Interaction between Interferon Consensus Sequence-binding Protein and COP9/Signalosome Subunit CSN2 (Trip15)

Abstract

Interferon consensus sequence-binding protein (ICSBP) is a member of the interferon regulatory factors (IRF) that has a pivotal role in mediating resistance to pathogenic infections in mice and in promoting the differentiation of myeloid cells. ICSBP exerts some of its transcriptional activities via association with other factors that enable its binding to a variety of promoters containing DNA composite elements. These interactions are mediated through a specific COOH-terminal domain termed IAD (IRF association domain). To gain a broader insight of the capacity of ICSBP to interact with other factors, yeast two-hybrid screens were performed using ICSBP-IAD as a bait against a B-cell cDNA library. Trip15 was identified as a specific interacting factor with ICSBP in yeast cells, which was also confirmed by in vitro glutathione S-transferase pull-down assays and by coimmunoprecipitation studies in COS7 cells. Trip15 was recently identified as a component of the COP9/ signalosome (CSN) complex composed of eight evolutionary conserved subunits and thus termed CSN2. This complex has a role in cell-signaling processes, which is manifested by its associated novel kinase activity and by the involvement of its subunits in regulating multiple cell-signaling pathways and cell-cycle progression. We show that in vitro association of ICSBP with the CSN leads to phosphorylation of ICSBP at a unique serine residue within its IAD. The phosphorylated residue is essential for efficient association with IRF-1 and thus for the repressor activity of ICSBP exerted on IRF-1. This suggests that the CSN has a role in integrating incoming signals that affect the transcriptional activity of ICSBP. Interferon (IFN)1 regulatory factors (IRFs) constitute a family of nine cellular transcription factors that share high homology at the first 115 amino acids which comprise the DNAbinding domain and therefore bind to similar DNA elements. These factors mediate numerous biological activities such as, anti-viral activity, IFN signaling, and immunomodulation (1, 2). IRFs are expressed in many cell types and tissues except for IFN consensus sequence-binding protein (ICSBP, also termed IRF-8) and IRF-4, which are expressed specifically in immune cells. IRFs act as transcriptional repressor or activator and the current data suggest that they harbor dual function. This dual functionality is in part due to interactions with different transcription factors resulting in the ability to interact with various promoters leading to alteration in transcriptional activities, i.e. repression or activation (for review, see Ref.

IFN-Stimulated Gene 15 Is Synergistically Activated Through Interactions Between the Myelocyte/Lymphocyte-Specific Transcription Factors, PU.1, IFN Regulatory Factor-8/IFN Consensus Sequence Binding Protein, and IFN Regulatory Factor-4: Characterization of a New Subtype of IFN-Stimulated Response Element

“…1A, compare lane 9 to lane 2). Further, we have also shown that serine 260 in IRF-8\ICSBP is essential for protein-protein interaction (21). Similar results are demonstrated here; mutation of this serine residue within the IAD of IRF-8\ICSBP eliminates its ability to negate IRF-1 (Fig.…”

“…Our data also show that serine 260, which is unique to the IAD of IRF-8\ICSBP, is essential for this interaction, suggesting that the phosphorylation state of this residue is essential (for more details see Ref. 21). Tyrosine phosphorylation is also essential for proper activity of IRF-8\ICSBP.…”

“…Mammalian expression vectors, which are also suitable for in vitro transcription (IVT), corresponding to IRF-1, IRF-4, PU.1, IRF-8\ICSBP, ICSBPL331P, and ICSBPS260A, were previously described (7,21).…”

The Jab1/COP9 signalosome subcomplex is a downstream mediator of Bcr-Abl kinase activity and facilitates cell-cycle progression

“…21,22 In mammals, CSN appears to be involved in important signal transduction pathways because of its ability to regulate protein kinases that phosphorylate c-Jun, p53, IB␣ and ICSBP/IRF-8. 21,[23][24][25] The phosphorylation of c-Jun and p53 requires direct binding to Jab1. 26 In addition, CSN cleaves the ubiquitinlike protein Nedd8 from the Cul1 subunit of the SCF 27 This "deneddylation" activity of CSN depends on the JAMM motif within the MPN domain of the Jab1/CSN5 subunit.…”

Toll-Like Receptor 9 Signaling Activates NF-κB through IFN Regulatory Factor-8/IFN Consensus Sequence Binding Protein in Dendritic Cells

“…It resides primarily in the nucleus, although it may temporarily be in the cytoplasm (26). IRF-8 interacts with proteins of the Ets family, such as PU.1 and TEL, and regulates the development of myeloid cells (25,27).…”