Ligand-induced Assembling of the Type I Interferon Receptor on Supported Lipid Bilayers

Type I interferons (IFNs) elicit antiviral, antiproliferative and immuno-modulatory responses through binding to a shared receptor consisting of the transmembrane proteins ifnar1 and ifnar2. Differential signaling by different interferons, in particular IFNαs and IFNβ, suggests different modes of receptor engagement. Using reflectometric interference spectroscopy (RIfS), we studied kinetics and affinities of the interactions between IFNs and the extracellular receptor domains of ifnar1 (ifnar1-EC) and ifnar2 (ifnar2-EC). For IFNα2, we determined a KD value of 3 nM and 5 μM for the interaction with ifnar2-EC and ifnar1-EC, respectively. As compared to IFNα2, IFNβ formed complexes with ifnar2-EC as well as ifnar1-EC with substantially higher affinity. For neither IFNα2 nor IFNβ was stabilization of the complex with ifnar1-EC in the presence of soluble ifnar2-EC observed. We investigated ligand-induced complex formation with ifnar1-EC and ifnar2-EC being tethered onto solid-supported, fluid lipid bilayers by RIfS and total internal reflection fluorescence spectroscopy. We observed very stable binding of IFNα2 at high receptor surface concentrations with an apparent kd value approximately 200 times lower than that for ifnar2-EC alone. The apparent kd value was strongly dependent on the surface concentration of the receptor components, suggesting kinetic stabilization. This was corroborated by the fast exchange of labeled IFNα2 bound to the receptor by unlabeled IFNα2. Taken together, our results indicate that IFN first binds to ifnar2 and subsequently recruits ifnar1 in a transient fashion. In particular, this second step is much more efficient for IFNβ than for IFNα2, which could explain differential activities observed for these IFNs.