A protein that has G protein-coupled receptor (GPCR) activity. Each GPCR consists of a single polypeptide with an extracellular N-terminus, an intracellular C-terminus and seven hydrophobic transmembrane domains (TM1-TM7) linked by three extracellular loops (ECL1-ECL3) and three intracellular loops (ICL1-ICL3). GPCRs function as part of a heterotrimeric complex to transmit an extracellular signal across the membrane by activating an associated G-protein via the exchange of GDP for GTP on the alpha subunit. [ http://en.wikipedia.org/wiki/GPCR ]