Proton-sensing G-protein coupled receptor couples to multiple intracellular signaling pathways, including GNAS/cAMP, GNAQ/phospholipase C (PLC), and GNA12/GNA13/Rho pathways (PubMed:12955148, PubMed:17462861, PubMed:22110680, PubMed:20211729). Acidosis-induced GPR4 activation increases paracellular gap formation and permeability of vascular endothelial cells through the GNA12/GNA13/Rho GTPase signaling pathway (PubMed:32058960). In the brain may mediate central respiratory sensitivity to CO(2)H(+) (By similarity). Belongs to the G-protein coupled receptor 1 family. Was originally thought to be a receptor for sphingosylphosphorylcholine and lysophosphatidylcholine (PubMed:11535583). However, this work has been retracted.