Modeling and mutagenesis of the human alpha 1a-adrenoceptor: orientation and function of transmembrane helix V sidechains.
A 3-dimensional model of the seven transmembrane helical segments (TMs) of the human alpha 1a-adrenoceptor was initially built by analogy to the known structure of bacteriorhodopsin. However, the rotational orientation of TM V about its helical axis, and the roles of several TM V residues in ligand binding and receptor activation remained in question. Accordingly, we determined the effects of six site-specific mutations in TM V on binding affinity and functional potency of a structurally diverse series of agonists and antagonists. Mutation of Ser 192 and Phe 193 disrupted the binding of many of the tested ligands, as measured by displacement of [3H]prazosin. In addition, mutation of Ser 188, Ser 192, and Phe 193 disrupted receptor activation, as measured by [3H]inositol phosphate formation. On the basis of these results, a specific rotational orientation of TM V is proposed as part of a revised receptor model, which also takes into account more recently reported information about the structure of rhodopsin. This revised alpha 1a-adrenoceptor model accounts for direct interactions which are proposed between Ser 188 and Ser 192 and the meta and para hydroxyl groups of norepinephrine, respectively, in the G-protein coupled receptor state.
Wetzel
J M
JM
Department of Chemistry, Synaptic Pharmaceutical Corporation, Paramus, NJ 07652, USA.
Salon
J A
JA
Tamm
J A
JA
Forray
C
C
Craig
D
D
Nakanishi
H
H
Cui
W
W
Vaysse
P J
PJ
Chiu
G
G
Weinshank
R L
RL
Hartig
P R
PR
Branchek
T A
TA
Gluchowski
C
C
D000595:Amino Acid Sequence; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011224:Prazosin / Q000494:pharmacology; D017433:Protein Structure, Secondary / Q000187:drug effects; D011869:Radioligand Assay; D018340:Receptors, Adrenergic, alpha-1 / Q000737:chemistry* / Q000187:drug effects / Q000235:genetics* / Q000502:physiology
D018340:Receptors, Adrenergic, alpha-1; D011224:Prazosin