Transmission of extracellular signals by G protein-coupled receptors typically relies on a cascade of intracellular events initiated by the activation of heterotrimeric G proteins or β-arrestins followed by effector activation/inhibition. Here, we report an alternative signal transduction mode used by the orphan GPR50 that relies on the nuclear translocation of its carboxyl-terminal domain (CTD). Activation of the calcium-dependent calpain protease cleaves off the CTD from the transmembrane-bound GPR50 core domain between Phe-408 and Ser-409 as determined by MALDI-TOF-mass spectrometry. The cytosolic CTD then translocates into the nucleus assisted by its 'DPD' motif, where it interacts with the general transcription factor TFII-I to regulate c-fos gene transcription. RNA-Seq analysis indicates a broad role of the CTD in modulating gene transcription with ~ 8000 differentially expressed genes. Our study describes a non-canonical, direct signaling mode of GPCRs to the nucleus with similarities to other receptor families such as the NOTCH receptor.
Keywords: Calpain; GPCR; GPR50; Orphan; Proteolytic cleavage; Signal transduction.