Md. Martin et A. Karlin, FUNCTIONAL-EFFECTS ON THE ACETYLCHOLINE-RECEPTOR OF MULTIPLE MUTATIONS OF GAMMA-ASP174 AND DELTA-ASP180, Biochemistry, 36(35), 1997, pp. 10742-10750
Residues gamma Asp174 and delta Asp180, previously implicated in the b
inding of acetylcholine (ACh) by the mouse muscle ACh receptor, were e
ach mutated to nine other residues, Asn, Glu, Thr, Ala, Cys, I-Iis, Va
l, Tyr, and Lys, The effects of the mutations on ACh-induced current w
as determined on surface receptors containing wild-type alpha and beta
subunits and mutant gamma and delta subunits. The mutations increased
the concentration of ACh eliciting half-maximal current (EC50) by fac
tors from 22 for the Glu mutant to 660 for the Lys mutant. Analysis of
the effects in terms of the difference in the accessible surface area
s of the mutant and wild-type side chains and the difference in side-c
hain charges Indicated that, per binding site, Delta Delta G(0) for ac
tivation was a sum of 10 cal mol(-1) Angstrom(-2) Of change in side-ch
ain accessible surface area and of 0.95 kcal mol(-1) positive step(-1)
in side-chain charge, equivalent to 1 mol of charge falling through 4
2 mV. The effects on the concentration of ACh (IC50,ACh) and of d-tubo
curarine (IC50,dTC) causing half-maximal retardation of cx-bungarotoxi
n binding were determined on complexes containing wild-type a and beta
subunits and either mutant gamma or mutant delta subunit. The effects
on IC50,ACh correlated well with the effects on EC50, with a similar
magnitude for the influence of side-chain charge on the free energy of
binding (in this case to the desensitized state) and on the electrost
atic potential at the binding site. The effects on IC50,dTC were in al
l cases less than the effects on IC50,ACh, and the two sets of effects
were poorly correlated. In line with the higher ACh affinity and lowe
r d-tubocurarine affinity of the alpha-delta binding site compared to
the alpha-gamma binding site, mutations of delta Asp180 had a greater
effect on IC50,ACh than did the same mutations of gamma Asp174, and vi
ce versa for effects on IC50,dTC. Consequently, all mutations decrease
d the asymmetry in the binding properties of the two types of sites.