Question

2. The receptor that binds the steroid hormone testosterone is located in the cytoplasm of its target cells. Vasopressin is a peptide hormone (nine amino acids long) that stimulates water reabsorption in the kidney. Its receptor is an integral protein in which the domain of the receptor responsible for binding vasopressin is located on the extracellular side of the cell membrane.

a) Why must the vasopressin-binding domain of the vasopressin receptor be located on the outside of a cell while the testosterone receptor can be located within a cell? Neither hormone binds to another molecule before it binds to its receptor. (If you are curious, the vasopressin sequence is Gly-Arg-Pro-Cys-Asn-Gln-Phe-Tyr-Cys – you can look up those side chains if you want.)

b) What type or types of amino acids do you expect to predominate in the vasopressin-binding domain of the vasopressin receptor? What type or types of amino acids do you expect to find the transmembrane region of the vasopressin receptor (this region has several alpha helices passing through the membrane)? Explain your reasoning.

c) Why are alpha-helices commonly found in the transmembrane region of integral proteins?