P-Rex1 Limits the Agonist-Induced Internalisation of GPCRs Independently of its Rac-GEF Activity

The Rac-GEF P-Rex1 mediates GPCR signalling by activating the small GTPase Rac. We show here that P-Rex1 also controls GPCR trafficking. P-Rex1 inhibits the agonist-stimulated internalisation of the GPCR S1PR1 independently of its Rac-GEF activity, through its PDZ, DEP and IP4P domains. P-Rex1 also limits the agonist-induced trafficking of CXCR4, PAR4, and GLP1R, but does not control steady-state GPCR levels, nor the agonist-induced internalisation of the RTKs PDGFR and EGFR. P-Rex1 blocks the phosphorylation required for GPCR internalisation. P-Rex1 binds Grk2, both in vitro and in cells, but does not appear to regulate Grk2 activity. We propose that P-Rex1 limits the agonist-induced internalisation of GPCRs through its interaction with Grk2 to maintain high levels of active GPCR at the plasma membrane. Therefore, P-Rex1 plays a dual role in promoting GPCR responses, by controlling GPCR trafficking through an adaptor function as well as by mediating GPCR signalling through its Rac-GEF activity.