Conductance mutations of the nicotinic acetylcholine receptor do not act by a simple electrostatic mechanism.
Fixed negative charges in many cation channels raise the single-channel conductance, apparently by an electrostatic mechanism: their effects are accentuated in solutions of low ionic strength and attenuated at high ionic strength. The charges of specific amino acids near the ends of the proposed pore-lining M2 segment of the nicotinic acetylcholine receptor, termed the extracellular and cytoplasmic rings, have recently been shown to influence the single-channel K+ conductance (Imoto, K., C. Busch, B. Sakmann, M. Mishina, T. Konno, J. Nakai, H. Bujo, Y. Mori, K. Fukuda and S. Numa. 1988. Nature 335:645-648). We examined whether these charges might act by a direct electrostatic effect on the energy of ions in the pore, rather than indirectly by inducing a structural change. To this end, we measured the conductances of charge mutants over a range of K+ concentrations (ionic strengths). As expected, we found that negative charge mutations raise the conductance, and positive charge mutations lower it. The effects of cytoplasmic-ring mutations are accentuated at low ionic strength, but they are not completely attenuated at high ionic strength. The effects of extracellular-ring mutations are independent of ionic strength. These results are inconsistent with the simplest electrostatic model. We suggest a modified model that qualitatively accounts for the data.
Kienker
P
P
Howard Hughes Medical Institute, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
Tomaselli
G
G
Jurman
M
M
Yellen
G
G
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D055592:Biophysical Phenomena; D001703:Biophysics; D004553:Electric Conductivity; D004563:Electrochemistry; D005260:Female; D066298:In Vitro Techniques; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D051379:Mice; D008956:Models, Chemical; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009994:Osmolar Concentration; D011188:Potassium / Q000378:metabolism; D011978:Receptors, Nicotinic / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D013816:Thermodynamics; D014982:Xenopus laevis
D011978:Receptors, Nicotinic; D008274:Magnesium; D011188:Potassium