(a) A side-view of the TM domains. TM2, TM3, and TM4 are above the plane of the paper, and TM6 and TM7 are below the plane. The following assignments have been made for TM helices: TM1, residues 27 to 57; TM2, residues 64 to 93; TM3, residues 97 to 130; TM4, residues 142 to 165; TM5, residues 191 to 219; TM6, residues 232 to 259; TM7, residues 279 to 300. (b) A top-view of the extra-cellular loop regions. The following assignments have been made for loops: N-terminus, 1 to 26; ECL1, residues 94 to 96; ECL2, residues 166 to 190; ECL3, residues 260 to 278.