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

Abstract

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 Ser/Thr 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 novel crystal structure of the transactivation domain of pseudophosphorylated human IRF5 reveals a striking 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 likely activates IRF5 and other family members by triggering remarkable conformational rearrangements that switch the C-terminal segment from an autoinihibitory to a dimerization role.

Protein kinase IKKβ-catalyzed phosphorylation of IRF5 at Ser462 induces its dimerization and nuclear translocation in myeloid cells

“…6B), whose phosphorylation by TBK1 is important for the nuclear translocation of IRF3. Ser462 of IRF5 is positioned within hydrogen-bonding distance of Arg354, an arginine residue that is conserved in IRF3 (24). The phosphorylation of Ser462 of IRF5 or Ser396 of IRF3 may therefore induce an interaction between the phosphoserine and arginine residues that stabilizes the dimeric form of IRF5 and permits its nuclear translocation and the stimulation of gene transcription.…”

Phosphorylation of IRF4 by ROCK2 regulates IL-17 and IL-21 production and the development of autoimmunity in mice

“…Consistent with previous studies demonstrating that IRF4-deficient CD4 + T cells cannot upregulate RORγt upon exposure to Th17 skewing conditions (11), IRF4 phosphorylation was also important for the ability of IRF4 to directly target the RORγt promoter and for the induction of RORγt. Interestingly, the 2 serines targeted by ROCK2, S446 and S447, are contained within an α-helical region of IRF4 located carboxyterminal of the IRF association domain (55). Structural studies of IRF3 and IRF5 have demonstrated that this region serves an autoinhibitory function and that its phosphorylation triggers a conformational rearrangement that relieves the autoinhibition (55)(56)(57).…”

“…Interestingly, the 2 serines targeted by ROCK2, S446 and S447, are contained within an α-helical region of IRF4 located carboxyterminal of the IRF association domain (55). Structural studies of IRF3 and IRF5 have demonstrated that this region serves an autoinhibitory function and that its phosphorylation triggers a conformational rearrangement that relieves the autoinhibition (55)(56)(57). Despite sharing structural similarities, the carboxyterminal regions of different IRFs exhibit very low sequence similarity (55).…”

Beneficial innate signaling interference for antibacterial responses by a Toll-like receptor–mediated enhancement of the MKP-IRF3 axis

“…The activation of the STING (or RLR) pathway enhances the association of IRF3 and TBK1, a process essential to IRF3 activation by TBK1. However, phosphorylated IRF3 must promptly dissociate from TBK1 to undergo the conformational changes required for its dimerization (5,25). Hence, if IRF3 remains associated with TBK1 by additionally interacting molecules induced/activated by TLR4-MyD88 signaling, the IRF3 activation processes would be inhibited.…”