High-level production and purification in a functional state of an extrasynaptic gamma-aminobutyric acid type A receptor containing a4ß3d subunits

Abstract

The inhibitory γ-aminobutyric acid type A receptors are implicated in numerous physiological processes, including cognition and inhibition of neurotransmission, rendering them important molecular targets for many classes of drugs. Functionally, the entire GABAAR family of receptors can be subdivided into phasic, fast acting synaptic receptors, composed of a-, ß- and δ-subunits, and tonic extrasynaptic receptors, many of which contain the d-subunit in addition to a- and ß-subunits. Whereas the subunit arrangement of the former group is agreed upon, that of the aßd GABAARs remains unresolved by electrophysiological and pharmacological research. To resolve such issues will require biophysical techniques that demand quantities of receptor that have been previously unavailable. Therefore, we have engineered a stable cell line with tetracycline inducible expression of human a4-, ß3- and N-terminally Flag-tagged d-subunits. This cell line achieved a specific activity between 15 and 20 pmol [3H]muscimol sites/mg of membrane protein, making it possible to obtain 1 nmole of purified a4ß3d GABAAR from sixty 15–cm culture dishes. When induced, these cells exhibited agonist–induced currents with characteristics comparable to those previously reported for this receptor and a pharmacology that included strong modulation by etomidate and the d-subunit-specific ligand, DS2. Immunoaffinity purification and reconstitution in CHAPS/asolectin micelles resulted in the retention of equilibrium allosteric interactions between the separate agonist, anesthetic and DS2 sites. Moreover, all three subunits retained glycosylation. The establishment of this well–characterized cell line will allow molecular level studies of tonic receptors to be undertaken.