S27 Of Ifn Alpha 1 Contributes To Its Low Affinity For Ifnar2 And Weak Antiviral Activity

Type I interferons (IFNs) signal by forming a high affinity IFN-IFNAR2 dimer, which subsequently recruits IFNAR1 to form a ternary complex that initiates JAK/STAT signaling. Among the 12 IFN alpha subtypes, IFN alpha 1 has a uniquely low affinity for IFNAR2 (<100 x of the other IFN alpha subtypes) and commensurately weak antiviral activity, suggesting an undefined function distinct from suppression of viral infections. Also unique in IFN alpha 1 is substitution of a serine for phenylalanine at position 27, a contact point that stabilizes the IFN alpha:IFNAR2 hydrophobic interface. To determine whether IFN alpha 1-S27 contributes to the low affinity for IFNAR2, we created an IFN alpha 1 mutein, IFN alpha 1-S27F, and compared it to wild-type IFN alpha 1 and IFN alpha 2. Substitution of phenylalanine for serine increased affinity for IFNAR2 similar to 4-fold and commensurately enhanced activation of STAT1, STAT3, and STAT5, transcription of a subset of interferon stimulated genes, and restriction of vesicular stomatitis virus infection in vitro. Structural modeling suggests that S27 of IFN alpha 1 disrupts the IFN alpha:IFNAR2 hydrophobic interface that is otherwise stabilized by F27 and that replacing S27 with phenylalanine partially restores the hydrophobic surface. Disruption of the hydrophobic IFN alpha:IFNAR2 interface by the unique S27 of IFN alpha 1 contributes to its low affinity and weak antiviral activity.