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Biophys J, March 2000, p. 1324-1334, Vol. 78, No. 3
Department of Molecular Physiology and Biophysics, Baylor College of Medicine, Houston, Texas 77030 USA
To determine the importance of electrostatic interactions
for agonist binding to the nicotinic acetylcholine receptor (AChR), we
examined the affinity of the fluorescent agonist dansyl-C6-choline for
the AChR. Increasing ionic strength decreased the binding affinity in a
noncompetitive manner and increased the Hill coefficient of binding.
Small cations did not compete directly for dansyl-C6-choline binding.
The sensitivity to ionic strength was reduced in the presence of
proadifen, a noncompetitive antagonist that desensitizes the receptor.
Moreover, at low ionic strength, the dansyl-C6-choline affinities were
similar in the absence or presence of proadifen, a result consistent
with the receptor being desensitized at low ionic strength. Similar
ionic strength effects were observed for the binding of the
noncompetitive antagonist [3H]ethidium when examined in
the presence and absence of agonist to desensitize the AChR. Therefore,
ionic strength modulates binding affinity through at least two
mechanisms: by influencing the conformation of the AChR and by
electrostatic effects at the binding sites. The results show that
charge-charge interactions regulate the desensitization of the
receptor. Analysis of dansyl-C6-choline binding to the desensitized
conformation using the Debye-Hückel equation was consistent with
the presence of five to nine negative charges within 20 Å of the
acetylcholine binding sites.
Biophys J, March 2000, p. 1324-1334, Vol. 78, No. 3
© 2000 by the Biophysical Society 0006-3495/00/03/1324/11 $2.00
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