The cytoplasmic domain of the pseudoprotease iRhom2 mediates distinct signaling mechanisms to control activation of the cell surface protease ADAM17

ADAM17 is a cell surface protease that controls the release of the ectodomains of signaling proteins, including epidermal growth factor receptor ligands and the primary inflammatory cytokine tumor necrosis factor. Reflecting this important role in signaling, dysregulated ADAM17 activity is linked to many human diseases, including immunodeficiency, inflammatory bowel disease, rheumatic arthritis, cancer, and Alzheimer's disease. iRhom2, a pseudoprotease of the rhomboid-like superfamily, has evolved to be a multifunctional regulatory cofactor of ADAM17. Recent structural and functional work has begun to reveal how the iRhom2 transmembrane and extracellular domains act to control ADAM17 activity. The cytoplasmic domain, however, remains less explored. Here, using a combination of proteomic, genetic, and biochemical approaches, we report three distinct mechanisms by which the cytoplasmic domain of iRhom2 contributes to ADAM17 regulation. First, upon oncogenic KRAS signaling, the serine/threonine kinase RSK2 is recruited to the iRhom2 cytoplasmic N terminus and coordinates with phosphorylated ERK kinase to activate the iRhom2-ADAM17 sheddase complex. Second, we show that iRhom2 may have an inhibitory function on ADAM17 at the cell surface: stabilizing iRhom2 at the cell surface by overexpressing iRhom2's cytoplasmic binding partner, FRMD8, inhibits PMA-stimulated ADAM17 activity. Third, we have identified a previously undefined motif (RKR) in the iRhom2 cytoplasmic domain that represses unstimulated ADAM17 activity. Overall, these findings reveal the complex regulatory system by which the iRhom2 cytoplasmic tail transduces cellular signals to regulate ADAM17 activation, potentially paving the way toward understanding and possibly manipulating the iRhom2-ADAM17 complex in health and disease.