Cell surface receptors that are coupled to G-proteins (GTP-binding protein). G-protein coupled receptors are thought to have seven membrane spanning domains and have been divided into 2 subclasses: those in which the binding site is in the extracellular domain for example receptors for glycoprotein hormones, such as thyroid stimulating hormone (TSH) and follicle-stimulating hormone (FSH) and those in which the ligand binding site is likely to be in the plane of the 7 transmembrane domains for example rhodopsin and receptors for small neurotransmitters and hormones for example muscarinic acetylcholine receptor. [ ]