Regulation of recombinant gamma-aminobutyric acid (GABA)(A) and GABA(C) receptors by protein kinase c

Activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate ind uced a continuous decrease in the gamma-aminobutyric acid (GABA)-activated current amplitude from recombinant GABA receptors (formed by rho 1 alpha be ta gamma subunits) expressed in Xenopus oocytes. This decline was due to in ternalization of receptors from the plasma membrane as confirmed by a decre ase in surface fluorescence with green fluorescence protein-tagged receptor s as well as a concomitant decrease in surface [H-3]GABA binding. PMA speci fically caused internalization of GABA receptors, but not neuronal acetylch oline receptors (alpha(7) or alpha(4)beta(2)), indicating the internalizati on was not a general, non-specific phenomenon. Mutation of rho 1 PKC phosph orylation sites, identified by in vitro phosphorylation, did not prevent GA BA receptor internalization, nor did coexpression of the rho 1 M3-M4 intrac ellular loop along with rho 1 GABA receptors. It is likely that PKC-mediate d phosphorylation of other proteins, rather than rho 1 itself, was required for the internalization. Both rho 1 and alpha beta gamma receptors did not degrade after phorbol 12-myristate 13-acetate-induced internalization, but returned to the membrane surface within 24 h. These data suggest internali zed receptors can exist in an intracellular compartment that can be deliver ed back to the plasma membrane. Thus, by regulating GABA receptor surface e xpression, PKC may play a key role in the regulation of GABA-mediated inhib ition.