Type I interferons (IFNs) are composed of various genes including IFN alpha (IFNA), beta (IFNB), omega, epsilon, and kappa. In humans the IFNA genes are composed of more than 13 subfamily genes, whereas there is only one IFNB gene. The large family of IFNA/B proteins all bind to a single receptor which is composed of two distinct chains: IFNAR1 and IFNAR2. The IFNA/B stimulation of the IFNA receptor complex leads to the formation of two transcriptional activator complexes: IFNA-activated-factor (AAF), which is a homodimer of STAT1 and IFN-stimulated gene factor 3 (ISGF3), which comprises STAT1, STAT2 and a member of the IRF family, IRF9/P48. AAF mediates activation of the IRF-1 gene by binding to GAS (IFNG-activated site), whereas ISGF3 activates several IFN-inducible genes including IRF3 and IRF7.
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Effects of IFNs result from induction of a subset of genes, called IFN stimulated genes (ISGs). These ISGs are mainly implicated in anti-viral, anti-angiogenic, immunomodulatory, cell cycle inhibitory effects and apoptotic functions. All IFNA/B-stimulated genes have a conserved region of about 15bp in their promoter called the Interferon Stimulation Response Element (ISRE). The transcription factor ISGF3 binds to this ISRE and induce the transcription of these genes by IFN.
Around 300 IFN-induced genes have been identified from different oligonucleotide microarray studies in melanoma (WM9) and fibrosarcoma (HT1080) cell lines as well as from human dendritic cells treated with IFN. Only the proteins which are well studied and their function characterized are represented here.
2-5A-dependent ribonuclease (RNASEL) is an endoribonuclease that is activated in the interferon (IFN) antiviral response. Its anti-viral effects are probably a combination of induction of apoptosis, cleavage of viral mRNA and induction of other anti-viral genes. ATP-binding cassette sub-family E member 1 (ABCE1, aka RNase L inhibitor, RLI) directly interacts with RNASEL and inhibits its endoribonuclease activity, thus antagonising the anti-viral effect of the IFN-regulated 2-5A/RNase L pathway (Martinand et al. 1998, Martinand et al. 1999, Le Roy et al. 2001).
Under certain conditions type I IFNs, IFNA/B are able to activate genes through a second STAT-based signaling cascade enabling the formation of p-STAT1:p-STAT1 homodimers called IFNA-activated-factor (AAF).
Phosphorylated tyrosine residue 466 on IFNAR1 acts as a docking site for STAT2. Latent STAT2 is recruited to this phosphotyrosine residue via its SH2 domain.
The ligand IFNalpha/beta (IFNA/B), interacts independently with the two interferon receptor subunits. Based on detailed binding studies with the extracellular domains of the receptor subunits tethered onto solid-supported membranes, a two-step binding mechanism was experimentally confirmed, where the ligand binds first to one of the receptor subunits and then recruits the second subunit (Gavutis et al. 2005). The efficiency of recruitment of the IFNA receptor subunits by the IFN ligand depends on the absolute and relative concentration of the receptor subunits. IFNAR2 chain constitutively associates with JAK1 kinase in its cytoplasmic domain. In addition IFNAR2 also binds STAT2 in a constitutive manner and this interaction is biochemically different from the interaction of STAT2 with phosphorylated IFNAR1. Although this interaction facilitates the recruitment of STAT2 to the receptors, the biological significance of this constitutive STAT2 interaction to IFNAR2 remains unclear (Nguyen et al, 2002). IFNAR2 not only associates with STAT2, but also with STAT1 and this binding of STAT1 to IFNAR2 depends on the presence of STAT2 but not vice versa. IFNA/B may first bind to the high-affinity subunit IFNAR2 and subsequently recruit IFNAR1 in a transient fashion (Lamken et al. 2004). Different type I IFNs interact differently with the two IFNA receptor (IFNAR) subunits, IFNB generates a more stable signaling complex than IFNA subtypes. The interaction between IFNalpha2 (IFNA2) and IFNAR2 has an affinity in the nM range, whereas the affinity of the interaction with INFB is about tenfold tighter.
The resultant ISGF3 trimeric complex then migrates to the nucleus and binds to interferon-stimulated response elements (ISREs). IRF9 is the DNA binding part of this ISGF3 complex. These ISREs are present in the promoters of a subset of ISGs (interferon stimulated genes), such as promyelocytic leukemia (PML), ISG15 ubiquitin-like modifier (ISG15), interferon-induced protein with tetratricopeptide repeats 2 (ISG54) and interferon alpha-inducible protein 6 (IFI6) to elicit an antiviral response.
The extracellular domain of IFNAR1 is atypical, consisting of a tandem array of four FNIII domains and the first three N-terminal FNIII domains are involved in ligand recognition. IFNAR1 is recruited to the binary complex (IFNA/B:IFNAR2) on the membrane to form the ternary complex (IFNAR2:IFNA/B:IFNAR1). TYK2 kinase is pre-associated with IFNAR1 and JAK1 with IFNAR2. The binding of IFNA/B to IFNA receptors brings these JAK kinase together, allowing cross-phosphorylation and kinase activation.
The phosphorylated STAT2:STAT1 heterodimer associates with interferon-regulating factor 9 (IRF9) to form the interferon-stimulated gene factor 3 (ISGF3) complex.
Phosphotyrosine on STAT2 acts as docking site for STAT1 molecules. STAT1 binds to phosphorylated STAT2 and this is followed by STAT1 phosphorylation on tyrosine residue 701 (Y701). These STATs recruited to the phosporylated IFNAR1 form two distinct transcriptional activator complexes, namely, IFN-alpha-activated factor (AAF) and IFN-stimulated gene factor 3 (ISGF3). AAF is a homodimer of STAT1, whereas ISGF3 is a heterotrimeric complex of STAT1, STAT2 and IRF9 (also known as p48 or ISGF3gamma) (Honda et al. 2005).
The two chains IFNAR1 and IFNAR2 are pre-associated with the JAK kinases TYK2 and JAK1, respectively. Receptor heterodimerization brings these JAK kinases into close proximity and they are activated by reciprocal trans-phosphorylation. Tyr-1054 and Tyr-1055 within the activation loop of TYK2 sub-domain VII are critical for TYK2 activation. For JAK1 two tyrosine residues with in the KEYY motif (Tyr 1034 and Tyr 1035) of the kinase domain are thought to be transphosphorylated.
TYK2 functions as part of a receptor complex to trigger intracellular signaling in response to IFNA/B. TYK2 bound to IFNAR1 subunit is activated in response to IFNA/B treatment and this in turn phosphorylates two tyrosine residues Y466 and Y481 in the juxta-membrane region of IFNAR1.
STAT2 recruited to the IFNAR1 subunit then becomes tyrosine phosphorylated on residue 690 by TYK2 kinase. This phosphotyrosine provides a docking site for recruitment of STAT1 to IFNAR1, which is then tyrosine phosphorylated and activated.
SOCS1/3 are the major negative regulators of IFNA/B signaling. They inhibit JAKs catalytic activity directly through their kinase inhibitory region (KIR) and turn off downstream IFNA/B signaling. SOCS1 may also prevent IFN signaling by targeting the signaling machinery to ubiquitin-proteasomal degradation pathway.
UBP43, a type I IFN-inducible cysteine protease acts as a negative regulator of type I IFN signaling. UBP43 binds directly to IFNAR2 and blocks JAK-receptor interaction leading to inhibition of downstream phosphorylation and other signaling events.
IFNA-activated-factor (AAF) translocates to nucleus and then promotes the expression of a distinct set of gamma activated sequence (GAS)-driven genes like IRF1. IRF1, in turn, induces the transcription of ISG15, ISG54 and IFI6 genes. This second pathway of STAT1 homodimer formation is primarily activated by IFNG and is likely to account for some of the functional overlap between type I and type II IFNs.
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IFN-regulated genesAnnotated Interactions
elements in type I
IFN-regulated genesIFNAR2 chain constitutively associates with JAK1 kinase in its cytoplasmic domain. In addition IFNAR2 also binds STAT2 in a constitutive manner and this interaction is biochemically different from the interaction of STAT2 with phosphorylated IFNAR1. Although this interaction facilitates the recruitment of STAT2 to the receptors, the biological significance of this constitutive STAT2 interaction to IFNAR2 remains unclear (Nguyen et al, 2002). IFNAR2 not only associates with STAT2, but also with STAT1 and this binding of STAT1 to IFNAR2 depends on the presence of STAT2 but not vice versa.
IFNA/B may first bind to the high-affinity subunit IFNAR2 and subsequently recruit IFNAR1 in a transient fashion (Lamken et al. 2004). Different type I IFNs interact differently with the two IFNA receptor (IFNAR) subunits, IFNB generates a more stable signaling complex than IFNA subtypes. The interaction between IFNalpha2 (IFNA2) and IFNAR2 has an affinity in the nM range, whereas the affinity of the interaction with INFB is about tenfold tighter.