Chimeras of alpha(1)-adrenergic receptor subtypes identify critical residues that modulate active state isomerization

We have identified previously two amino acids, one in each of the fifth and sixth transmembrane segments of both the alpha(1a)-adrenergic receptor and the alpha(1b)-adrenergic receptor (AR), that account almost entirely for the selectivity of agonist binding by these receptor subtypes (Hwa, J., Graham, R. M., and Perez, D. M. (1995) J. Biol. Chem. 270, 23189-23195). Thus reversal of these two residues, from those found in the native receptor of one subtype to those in the other subtype, produces complementary changes in subtype selectivity of agonist binding. Here we show that mutating only one of these residues in either the alpha(1b)-AR or the alpha(1a)-AR to the corresponding residue in the other subtype (Ala(204) --> Val for the alpha(1b); Met(292) --> Leu for the alpha(1a)-AR) results in chimeras that are constitutively active for signaling by both the phospholipase C and phospholipase A(2) pathways. This is evident by an increased affinity for agonists, increased basal phospholipase C and phospholipase A(2) activation, and increased agonist potency. Although mutation of the other residue involved in agonist binding selectivity, to the corresponding residue in the other subtype (Leu(314) --> Met for the alpha(1b)-AR; Val(185) --> Ala for the alpha(1a)-AR) does not alter receptor binding or signaling, per se, when combined with the corresponding constitutively activating mutations, the resulting chimeras, Ala(204) --> Val/Leu(314) --> Met (alpha(1b)-AR) and Val(185) --> Ala/Met(292) --> Leu ((alpha(1a)-AR), display wild type ligand binding and signaling. A simple interpretation of these results is that the alpha(1a)- and alpha(1b)-ARs possess residues that critically modulate isomerization from the basal state, R, to the active state R*, and that the native receptor structures have evolved to select residues that repress active state isomerization. It is likely that the residues identified here modulate important interhelical interactions between the fifth and sixth transmembrane segments that inhibit or promote receptor signaling.