36 M. H. Akabas
identified by this approach are a subset of those identified by SCAM studies pro-
viding validation for both approaches. Using a different experimental approach,
the effect of protonation of substituted ionizable residues, Grosman and colleagues
identified a similar subset of residues as lining the open state of the nACh recep-
Fig. 3 Helical wheel
representations of the M2
membrane-spanning seg-
ments of the nACh receptor
α1 ( top), GABAA receptor
α1 ( middle), and 5-HT 3 A
( bottom) illustrating the
sulfhydryl-reactive residues.
The sulfhydryl reagent
used was MTSEA (nAChR,
Akabas et al. 1994a), pCMBS
(GABAA, Xu and Akabas
1996 ) and MTSET (5-HT 3 A,
Reeves et al. 2001 ). The left
side of each helix is inferred
was inferred to be the
channel-lining face of each
M2 segment