Incorporation of the π subunit into functional γ-aminobutyric acidA receptors

mRNA encoding the recently cloned gamma-aminobuytyric acid, receptor (GABAR) pi subunit is expressed in the hippocampus and in several non-neuronal tissues including the uterus and ovaries. Whereas native GABARs are pentamers composed primarily of alpha beta gamma, alpha beta delta, or alpha beta epsilon subunits, it has not been demonstrated clearly that the pi subunit incorporates into functional GABARs to form cup rr receptors and, if so, with what properties. We provide electrophysiological evidence that the rr subunit can coassemble with either alpha 5 beta 3 or alpha 5 beta 3 gamma 3 subunits to produce recombinant GABARs with distinct pharmacological and biophysical properties. Compared with alpha 5 beta 3 receptors, GABARs produced by coexpression of alpha 5 beta 3 pi subunits had a lower GABA EC50 value, were enhanced to a lesser extent by loreclezole, had different IC50 values for pregnenolone sulfate and lanthanum, and were insensitive to benzodiazepines. Incorporation of both pi and gamma 3 subunits into an alpha 5 beta 3 gamma 3 pi isoform was suggested by reduced enhancement by diazepam and a high zinc IC50 value. Current-voltage relations for the alpha 5 beta 3 pi subunit combination outwardly rectified more than currents from alpha 5 beta 3 gamma 3 but less than alpha 5 beta 3 combination GABARs. Single-channel alpha 5 beta 3 GABAR currents had a main conductance state of 15.2 picoSeimens (pS). Coexpression of the pi subunit with alpha 5 beta 3 subtypes increased the conductance level to 23.8 pS, similar to the conductance level of alpha 5 beta 3 gamma 3 GABARs (26.9 pS). We conclude that the rr subunit coassembles with alpha, beta, and gamma subunits to form functional alpha beta pi or alpha beta gamma pi GABARs and, thus, could have a significant impact on the function of native GABARs expressed in the brain or non-neuronal tissue.