title: Lys-63-linked Ubiquitination of γ-Aminobutyric Acid (GABA), Type B1, at Multiple Sites by the E3 Ligase Mind Bomb-2 Targets GABAB Receptors to Lysosomal Degradation.
abstract: GABABreceptors are heterodimeric G protein-coupled receptors, which control neuronal excitability by mediating prolonged inhibition. The magnitude of GABABreceptor-mediated inhibition essentially depends on the amount of receptors in the plasma membrane. However, the factors regulating cell surface expression of GABABreceptors are poorly characterized. Cell surface GABABreceptors are constitutively internalized and either recycled to the plasma membrane or degraded in lysosomes. The signal that sorts GABABreceptors to lysosomes is currently unknown. Here we show that Mind bomb-2 (MIB2)-mediated Lys-63-linked ubiquitination of the GABAB1subunit at multiple sites is the lysosomal sorting signal for GABABreceptors. We found that inhibition of lysosomal activity in cultured rat cortical neurons increased the fraction of Lys-63-linked ubiquitinated GABABreceptors and enhanced the expression of total as well as cell surface GABABreceptors. Mutational inactivation of four putative ubiquitination sites in the GABAB1subunit significantly diminished Lys-63-linked ubiquitination of GABABreceptors and prevented their lysosomal degradation. We identified MIB2 as the E3 ligase triggering Lys-63-linked ubiquitination and lysosomal degradation of GABABreceptors. Finally, we show that sustained activation of glutamate receptors, a condition occurring in brain ischemia that down-regulates GABABreceptors, considerably increased the expression of MIB2 and Lys-63-linked ubiquitination of GABABreceptors. Interfering with Lys-63-linked ubiquitination by overexpressing ubiquitin mutants or GABAB1mutants deficient in Lys-63-linked ubiquitination prevented glutamate-induced down-regulation of the receptors. These findings indicate that Lys-63-linked ubiquitination of GABAB1at multiple sites by MIB2 controls sorting of GABABreceptors to lysosomes for degradation under physiological and pathological conditions.
authors: From the Institute of Pharmacology and Toxicology, University of Zurich.
authors: Khaled
authors: K
authors: Zemoura
authors: From the Institute of Pharmacology and Toxicology, University of Zurich.
authors: Claudia
authors: C
authors: Trümpler
authors: From the Institute of Pharmacology and Toxicology, University of Zurich, benke@pharma.uzh.ch.|the Neuroscience Center Zurich, University of Zurich and ETH Zurich, and.|the Drug Discovery Network Zurich (DDNZ), Winterthurerstrasse 190, CH-8057 Zurich, Switzerland.
authors: Dietmar
authors: D
authors: Benke
mesh_terms: D000818:Animals; D002545:Brain Ischemia / Q000235:genetics / Q000378:metabolism*; D001933:Brain Stem / Q000378:metabolism*; D015971:Gene Expression Regulation, Enzymologic; D008247:Lysosomes / Q000235:genetics / Q000378:metabolism*; D059748:Proteolysis; D051381:Rats; D017208:Rats, Wistar; D018080:Receptors, GABA-B / Q000235:genetics / Q000378:metabolism*; D044767:Ubiquitin-Protein Ligases / Q000096:biosynthesis* / Q000235:genetics; D054875:Ubiquitination / Q000235:genetics
chemical_list: D018080:Receptors, GABA-B; D044767:Ubiquitin-Protein Ligases
keywords: E3 ubiquitin ligase; GABA receptor; lysosome; protein degradation; ubiquitylation (ubiquitination)