Non-canonical β-adrenergic activation of ERK at endosomes.

G-protein-coupled receptors (GPCRs), the largest family of signalling receptors, as well as important drug targets, are known to activate extracellular-signal-regulated kinase (ERK)-a master regulator of cell proliferation and survival. However, the precise mechanisms that underlie GPCR-mediated ERK activation are not clearly understood. Here we investigated how spatially organized β-adrenergic receptor (βAR) signalling controls ERK. Using subcellularly targeted ERK activity biosensors, we show that βAR signalling induces ERK activity at endosomes, but not at the plasma membrane. This pool of ERK activity depends on active, endosome-localized Gα and requires ligand-stimulated βAR endocytosis. We further identify an endosomally localized non-canonical signalling axis comprising Gα, RAF and mitogen-activated protein kinase kinase, resulting in endosomal ERK activity that propagates into the nucleus. Selective inhibition of endosomal βAR and Gα signalling blunted nuclear ERK activity, MYC gene expression and cell proliferation. These results reveal a non-canonical mechanism for the spatial regulation of ERK through GPCR signalling and identify a functionally important endosomal signalling axis.