domains, respectively. As no other obvious sequence similarities are observed among PB1 domains, it is difficult to establish the structural determinants of the specificity that directs PB1-PB1 interactions. However, since PB1 domains conserve the essential residues that provide the scaffold for their interactions, it is possible to generate templates to build homology models for PB1 domains of unknown structure, and to predict the structure of unknown heterodimers. Two-hybrid screens in yeast, using as bait the first 126 amino acids of PKCλ/ι (which includes its PB1 domain), identified p62 (also known as sequestosome-1) as a partner of this kinase (Moscat and Diaz-Meco, 2000Moscat J. Diaz-Meco M.T. The atypical protein kinase Cs. Functional specificity mediated by specific protein adapters.EMBO Rep. 2000; 1: 399-403Crossref PubMed Scopus (188) Google Scholar). p62 specifically binds PKCλ/ι and PKCζ and harbors a number of domains that support its role as scaffold in aPKC signaling (Moscat and Diaz-Meco, 2000Moscat J. Diaz-Meco M.T. The atypical protein kinase Cs. Functional specificity mediated by specific protein adapters.EMBO Rep. 2000; 1: 399-403Crossref PubMed Scopus (188) Google Scholar) (Figure 1). The first clue about the function and mechanism of action of p62 came from the identification of receptor-interacting protein-1 (RIP1) as a p62-interacting protein (Moscat and Diaz-Meco, 2000Moscat J. Diaz-Meco M.T. The atypical protein kinase Cs. Functional specificity mediated by specific protein adapters.EMBO Rep. 2000; 1: 399-403Crossref PubMed Scopus (188) Google Scholar). RIP1 is a death domain (DD)-containing kinase that interacts, through homotypic interactions, with the DD-containing adaptor TRADD, which also bridges RIP1 to the TNFα receptor-1 (TNFR1) through the interactions of their respective DDs (Chen and Goeddel, 2002Chen G. Goeddel D.V. TNF-R1 signaling: a beautiful pathway.Science. 2002; 296: 1634-1635Crossref PubMed Scopus (1381) Google Scholar). Activated TNFR1 assembles a prosurvival signaling complex involving TRADD, RIP, and the NF-κB and JNK activators TRAF2 and TRAF5 (Chen and Goeddel, 2002Chen G. Goeddel D.V. TNF-R1 signaling: a beautiful pathway.Science. 2002; 296: 1634-1635Crossref PubMed Scopus (1381) Google Scholar) (Figure 3A). Genetic evidence demonstrates that the loss of RIP1, as well as the simultaneous genetic inactivation of TRAF2 and TRAF5, dramatically impairs NF-κB (Meylan and Tschopp, 2005Meylan E. Tschopp J. The RIP kinases: crucial integrators of cellular stress.Trends Biochem. Sci. 2005; 30: 151-159Abstract Full Text Full Text PDF PubMed Scopus (312) Google Scholar). RIP1 kinase activity is not required for NF-κB activation, but its intermediary region is essential for its TRAF2 binding function that helps to recruit the IKK complex via interaction with its IKKγ regulatory subunit (Chen and Goeddel, 2002Chen G. Goeddel D.V. TNF-R1 signaling: a beautiful pathway.Science. 2002; 296: 1634-1635Crossref PubMed Scopus (1381) Google Scholar, Meylan and Tschopp, 2005Meylan E. Tschopp J. The RIP kinases: crucial integrators of cellular stress.Trends Biochem. Sci. 2005; 30: 151-159Abstract Full Text Full Text PDF PubMed Scopus (312) Google Scholar) (Figure 3A). Interestingly, p62 also binds the intermediary domain of RIP1 through its ZZ region, further suggesting that p62 is a relevant player in RIP1 function (Moscat and Diaz-Meco, 2000Moscat J. Diaz-Meco M.T. The atypical protein kinase Cs. Functional specificity mediated by specific protein adapters.EMBO Rep. 2000; 1: 399-403Crossref PubMed Scopus (188) Google Scholar). Consistent with this hypothesis, the knockdown of p62 with an antisense construct severely impairs NF-κB activation in response to TNFα. Biochemical in vitro studies showed that recombinant pure aPKCs were able to phosphorylate the activation-loop residues of IKKβ, but not of IKKα (Moscat and Diaz-Meco, 2000Moscat J. Diaz-Meco M.T. The atypical protein kinase Cs. Functional specificity mediated by specific protein adapters.EMBO Rep. 2000; 1: 399-403Crossref PubMed Scopus (188) Google Scholar), offering a mechanistic explanation for the potential involvement of the p62/aPKC complex in RIP1-mediated NF-κB activation. Similar to the TNFR1/TRADD/RIP1/TRAF2 complex, IL-1 activates a module including the IL-1 receptor proximal adaptor MyD88, the kinases IRAK1 and IRAK4, and TRAF6 (Chen et al., 2006Chen Z.J. Bhoj V. Seth R.B. Ubiquitin, TAK1 and IKK: is there a connection?.Cell Death Differ. 2006; 13: 687-692Crossref PubMed Scopus (97) Google Scholar) (Figure 3B). The interaction of IRAK1 with TRAF6, once TRAF6 has been phosphorylated by IRAK4 and dislodged from MyD88, serves to assemble a complex including the kinase TAK1 and the scaffold proteins TAB1, TAB2, and TAB3 (Chen et al., 2006Chen Z.J. Bhoj V. Seth R.B. Ubiquitin, TAK1 and IKK: is there a connection?.Cell Death Differ. 2006; 13: 687-692Crossref PubMed Scopus (97) Google Scholar). The role of TRAF6 in this pathway has been proven genetically, and p62 has been identified as a relevant TRAF6-interacting partner in IL-1-activated cells (Moscat and Diaz-Meco, 2000Moscat J. Diaz-Meco M.T. The atypical protein kinase Cs. Functional specificity mediated by specific protein adapters.EMBO Rep. 2000; 1: 399-403Crossref PubMed Scopus (188) Google Scholar). Thus, p62 emerges as a component of both the TNFR1 and IL-1 receptor signaling complex due to its ability to interact with different adaptors of the two complexes (Figure 3B). The p62-TRAF6 interaction involves the TRAF domain of TRAF6 and a short stretch of amino acids in the p62 molecule that conforms to the TRAF binding consensus sequence (Moscat et al., 2006Moscat J. Rennert P. Diaz-Meco M.T. PKCzeta at the crossroad of NF-kappaB and Jak1/Stat6 signaling pathways.Cell Death Differ. 2006; 13 (Published online December 2, 2005): 702-711https://doi.org/10.1038/sj.cdd.4401823Crossref PubMed Scopus (101) Google Scholar). The role of the p62/aPKC cassette in NF-κB activation is remarkably conserved in Drosophila. Thus, the p62 homolog Ref(2)P has a strikingly similar domain organization to that of p62 (Moscat et al., 2006Moscat J. Rennert P. Diaz-Meco M.T. PKCzeta at the crossroad of NF-kappaB and Jak1/Stat6 signaling pathways.Cell Death Differ. 2006; 13 (Published online December 2, 2005): 702-711https://doi.org/10.1038/sj.cdd.4401823Crossref PubMed Scopus (101) Google Scholar) and has been shown to be required along with DaPKC for the activation of the Drosophila homolog of NF-κB in RNAi depletion experiments, both in cell culture and in vivo (Moscat et al., 2006Moscat J. Rennert P. Diaz-Meco M.T. PKCzeta at the crossroad of NF-kappaB and Jak1/Stat6 signaling pathways.Cell Death Differ. 2006; 13 (Published online December 2, 2005): 702-711https://doi.org/10.1038/sj.cdd.4401823Crossref PubMed Scopus (101) Google Scholar). Two additional isoforms have been described and are named ZIP2 and ZIP3 (Croci et al., 2003Croci C. Brandstatter J.H. Enz R. ZIP3, a new splice variant of the PKC-zeta-interacting protein family, binds to GABAC receptors, PKC-zeta, and Kvbeta 2.J. Biol. Chem. 2003; 278 (Published online November 12, 2002): 6128-6135https://doi.org/10.1074/jbc.M205162200Crossref PubMed Scopus (47) Google Scholar, Gong et al., 1999Gong J. Xu J. Bezanilla M. van Huizen R. Derin R. Li M. Differential stimulation of PKC phosphorylation of potassium channels by ZIP1 and ZIP2.Science. 1999; 285: 1565-1569Crossref PubMed Scopus (113) Google Scholar). ZIP2 lacks the TRAF6-interacting sequence, whereas ZIP3 lacks part of the C-terminal region. As both sequences are important for signaling by p62, the expression of these isoforms may have important impacts in p62-mediated signaling. TRAF6 and RIP1 also orchestrate the activation of the MAPK cascades, including those for JNK and p38 (Chen and Goeddel, 2002Chen G. Goeddel D.V. TNF-R1 signaling: a beautiful pathway.Science. 2002; 296: 1634-1635Crossref PubMed Scopus (1381) Google Scholar, Meylan and Tschopp, 2005Meylan E. Tschopp J. The RIP kinases: crucial integrators of cellular stress.Trends Biochem. Sci. 2005; 30: 151-159Abstract Full Text Full Text PDF PubMed Scopus (312) Google Scholar). However, p62 does not appear to play a role in those signaling cascades (Sanz et al., 1999Sanz L. Sanchez P. Lallena M.J. Diaz-Meco M.T. Moscat J. The interaction of p62 with RIP links the atypical PKCs to NF-kappaB activation.EMBO J. 1999; 18: 3044-3053Crossref PubMed Scopus (314) Google Scholar, Sanz et al., 2000Sanz L. Diaz-Meco M.T. Nakano H. Moscat J. The atypical PKC-interacting protein p62 channels NF-kappaB activation by the IL-1-TRAF6 pathway.EMBO J. 2000; 19: 1576-1586Crossref PubMed Google Scholar). Genetic and biochemical data demonstrate that TRAF6 is also involved in the RANK pathway (Lomaga et al., 1999Lomaga M.A. Yeh W.C. Sarosi I. Duncan G.S. Furlonger C. Ho A. Morony S. Capparelli C. Van G. Kaufman S. et al.TRAF6 deficiency results in osteopetrosis and defective interleukin-1, CD40, and LPS signaling.Genes Dev. 1999; 13: 1015-1024Crossref PubMed Scopus (1018) Google Scholar), another signaling system that controls, among other functions, the physiology of bone. Cells activated with RANK ligand (RANK-L) trigger RANK-associated TRAF6 molecules to activate NF-κB and JNK, but, in this case, without the intervention of MyD88 or IRAK (Karsenty and Wagner, 2002Karsenty G. Wagner E.F. Reaching a genetic and molecular understanding of skeletal development.Dev. Cell. 2002; 2: 389-406Abstract Full Text Full Text PDF PubMed Scopus (1143) Google Scholar). Mice deficient in p62 are not osteopetrotic like the TRAF6 or RANK KO mice, but they do show impaired osteoclastogenesis following stimulation by injection of the calciotropic hormone PTHrP. This indicates that p62 is required for efficient osteoclastogenesis under activated or stress conditions (Duran et al., 2004bDuran A. Serrano M. Leitges M. Flores J.M. Picard S. Brown J.P. Moscat J. Diaz-Meco M.T. The atypical PKC-interacting protein p62 is an important mediator of RANK-activated osteoclastogenesis.Dev. Cell. 2004; 6: 303-309Abstract Full Text Full Text PDF PubMed Scopus (252) Google Scholar) (Figure 3B). In vitro studies demonstrate that p62 levels dramatically increase when precursor cells are induced to differentiate into osteoclasts. Also, the loss of p62 severely impairs the sustained phase of NF-κB activation in RANK-stimulated osteoclasts. The mechanism whereby the p62/aPKC cassette regulates NF-κB by RIP1 and TRAF6 complexes, as well as its relative contribution to this pathway when compared to that of the TAB2/TAK1 module, is a very interesting question. Whereas p62−/− mice display a clear defect in RANK-induced NF-κB activation, TAB1−/− and TAB2−/− cells show normal IL-1 and TNFα-induced NF-κB (Sanjo et al., 2003Sanjo H. Takeda K. Tsujimura T. Ninomiya-Tsuji J. Matsumoto K. Akira S. TAB2 is essential for prevention of apoptosis in fetal liver but not for interleukin-1 signaling.Mol. Cell. Biol. 2003; 23: 1231-1238Crossref PubMed Scopus (101) Google Scholar, Shim et al., 2005Shim J.-H. Xiao C. Paschal A.E. Bailey S.T. Rao P. Hayden M.S. Lee K.-Y. Bussey C. Steckel M. Tanaka N. et al.TAK1, but not TAB1 or TAB2, plays an essential role in multiple signaling pathways in vivo.Genes Dev. 2005; 19 (Published online October 31, 2005): 2668-2681https://doi.org/10.1101/gad.1360650Crossref PubMed Google Scholar). This finding raises the question of what is the adaptor of TAK1 in this pathway. A possible explanation is the presence of TAB3 that may functionally compensate for the loss of TAB2 in the KO mice cells. TAK1 has recently been shown in biochemical and genetic experiments to be required for NF-κB and IKK activation in IL-1- and TNFα-activated cells (Chen et al., 2006Chen Z.J. Bhoj V. Seth R.B. Ubiquitin, TAK1 and IKK: is there a connection?.Cell Death Differ. 2006; 13: 687-692Crossref PubMed Scopus (97) Google Scholar, Sato et al., 2005Sato S. Sanjo H. Takeda K. Ninomiya-Tsuji J. Yamamoto M. Kawai T. Matsumoto K. Takeuchi O. Akira S. Essential function for the kinase TAK1 in innate and adaptive immune responses.Nat. Immunol. 2005; 6: 1087-1095Crossref PubMed Scopus (688) Google Scholar, Shim et al., 2005Shim J.-H. Xiao C. Paschal A.E. Bailey S.T. Rao P. Hayden M.S. Lee K.-Y. Bussey C. Steckel M. Tanaka N. et al.TAK1, but not TAB1 or TAB2, plays an essential role in multiple signaling pathways in vivo.Genes Dev. 2005; 19 (Published online October 31, 2005): 2668-2681https://doi.org/10.1101/gad.1360650Crossref PubMed Google Scholar). The role of the aPKCs as potential IKK kinases in the p62-driven activation of NF-κB in osteoclasts is less clear due to the potential redundancies played by the existence of two highly related aPKCs. In consequence, although cellular and biochemical studies demonstrate that RANK-L triggers the formation of a TRAF6/p62/aPKC complex in osteoclasts (Duran et al., 2004bDuran A. Serrano M. Leitges M. Flores J.M. Picard S. Brown J.P. Moscat J. Diaz-Meco M.T. The atypical PKC-interacting protein p62 is an important mediator of RANK-activated osteoclastogenesis.Dev. Cell. 2004; 6: 303-309Abstract Full Text Full Text PDF PubMed Scopus (252) Google Scholar), PKCζ−/− mice do not show alterations in the osteoclastogenic response. This indicates that p62 actions are largely PKCζ-independent but might rely on PKCλ/ι (Duran et al., 2004bDuran A. Serrano M. Leitges M. Flores J.M. Picard S. Brown J.P. Moscat J. Diaz-Meco M.T. The atypical PKC-interacting protein p62 is an important mediator of RANK-activated osteoclastogenesis.Dev. Cell. 2004; 6: 303-309Abstract Full Text Full Text PDF PubMed Scopus (252) Google Scholar). Future studies using conditional PKCλ/ι−/− mice should resolve this important issue. It is possible that p62 contributes to the activation of NF-κB as part of the RIP1 and TRAF6 complex via a mechanism that is independent of the aPKCs. That is, the ability of the C-terminal UBA domain of p62 to bind polyubiquitin chains (Wooten et al., 2005Wooten M.W. Geetha T. Seibenhener M.L. Babu J.R. Diaz-Meco M.T. Moscat J. The p62 scaffold regulates nerve growth factor-induced NF-kappaB activation by influencing TRAF6 polyubiquitination.J. Biol. Chem. 2005; 280: 35625-35629Crossref PubMed Scopus (158) Google Scholar), together with evidence that ubiquitination plays a positive role in NF-κB activation through the TRAF6 and RIP1 complexes (Chen et al., 2006Chen Z.J. Bhoj V. Seth R.B. Ubiquitin, TAK1 and IKK: is there a connection?.Cell Death Differ. 2006; 13: 687-692Crossref PubMed Scopus (97) Google Scholar), indicates that p62 may be involved in the regulation of the IKK complex via ubiquitin. This notion stems from a series of pioneering in vitro biochemical reconstitution experiments by Chen and coworkers. They identified TRAF6 as an E3 ubiquitin ligase that catalyzes the synthesis of polyubiquitin chains linked through lysine 63 (K63) (Chen et al., 2006Chen Z.J. Bhoj V. Seth R.B. Ubiquitin, TAK1 and IKK: is there a connection?.Cell Death Differ. 2006; 13: 687-692Crossref PubMed Scopus (97) Google Scholar, Gao and Karin, 2005Gao M. Karin M. Regulating the regulators: control of protein ubiquitination and ubiquitin-like modifications by extracellular stimuli.Mol. Cell. 2005; 19: 581-593Abstract Full Text Full Text PDF PubMed Scopus (132) Google Scholar) that, in contrast to the more classical K48-linked chains, do not target the polyubiquitinated protein to the proteasome but rather serve as a positive switch in the activation of IKK (Chen et al., 2006Chen Z.J. Bhoj V. Seth R.B. Ubiquitin, TAK1 and IKK: is there a connection?.Cell Death Differ. 2006; 13: 687-692Crossref PubMed Scopus (97) Google Scholar). The current model proposes that TAK1 would be activated by the autopolyubiquitination of TRAF6. TAB2 and/or TAB3 has been demonstrated in in vitro biochemical studies to be important in that process, probably through their ability to interact with K63 polyubiquitin chains by means of their well-conserved C-terminal zinc-finger domains (Chen et al., 2006Chen Z.J. Bhoj V. Seth R.B. Ubiquitin, TAK1 and IKK: is
Referência(s)