Three classes of propofol binding sites on GABA_A receptors

Propofol is a widely used anesthetic and sedative that acts as a positive allosteric modulator of gamma-aminobutyric acid type A (GABA_A) receptors. Several potential propofol binding sites that may mediate this effect have been identified using propofol-analogue photoaffinity labeling. Ortho-propofol diazirine (o-PD) labels β-H267, a pore-lining residue, whereas AziPm labels residues β-M286, β-M227, and α-I239 in the two membrane-facing interfaces [β(+)/α(-) and α(+)/β(-)] between α and β subunits. This study used photoaffinity labeling of α₁β₃ GABA_A receptors to reconcile the apparently conflicting results obtained with AziPm and o-PD labeling, focusing on whether β₃-H267 identifies specific propofol binding site(s). The results show that propofol, but not AziPm protects β₃-H267 from labeling by o-PD, whereas both propofol and o-PD protect against AziPm labeling of β₃-M286, β₃-M227, and α₁I239. These data indicate that there are three distinct classes of propofol binding sites, with AziPm binding to two of the classes and o-PD to all three. Analysis of binding stoichiometry using native mass spectrometry in β₃ homomeric receptors, demonstrated a minimum of five AziPm labeled residues and three o-PD labeled residues per pentamer, suggesting that there are two distinct propofol binding sites per β-subunit. The native mass spectrometry data, coupled with photolabeling performed in the presence of zinc, indicate that the binding site(s) identified by o-PD are adjacent to, but not within the channel pore, since the pore at the 17' H267 residue can accommodate only one propofol molecule. These data validate the existence of three classes of specific propofol binding sites on α₁β₃ GABA_A receptors.