J. Ellis et M. Seidenberg, Site-directed mutagenesis implicates a threonine residue in TM6 in the subtype selectivities of UH-AH 37 and pirenzepine at muscarinic receptors, PHARMACOL, 61(2), 2000, pp. 62-69
The structural basis for the selectivity of the antagonist UH-AH 37 at huma
n muscarinic acetylcholine receptors was investigated by expressing mutant
receptors in COS-7 cells. Previous studies have demonstrated that the inter
action between UH-AH 37 and [H-3]N-methylscopolamine in equilibrium assays
is competitive and that the high affinity of UH-AH 37 for the M-5 subtype,
compared to M-2, is due to an epitope in the sixth transmembrane domain (TM
6) or the third outer loop of the receptor. By mutating each nonconserved r
esidue in this region of M-2 and M-5 to its counterpart in the other recept
or, we identified a threonine residue in the middle of TM6 uniquely respons
ible for the higher affinity of the Mg receptor (M-1, M-3, and M-4 receptor
s also carry a threonine at that location and also have high affinity for U
H-AH 37). The mutant receptor in which the corresponding alanine of the M-2
receptor was replaced by threonine, M(2)(401)ala double right arrow thr, e
xpressed enhanced affinity for pirenzepine as well as for UH-AH 37. The chi
ck M-2 receptor, which expresses anomalously high affinity for pirenzepine,
differs from its mammalian counterparts by the presence of a threonine at
this position. Affinities of AF-DX 116 and 4-DAMP, as well as the allosteri
c potency of UH-AH 37, were not sensitive to the M-2(401) ala double right
arrow thr mutation. Copyright (C) 2000 S. Karger AG, Basel.