Members of the multi pass, G protein-linked family of receptors include the muscarinic cholinergic and J3-adrenergic receptors and rhodopsin. Which of the following characterizes these receptors?

Binding of ligand to the multipass G protein-linked receptors activates or inactivates enzymes bound to the plasma membrane (adenylyl cyclase or phospholipase C) or opens or closes ion channels using G proteins. A table of G proteins and their functions appears below. The muscarinic cholinergic and β receptors, as well as rhodopsin, are multipass transmembrane proteins consisting specifically of seven hydrophobic spanning segments of the single polypeptide chain. The peptide bonds of the spanning segments are polar. In the hydrophobic environment of the lipid bilayer, in the absence of water, they form hydrogen bonds with each other. In the vast majority of cases this leads to the formation of ex helices. In the case of the porins, found in the outer mitochondrial membrane, a 0-barrel structure is established. There is a remarkable homology between the cell-surface receptors linked to the G proteins. Between the segments, the polypeptide chain loops on both the extracellular and the intracellular sides of the membrane. Ligand binding occurs on the extracellular surface. Receptors with intrinsic enzyme activity belong to a separate class of singlepass transmembrane proteins. All of these transmembrane proteins show a carboxyl terminus on the cytosolic side and N-linked glycosylation sites on the extracellular surface.