Insights into interferon regulatory factor activation from the crystal structure of dimeric IRF5

Department of Biochemistry and Molecular Pharmacology, 364 Plantation Street, Worcester, Massachusetts 01605, USA.
Nature Structural & Molecular Biology (Impact Factor: 13.31). 11/2008; 15(11):1213-20. DOI: 10.1038/nsmb.1496
Source: PubMed


Interferon regulatory factors (IRFs) are essential in the innate immune response and other physiological processes. Activation of these proteins in the cytoplasm is triggered by phosphorylation of serine and threonine residues in a C-terminal autoinhibitory region, which stimulates dimerization, transport into the nucleus, assembly with the coactivator CBP/p300 and initiation of transcription. The crystal structure of the transactivation domain of pseudophosphorylated human IRF5 strikingly reveals a dimer in which the bulk of intersubunit interactions involve a highly extended C-terminal region. The corresponding region has previously been shown to block CBP/p300 binding to unphosphorylated IRF3. Mutation of key interface residues supports the observed dimer as the physiologically activated state of IRF5 and IRF3. Thus, phosphorylation is likely to activate IRF5 and other family members by triggering conformational rearrangements that switch the C-terminal segment from an autoinihibitory to a dimerization role.

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Available from: Celia A Schiffer, Dec 31, 2013
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    • "The C-terminal IAD domain in IRF5, which we show to mediate the interaction between IRF5 and TRIM21, has indeed been shown to undergo structural changes following TLR-mediated phosphorylation of conserved serine residues in this region. Phosphorylation-dependent dislocation of an autoinhibitory helix is necessary to expose IRF5 dimerization domain and to allow the formation of homo- and heterodimers which can then associate with other transcriptional co-activators such as CBP/p300 [12], [37]. Furthermore, analysis of the crystal structure of the closely related IRF3 and other IRF family members suggests the possibility that, in an inactive state, the N-terminal DNA binding domain of IRFs may be folded upon the C-terminal interaction domain [38]–[40]. "
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    ABSTRACT: IRF5 is a member of the Interferon Regulatory Factor (IRF) family of transcription factors activated downstream of the Toll-Like receptors (TLRs). Polymorphisms in IRF5 have been shown to be associated with the autoimmune disease Systemic Lupus Erythematosus (SLE) and other autoimmune conditions, suggesting a central role for IRF5 in the regulation of the immune response. Four different IRF5 isoforms originate due to alternative splicing and to the presence or absence of a 30 nucleotide insertion in IRF5 exon 6. Since the polymorphic region disturbs a PEST domain, a region associated with protein degradation, we hypothesized that the isoforms bearing the insertion might have increased stability, thus explaining the association of individual IRF5 isoforms with SLE. As the E3 ubiquitin ligase TRIpartite Motif 21 (TRIM21) has been shown to regulate the stability and hence activity of members of the IRF family, we investigated whether IRF5 is subjected to regulation by TRIM21 and whether dysregulation of this mechanism could explain the association of IRF5 with SLE. Our results show that IRF5 is degraded following TLR7 activation and that TRIM21 is involved in this process. Comparison of the individual IRF5 variants demonstrates that isoforms generated by alternative splicing are resistant to TRIM21-mediated degradation following TLR7 stimulation, thus providing a functional link between isoforms expression and stability/activity which contributes to explain the association of IRF5 with SLE.
    Full-text · Article · Aug 2014 · PLoS ONE
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    • "Although the detailed mechanism for the activation of IRF5 is still not fully elucidated, it is suggested that TRAF6-mediated K63-linked ubiquitination is important for IRF5 nuclear translocation in TLR7/9– MyD88-de- pendent signaling (Balkhi et al. 2008). In addition, phosphorylation of serine/threonine residues in a carboxyterminal auto-inhibitory region was shown to be a crucial step for dimer formation of IRF5 and interaction with CBP/p300 in the nucleus (Chen et al. 2008). Like IRF5, IRF1 also interacts with the central region of MyD88 (Negishi et al. 2006). "
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    ABSTRACT: The interferon-regulatory factor (IRF) family, originally identified as transcriptional regulators of the type I interferon system, consists of nine members in mammals. A large number of studies have revealed the versatile and critical functions performed by this transcription factor family in immunity and other biological processes. Most notably, the advances in the study of signal transducing innate immune receptors have placed many IRF members as central mediators in the regulation of innate immune responses. In parallel, mechanistic studies have made it clearer that many IRFs exert their function either in cooperation or competition with other factors. In this article, we discuss current advances on the multipurpose and critical functions of IRFs in the regulation of innate immunity, particularly as they instruct adaptive immunity.
    Preview · Article · Oct 2013 · Cold Spring Harbor Symposia on Quantitative Biology
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    • "Our results with a phosphomimetic of this serine (S415D) showed an increase in transcriptional activity and a modest increase in nuclear accumulation. The crystal structure predicts phosphorylation of serine 462v5 plays a significant role in stabilization of the formed IRF5 dimers [53]. The serine 462v5 is positioned within hydrogen bonding distance of arginine 354v5, an arginine that is conserved in human IRF3 and IRF7. "
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    ABSTRACT: The cellular defense to infection depends on accurate activation of transcription factors and expression of select innate immunity genes. Interferon regulatory factor 5 (IRF5), a risk factor for systemic lupus erythematosus, is activated in response to pathogen recognition receptor engagement and downstream effector molecules. We find the nucleotide-binding oligomerization domain containing protein 2 (NOD2) receptor to be a significant activator of IRF5. Phosphorylation is key to the regulation of IRF5, but the precise phosphorylation sites in IRF5 remained to be identified. We used mass spectrometry to identify for the first time specific residues that are phosphorylated in response to TANK-binding kinase-1 (TBK-1), tumor necrosis factor receptor-associated factor 6 (TRAF6), or receptor interacting protein 2 (RIP2). RIP2, a kinase known to function downstream of NOD2, was the most effective activator of IRF5-regulated gene expression. To determine if the phosphorylated residues are required or sufficient for IRF5 activity, aspartic acid phosphomimetic substitutions or inactivating alanine substitutions were tested. Phosphorylation of carboxyl serines 451 and 462 appear the primary trigger of IRF5 function in nuclear accumulation, transcription, and apoptosis. Results indicate polyubiquitination of IRF5 does not play a major role in its transcriptional activity, and that ubiquitination and phosphorylation are independent modifications.
    Full-text · Article · Mar 2012 · PLoS ONE
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