Endocytosis of GABA(C) receptors depends on subunit composition and is regulated by protein kinase C-ζ and protein phosphatase 1.

Neuronal excitability depends on the surface concentration of neurotransmitter receptors. Type C gamma-aminobutyric acid receptors (GABA(C)R) are composed of ρ subunits that are highly expressed in the retina. Molecular mechanisms that guide the surface concentration of this receptor type are largely unknown. Previously, we reported physical interactions of GABA(C)R ρ subunits with protein kinase C-ζ (PKCζ) via adapter proteins of the ZIP protein family, as well as of protein phosphatase 1 (PP1) via PNUTS. Here, we demonstrate that co-expressing ρ1 with ZIP3 and PKCζ enhanced basal internalization of GABA(C)R, while receptor internalization was reduced in the presence of PNUTS and PP1. Co-expression of ρ1 with individual binding partners showed no alterations, except for PP1. Heterooligomeric GABA(C)R composed of ρ1 and ρ2 subunits had a significant higher endocytosis rate than ρ1 containing homooligomeric receptors. Mutant constructs lacking binding sites for protein interactions ensured the specificity of our data. Finally, substitution of serine and threonine residues with alanines indicated that GABA(C)R internalization depends on serine/threonine kinases and phosphatases, but not on tyrosine phosphorylation. We conclude that GABA(C)R internalization is reciprocally regulated by PKCζ and PP1 that are anchored to the receptor via ZIP3 or PNUTS respectively.