Trends in Immunology
ReviewThe Pellino family: IRAK E3 ligases with emerging roles in innate immune signalling
Introduction
Toll-like receptors (TLRs) function as primary sensors of conserved microbial structures known as pathogen-associated molecular patterns (PAMPs) [1]. Ten TLRs have been identified in humans, each representing a transmembrane protein with an ectodomain comprising leucine-rich repeats and a cytoplasmic Toll/interleukin (IL)-1 receptor (TIR) domain. TLRs 1, 2, 4, 5 and 6 are expressed on the cell surface and their ectodomains bind PAMPs such as lipopolysaccharide (LPS) and flagellin, which are exposed on the exterior of the microbe. Other TLRs, such as TLR3, TLR7, TLR8 and TLR9, are found in intracellular endosomal compartments in which they serve to detect nucleic acids from invading microbes. The engagement of TLR ectodomains by cognate ligands leads to the recruitment of TIR-domain-containing adaptor proteins to the cytoplasmic TIR domains of TLRs. This triggers activation of transcription factors, such as nuclear factor (NF)-κB and interferon regulatory factors (IRFs), and mitogen-activated protein kinase (MAPK) cascades. These pathways work in a highly co-ordinated fashion to promote expression of gene profiles that are tailored towards efficient removal of the invading microbe.
Section snippets
The importance of Pellino proteins for TLR signalling
Much research effort has focused on the signal transduction cascades that are employed by TLRs in activating transcription factors and MAPK pathways. Ubiquitylation, a form of post-translational modification, is crucially important in the regulation of proteins in these pathways and there is much interest in identifying the enzymes that control such ubiquitylation. The recent discovery of Pellino proteins as a family of E3 ubiquitin ligases that catalyse polyubiquitylation of interleukin-1
TLRs and activation of NF-κB
The cytoplasmic TIR domain of the TLRs is key to initiating all TLR signal transduction cascades. This domain is also found in the IL-1 receptor (IL-1R) [2]. The engagement of the ectodomains of TLRs and IL-1R by their cognate ligands leads to recruitment of soluble TIR-domain-containing adaptor proteins to their cytoplasmic domains. With the exception of TLR3, all TLRs and IL-1R recruit the TIR adaptor myeloid differentiation factor 88 (MyD88) [3]. Although TLR3 and TLR4 can use other
IRAK polyubiquitylation and TLR/IL-1R signalling
Although TAK1 promotes phosphorylation and activation of IKKα and IKKβ 13, 20, the exact mechanism by which TAK1 and IKKs are brought into close proximity is not clearly understood. The above-mentioned model favours the dissociation of the multi-molecular TRAF6–TAK1–TAB1–TAB2–TAB3 complex from IRAK1 at the membrane followed by its translocation to the cytosol, at which TAK1 becomes active, TAB2 promotes assembly of TRAF6 with the IKKs and TAK1 activates the IKKs [17]. Thus, IRAK1 remains at the
Bi-directional communication in the IRAK–Pellino complex
Pellino was first identified in Drosophila melanogaster by virtue of its association with Pelle, the Drosophila orthologue of IRAK [34]. Three mammalian homologues (Pellino1, 2 and 3) have since been identified, with Pellino3 being expressed in two spliced forms 35, 36, 37, 38, 39. Given that Drosophila Pellino was discovered based on its interaction with Pelle, mammalian Pellino proteins have been characterized by their potential to interact with IRAK molecules. Overexpression studies have
Pellino proteins interact with other TLR and IL1-R signalling proteins
Pellino proteins can also interact with other TLR signalling molecules. None of the Pellino proteins have been reported to interact with TLR or IL-1R complexes, however, one report indicates that IL-1 signalling can promote the interaction of overexpressed Pellino1 with MyD88 [44]. However, other reports have failed to detect any basal interactions between overexpressed MyD88 and Pellino proteins 37, 47. Many groups have described associations of each of the Pellino proteins with TRAF6 and
Functional roles of Pellino proteins in TLR and IL-1R signalling pathways
The association of Pellino proteins with key signalling molecules in the TLR and IL-1R pathways has prompted characterization of their involvement in mediating downstream signalling cascades. Although each of the Pellino proteins interact with common signalling molecules, such as IRAKs and TRAF6, and each has E3 ubiquitin ligase activity that can promote polyubiquitylation of IRAK1, emerging data clearly show that the Pellino proteins are not functionally redundant but instead display signs of
Pellino proteins as novel targets for negative regulation of TLR signalling
The functional characterization of the Pellino family to date highlights their diverse and varying roles in innate immune signalling. Intriguingly, some studies have demonstrated that members of the Pellino family can be targeted by regulatory strategies to suppress TLR signalling 33, 44, 49. Indeed, the role of Pellino3 as a direct negative regulator of innate immune signalling has already been described [33] (Figure 4a). However, two additional reports have identified Pellino1 and Pellino2 as
Concluding remarks and future perspectives
The discovery of the Pellino family has provided new insight into innate immunity and informed an understanding of previously unanswered questions in the field of TLR signalling. The role of the kinase activity of IRAK1 has remained an enigma since its original characterization, but Pellino-related studies now strongly indicate that IRAK1 and/or IRAK4 phosphorylate members of the Pellino family and enhance their E3 ligase activity, leading to polyubiquitylation of IRAK. Furthermore, the kinase
Note added in proof
A study published after this article had been accepted revealed for the first time that Pellino proteins contain a cryptic FHA domain that mediates interaction with phosphorylated IRAK1.
Lin et al., Pellino Proteins Contain a Cryptic FHA Domain that Mediates Interaction with Phosphorylated IRAK1, Structure (2008), doi:10.1016/j.str.2008.09.011
Acknowledgements
P.N.M. is funded by Science Foundation Ireland, Health Research Board of Ireland and Enterprise Ireland.
References (64)
- et al.
Summary and comparison of the signaling mechanisms of the Toll/interleukin-1 receptor family
Biochim. Biophys. Acta
(2002) MyD88 is an adaptor protein in the hToll/IL-1 receptor family signaling pathways
Mol. Cell
(1998)The role of interleukin 1 receptor-associated kinase-4 (IRAK-4) kinase activity in IRAK-4-mediated signaling
J. Biol. Chem.
(2004)Sequential autophosphorylation steps in the interleukin-1 receptor-associated kinase-1 regulate its availability as an adapter in interleukin-1 signaling
J. Biol. Chem.
(2004)Identification of TIFA as an adapter protein that links tumor necrosis factor receptor-associated factor 6 (TRAF6) to interleukin-1 (IL-1) receptor-associated kinase-1 (IRAK-1) in IL-1 receptor signaling
J. Biol. Chem.
(2003)Activation of the IκB kinase complex by TRAF6 requires a dimeric ubiquitin-conjugating enzyme complex and a unique polyubiquitin chain
Cell
(2000)TAB2 and TAB3 activate the NF-κB pathway through binding to polyubiquitin chains
Mol. Cell
(2004)- et al.
The interleukin-1 receptor-associated kinase is degraded by proteasomes following its phosphorylation
J. Biol. Chem.
(1997) - et al.
Dissection of a signaling pathway by which pathogen-associated molecular patterns recruit the JNK and p38 MAPKs and trigger cytokine release
J. Biol. Chem.
(2007) Activation of IKK by TNFα requires site-specific ubiquitination of RIP1 and polyubiquitin binding by NEMO
Mol. Cell
(2006)