Abstract
The properties of the human α1β1γ-aminobutyric acid (GABA)A receptors were investigated after mutation of a highly conserved leucine residue at the 9′ position in the second membrane-spanning region (TM2). The role of this residue in α1 and β1 subunits was examined by mutating the 9′ leucine to phenylalanine, tyrosine, or alanine. The mutations were in either the α1 subunit (α∗β), the β1 subunit (αβ∗), or in both subunits (α∗β∗), and the receptors were expressed in Sf9 cells. Our results show that the rate of desensitization is increased as the size and hydrophobicity of the 9′ residue in the α1 subunit is increased: Y, F > L > A, T. Mutation of L9′ in only the β1 subunit (αβ∗) to either phenylalanine or tyrosine increased the EC50 value for GABA at least 100 times, but the EC50 was unchanged in αβ∗ alanine mutants. In the 9′ α1 mutants (α∗β, α∗β∗) the GABA EC50 was minimally affected. In α∗β and α∗β∗, but not αβ∗, the peak currents evoked by millimolar concentrations of GABA were greatly reduced. The reduction in currents could only be partially accounted for by decreased expression of the receptors These findings suggest different roles for the two types of subunits in GABA activation and later desensitization of α1β1 receptors. In addition, an increase in the resting membrane conductance was recorded in alanine but not in phenylalanine and tyrosine mutants, indicating that the side chain size at the 9′ position is a major determinant of current flow in the closed conformation.
Footnotes
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Send reprint requests to: Dr. Bryndis Birnir, Department of Cell and Molecular Physiology, Institute of Physiological Sciences, Lund University, Sölvegatan 19, S-223 62 Lund, Sweden. E-mail:bryndis.birnir{at}mphy.lu.se
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↵1 Present address: Department of Molecular and Cellular Physiology, Howard Hughes Medical Institute, Stanford University, Stanford, CA.
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↵2 Present address: Department of Cell and Molecular Physiology, Institute of Physiological Sciences, Lund University, Lund S-223 62, Sweden.
- Abbreviations:
- GABA
- γ-aminobutyric acid
- TM2
- second membrane-spanning region
- nACh
- nicotinic acetylcholine
- MES
- 2-(N-morpholino)ethanesulfonic acid
- Sf9
- Spodoptera frugiperda
- TES
- N-tris(hydroxymethyl)methyl-2-aminoethanesulfonic acid
- Received October 27, 1999.
- Accepted February 3, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
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