Interleukin (IL)-11 is an under-characterised member of the IL-6 family of cytokines that signals through a cell-type specific a-subunit receptor, IL-11Ra, and a ubiquitously expressed transmembrane b-subunit receptor, GP130. The formation of the active hexameric IL-11/IL-11Ra/GP130 (2:2:2) signaling complex leads to the recruitment of intracellular Janus Kinases (JAKs) which phosphorylate the cytoplasmic tail of GP130 providing docking sites for the signal transducer and activator of transcription (STAT) proteins. Once phosphorylated, STATs form active dimers that translocate to the nucleus where they regulate the expression of numerous genes involved in cell survival and proliferation. Recently, mutations in components of the IL-11 signaling complex have been linked to a range of human disorders including craniosynostosis.

Our current understanding of the structural mechanism of IL-11 signaling has been based on homology models using IL-6, despite low sequence similarity (~20%). IL-6, signals through a hexameric IL-6/IL-6Ra/GP130 (2:2:2) signaling complex, for which high resolution crystal structures are available. We have recently solved the first crystal structure for human IL-11, which suggests important differences between these two cytokines including how they engage GP130 and trigger the downstream signaling cascade. In order to understand how mutations in components of the IL-11 signaling complex identified in human disease impact on signal transduction, we have performed site-directed mutagenesis of key residues at the IL-11/IL-11Ra and IL-11Ra/GP130 interface. We characterised the impact of each mutation on the phosphorylation of STAT proteins in BA/F3 cells, which otherwise lack IL-11/IL-11Ra/GP130 components. Our results identify regions of the IL-11 signaling complex that are crucial for signaling, and represent druggable target sites to agonise or antagonise this signaling pathway.