GABRB3 mutation, G32R, associated with childhood absence epilepsy alters alpha1beta3gamma2L gamma-aminobutyric acid type A (GABAA) receptor expression and channel gating.
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Gurba KN, Hernandez CC, Hu N, Macdonald RL
GABRB3 mutation, G32R, associated with childhood absence epilepsy alters alpha1beta3gamma2L gamma-aminobutyric acid type A (GABAA) receptor expression and channel gating.
J Biol Chem. 2012 Apr 6;287(15):12083-97. doi: 10.1074/jbc.M111.332528. Epub 2012 Feb 2.
- PubMed ID
- 22303015 [ View in PubMed]
- Abstract
A GABA(A) receptor beta3 subunit mutation, G32R, has been associated with childhood absence epilepsy. We evaluated the possibility that this mutation, which is located adjacent to the most N-terminal of three beta3 subunit N-glycosylation sites, might reduce GABAergic inhibition by increasing glycosylation of beta3 subunits. The mutation had three major effects on GABA(A) receptors. First, coexpression of beta3(G32R) subunits with alpha1 or alpha3 and gamma2L subunits in HEK293T cells reduced surface expression of gamma2L subunits and increased surface expression of beta3 subunits, suggesting a partial shift from ternary alphabeta3gamma2L receptors to binary alphabeta3 and homomeric beta3 receptors. Second, beta3(G32R) subunits were more likely than beta3 subunits to be N-glycosylated at Asn-33, but increases in glycosylation were not responsible for changes in subunit surface expression. Rather, both phenomena could be attributed to the presence of a basic residue at position 32. Finally, alpha1beta3(G32R)gamma2L receptors had significantly reduced macroscopic current density. This reduction could not be explained fully by changes in subunit expression levels (because gamma2L levels decreased only slightly) or glycosylation (because reduction persisted in the absence of glycosylation at Asn-33). Single channel recording revealed that alpha1beta3(G32R)gamma2L receptors had impaired gating with shorter mean open time. Homology modeling indicated that the mutation altered salt bridges at subunit interfaces, including regions important for subunit oligomerization. Our results suggest both a mechanism for mutation-induced hyperexcitability and a novel role for the beta3 subunit N-terminal alpha-helix in receptor assembly and gating.