Figure 1. Type I and type II IFN receptors and the molecules involved in signal transduction. Binding of the monomeric form of the type I IFN (a) and the asymmetrical heterodimer of IFN (b) to their respective receptors results in activation of a tyrosine phosphorylation cascade involving the Janus kinases (Tyk2 and JAK2 for type I IFNs; JAK1 and JAK2 for IFN). Phosphorylation of the STAT transcription factors results in the formation of a heterodimer consisting of STAT 1 and STAT2 for the type I IFN, or a homodimer of STAT1 for IFN.

Figure 2. Proposed model for ligand (IFN) binding to receptor and translocation of STAT transcription factors to the nucleus. Data demonstrate that binding of IFN via the C-terminus to the cytoplasmic domain of the receptor (IFNR–cyto) enhances Janus kinase (JAK) binding. This, in turn, causes signal transducer and activator of transcription (STAT) binding. IFN provides the nuclear localization sequence (NLS) for translocation to the nucleus, none of the other participants have been shown to possess an NLS. Data suggest that the chain of the receptor does not undergo nuclear translocation. , importin ; , importin ; GAS, -activated sequence; GTP, guanosine triphosphate.