G-protein coupled receptor.

G - protein coupled receptors form a large super - family of proteins composed of three major classes and more than 30 subfamilies. They are integral membrane proteins characterized by seven membrane- spanning (transmembrane; TM) regions. They are involved with signal transduction across cell membranes. Many medically and pharmacologically important proteins are included in this super-family: e.g., Acetylcholine receptors, Dopamine receptors, and Opioid receptors.