Assembly of GABAA receptor subunits: alpha 1 beta 1 and alpha 1 beta 1 gamma 2S subunits produce unique ion channels with dissimilar single-channel properties.

Recent experimental evidence has led to the hypothesis that GABAA receptor channel (GABAR) heterogeneity or receptor channel subtypes may occur by differential assembly of a given set of subunits into various configurations. Alternatively, assembly of subunits into mature GABARs may arise from an ordered process to produce a preferred form of the receptor channel, as seen for nicotinic ACh receptors. In the preceding article, we demonstrated that transient expression of GABAR alpha 1 and beta 1 subunits in mouse L929 fibroblast cells produced two different types of GABARs, when coexpressed with and without the gamma 2S subunit. Not only did these GABARs differ in their GABA and diazepam pharmacology, but initial single-channel recordings suggested that the two types of GABARs (alpha 1 beta 1 and alpha 1 beta 1 gamma 2S) had different conductance and gating properties. It also appeared that alpha 1 beta 1 gamma 2S GABARs were preferentially formed over alpha 1 beta 1 GABARs, but it was not completely shown if both forms of GABARs were produced when a cell expressed all three subunits. To characterize further the assembly process and determine the preferred form, if it existed, it was necessary to obtain a kinetic "fingerprint" for both alpha 1 beta 1 and alpha 1 beta 1 gamma 2S GABARs. Thus, single-channel patch-clamp recording and kinetic analysis of receptor channel gating were performed. For both alpha 1 beta 1 and alpha 1 beta 1 gamma 2S GABARs, GABA evoked single-channel openings to both a main conductance (15 and 29 pS, respectively) and a subconductance level (10 and 21 pS, respectively) with greater than 90% of the total current through the main conductance level openings. The two GABAR populations were further differentiated by their open and burst properties. On average, alpha 1 beta 1 gamma 2S GABARs opened for almost three times the duration as alpha 1 beta 1 GABARs (6.0 vs 2.3 msec, respectively) and had three openings per burst. alpha 1 beta 1 GABARs opened predominantly as single opening bursts. Using the conductance and gating properties to differentiate the two GABAR populations, we determined that alpha 1 beta 1 GABARs were rarely, if ever, formed upon coexpression of all three subunits, suggesting that alpha 1 beta 1 gamma 2S GABARs were the preferred final form of the receptor channel. Also, the homogeneity of the conductance and gating properties of alpha 1 beta 1 gamma 2S GABARs among the different patches studied implied that a single preferred configuration of GABARs may exist.(ABSTRACT TRUNCATED AT 400 WORDS)