IRAK4 autophosphorylation controls inflammatory signaling by activating IRAK oligomerization

The controlled oligomerization of signaling proteins is an essential feature of many inflammatory signaling pathways. An example is IL-1 receptor signaling, which relies on the oligomerization of the Death Domain (DD)-containing proteins MyD88 and IRAK family kinases. This process leads to the assembly of the Myddosome signaling complex, and disrupting assembly holds potential for anti-inflammatory treatments. However, IRAKs signaling activity is also regulated by auto-/trans-phosphorylation, and it is unclear if these processes operate at or downstream of Myddosome assembly. Here, we find that the initial stage of Myddosome assembly is solely controlled by MyD88:IRAK4 DD interactions. In later stages, IRAK4 auto-phosphorylation serves as a switch, regulating IRAK1/2/3 incorporation and DD oligomerization. Small molecule inhibitors of IRAK4 kinase activity block this later stage of assembly, explaining how they dampen inflammatory signaling. Our data reveals IRAK4 auto-phosphorylation as an energy-dependent switch activating the heterotypic assembly of IRAKs DDs and downstream inflammatory IL-1 signaling. This highlights how a signaling cascade integrates phosphorylation and protein oligomerization steps.