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Subunit
Decreases the Rate of Agonist Dissociation
From the * Receptor Biology Laboratory, We describe the kinetic consequences of the mutation N217K in the M1 domain of the acetylcholine
receptor (AChR)
Muscle Research Laboratory, Department of
Neurology, Mayo Foundation, Rochester, Minnesota 55905; and § Department of Biophysical Sciences, State University of New York at
Buffalo, Buffalo, New York 14214
subunit that causes a slow channel congenital myasthenic syndrome (SCCMS). We previously
showed that receptors containing N217K expressed in 293 HEK cells open in prolonged activation episodes strikingly similar to those observed at the SCCMS end plates. Here we use single channel kinetic analysis to show that
the prolonged activation episodes result primarily from slowing of the rate of acetylcholine (ACh) dissociation
from the binding site. Rate constants for channel opening and closing are also slowed but to much smaller extents. The rate constants derived from kinetic analysis also describe the concentration dependence of receptor activation, revealing a 20-fold shift in the EC50 to lower agonist concentrations for N217K. The apparent affinity of
ACh binding, measured by competition against the rate of 125I--bungarotoxin binding, is also enhanced 20-fold
by N217K. Both the slowing of ACh dissociation and enhanced apparent affinity are specific to the lysine substitution, as the glutamine and glutamate substitutions have no effect. Substituting lysine for the equivalent asparagine in the 
, 
, or 
subunits does not affect the kinetics of receptor activation or apparent agonist affinity. The
results show that a mutation in the amino-terminal portion of the M1 domain produces a localized perturbation
that stabilizes agonist bound to the resting state of the AChR.
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