Arrangement of transmembrane domains in adrenergic receptors. Similarity to bacteriorhodopsin.
G protein-coupled receptors (GPCRs) have seven hydrophobic domains, which are thought to span the lipid bilayer as alpha helical transmembrane domains (TMDs). The tertiary structure of GPCRs has not been determined; however, molecular models of GPCRs have generally been based on bacteriorhodopsin, which is functionally unrelated to GPCRs but has a similar secondary structure. We sought to examine the validity of using bacteriorhodopsin as a scaffold for GPCR model building by experimentally determining the orientation of the TMDs of adrenergic receptors in the plasma membrane. In separate experiments, three sequential amino acid residues (Leu-310, Leu-311, Asn-312) in TMD VII of the beta 2 adrenoreceptors were mutated to the amino acids found in the homologous domain of the alpha 2 adrenoceptor (Phe, Phe, Phe). Exchange of Asn-312 and Leu-311 in the beta 2 adrenoceptor resulted in nonfunctional proteins, most likely due to incompatibility of the introduced bulky phenylalanine side chain with adjacent structural domains in the beta 2 adrenoreceptor. This structural incompatibility was "repaired" by replacing the specific beta 2 TMD sequence with an alpha 2 receptor sequence. TMD I and TMD II complemented the Asn-312-->Phe mutation, and TMD III and TMD VI complemented the Leu-311-->Phe mutation. These results indicate that TMDs I, II, III, and VI surround TMD VII in a counter-clockwise orientation analogous to the orientation of TMDs in bacteriorhodopsin.
Mizobe
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T
Department of Anesthesia, Stanford University, California 94305, USA.
Maze
M
M
Lam
V
V
Suryanarayana
S
S
Kobilka
B K
BK
D000818:Animals; D001436:Bacteriorhodopsins / Q000737:chemistry*; D002460:Cell Line; D003001:Cloning, Molecular; D006801:Humans; D011869:Radioligand Assay; D018341:Receptors, Adrenergic, alpha-2 / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D018343:Receptors, Adrenergic, beta-2 / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
D018341:Receptors, Adrenergic, alpha-2; D018343:Receptors, Adrenergic, beta-2; D001436:Bacteriorhodopsins