Molecular and Structural Basis of Receptor Binding and Signaling of a Fish Type I IFN with Three Disulfide Bonds.

In mammals, type I IFNs, which commonly contain one or two disulfide bonds, activate the JAK-STAT signaling pathway through binding to the common cell surface receptor formed by IFN-α/β receptor (IFNAR)1 and IFNAR2 subunits. Although type I IFNs are also known to be essential for antiviral defense in teleost fish, very little is known about mechanisms underlying the recognition of fish type I IFNs by associated receptors. In this study, we demonstrate that a type I IFN of large yellow croaker (IFNi), belonging to a new subgroup of fish type I IFNs, triggers antiviral response via the conserved JAK-STAT pathway through stable binding with a heterodimeric receptor comprising subunits CRFB5 and CRFB2. IFNi binds to CRFB5 with a much higher affinity than to CRFB2. Furthermore, we determined the crystal structure of IFNi at a 1.39 Å resolution. The high-resolution structure is, to our knowledge, the first reported structure of a type I IFN with three disulfide bonds, all of which were found to be indispensable for folding and stability of IFNi. Using structural analysis, mutagenesis, and biochemical assays, we identified key IFNi residues involved in receptor interaction and proposed a structural model of IFNi bound to the CRFB2-CRFB5 receptor. The results show that IFNi-CRFB2 exhibits a similar binding pattern to human IFN-ω-IFNAR2, whereas the binding pattern of IFNi-CRFB5 is quite different from that of IFN-ω-IFNAR1. Altogether, our findings reveal the structural basis for receptor interaction and signaling of a type I IFN with three disulfide bonds and provide new insights into the mechanisms underlying type I IFN recognition in teleosts.