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 induced a continuous decrease in the gamma-aminobutyric acid (GABA)-activated current amplitude from recombinant GABA receptors (formed by rho1 or alphabetagamma subunits) expressed in Xenopus oocytes. This decline was due to internalization of receptors from the plasma membrane as confirmed by a decrease in surface fluorescence with green fluorescence protein-tagged receptors as well as a concomitant decrease in surface [(3)H]GABA binding. PMA specifically caused internalization of GABA receptors, but not neuronal acetylcholine receptors (alpha(7) or alpha(4)beta(2)), indicating the internalization was not a general, nonspecific phenomenon. Mutation of rho1 PKC phosphorylation sites, identified by in vitro phosphorylation, did not prevent GABA receptor internalization, nor did coexpression of the rho1 M3-M4 intracellular loop along with rho1 GABA receptors. It is likely that PKC-mediated phosphorylation of other proteins, rather than rho1 itself, was required for the internalization. Both rho1 and alphabetagamma receptors did not degrade after phorbol 12-myristate 13-acetate-induced internalization, but returned to the membrane surface within 24 h. These data suggest internalized receptors can exist in an intracellular compartment that can be delivered back to the plasma membrane. Thus, by regulating GABA receptor surface expression, PKC may play a key role in the regulation of GABA-mediated inhibition.