Signalling by cytokine receptor systems is not an “all or none” phenomenon and is influenced by parameters that include the orientation of the receptor complex as well as the interaction between the cytokine and its receptor. Formation of an active signalling complex involves precise arrangement of receptor subunits either through formation of receptor homodimers or heterodimers. As such, dissecting the extracellular assembly of a cytokine-receptor complex can be a very insightful approach to identifying the key molecular determinants that play a role in dictating signal outcomes.
Interleukin-3 (IL-3) is a cytokine produced largely by antigen-activated T cells that links immunity to the haemopoietic system and plays a significant role in leukaemia as well as allergic inflammation, autoimmune diseases, infection and oncogenesis. However, little is known about how the IL-3 receptor complex is able to initiate alternate signalling pathways. We have solved the three-dimensional crystal structure of IL-3 in complex with the extracellular domains of IL3Rα and βc receptor subunits. Surprisingly, the IL-3 receptor complex adopts a higher-order, dodecameric receptor arrangement that is also maintained in solution as evidenced by transmission electron microscopy (TEM), analytical ultracentrifugation (AUC) and small-angle X-ray scattering (SAXS). The crystal structure reveals salient features that mediate dodecamer formation and play a significant role in dictating signalling outcomes. Through structural and functional studies, we have identified a molecular switch that dictates downstream signalling events mediated by IL-3. The structure of the IL-3 signalling complex provides insight into how the IL-3 receptor signals and strategies for designing a new class of peptide-based antagonists to modulate IL-3 mediated signalling.