the rhodopsin family of proteins -- with seven hydrophobic , alpha-helical transmembrane domains , four extracellular domains , and four intracytoplasmic domains -- indeed arose before the division of prokaryotes from eukaryotes and later gave rise to muscarinic acetylcholine receptor and beta-adrenergic receptor among others .
C1519623
transmembrane domains
C0001643
beta-adrenergic