Consensus phosphorylation sites of human GABA(C)/GABA(ρ) receptors are not critical for inhibition by protein kinase C activation

The mechanism of inhibition of human GABA(C)/GABA(ρ) receptors by protein kinase C (PKC) activation was investigated in Xenopus oocytes. Phorbol 12-myristate 13 acetate (PMA), a potent PKC activator, at 25 nM inhibited the currents through GABA(ρ2) receptors, which have one consensus phosphorylation site by PKC in the predicted intracellular loops. The time- courses and amplitudes of inhibition were not significantly different from those occurring through GABA(ρ1) receptors, which have six such sites. The inhibitory effect of PMA was also observed after removing each consensus phosphorylation site in both GABA(ρ1) and ρ2 receptors by site-directed mutagenesis. These results suggest that phosphorylation of consensus sites in the intracellular loops is not involved in the inhibition of human GABA(C)/GABA(ρ) receptors by PKC activation.