Membrane Protein Topology The receptor for the hormone epinephrine in animal cells is an inte- gral membrane protein (M, 64,000) that is believed to have seven membrane-spanning regions. (a) Show that a protein of this size is capable of spanning the membrane seven times. (b) Given the amino acid sequence of this protein, how would you predict which regions of the protein form the membrane-spanning helices? (e) Go to the Protein Data Bank (www.rcsb.org). Use the PDB identifier 1DEP to retrieve the data page for a portion of the -adrenergic receptor (one type of epinephrine receptor) from a turkey. Using Jmol to explore the structure, predict where this portion of the receptor is located: within the membrane or at the membrane surface. Explain. (d) Retrieve the data for a portion of another receptor, the acetylcholine receptor of neurons and myocytes, using the PDB identifier 1A11. As in (e), predict where this portion of the receptor is located and explain your answer. If you have not used the PDB, see Box 4-4 (p. 129) for more information.