ID: 86: Defining the functional binding sites of IL-12Rβ1 and IL-23R to Janus kinases
The Interleukin (IL)-12-type cytokines IL-12 and IL-23 show structural similarities but have different functions in the immune system. They are involved in T helper (Th) 1 or Th17 immunity, respectively. Both cytokines are heterodimers composed of the shared IL-12p40 subunit and IL-12p35 or IL-23p19, respectively, and share the IL-12Rβ1 as one component of their receptor signaling complexes. The second receptor for IL-12 signaling is IL-12Rβ2 and for IL-23 signal transduction IL-23R. Stimulation of cells containing the appropriate receptors with IL-12 or IL-23 results in activation of receptor-associated Janus kinases (Jak) and phosphorylation of STAT proteins in target cells. The Janus kinase Tyrosine kinase (Tyk) 2 was shown to associate with IL-12Rβ1, whereas Jak2 binds to the IL-23R and also to the IL-12Rβ2. Association of Janus kinases to receptors is mediated by so-called Box1 and Box2 motifs within the intracellular domain of the receptor chains. We have defined the hypothesized Box1 and Box2 binding sites in the IL-12Rβ1 and an unusual Jak2 binding site in the IL-23R by the use of deletion variants and site-directed mutagenesis. Non-functional box motifs abrogate IL-12 and IL-23 induced STAT phosphorylation and cytokine-dependent proliferation of respective generated Ba/F3 cells. Co-immunoprecipitation of Tyk2 by IL-12Rβ1 and Jak2 by IL 23R support our findings. In summary, we characterize the functional association of IL 12Rβ1 with Tyk2 and IL-23R with Jak2, which are mandatory for IL-12 and/or IL-23 signaling.