KLHL12 Promotes Non-Lysine Ubiquitination of the Dopamine Receptors D4.2 and D4.4, but Not of the ADHD-Associated D4.7 Variant

Dopamine D4 Receptor PolymorphismThe dopamine D4 receptor has an important polymorphism in its third intracellular loop that is intensively studied and has been associated with several abnormal conditions, among others, attention deficit hyperactivity disorder.KLHL12 Promotes Ubiquitination of the Dopamine D4 Receptor on Non-Lysine ResiduesIn previous studies we have shown that KLHL12, a BTB-Kelch protein, specifically interacts with the polymorphic repeats of the dopamine D4 receptor and enhances its ubiquitination, which, however, has no influence on receptor degradation. In this study we provide evidence that KLHL12 promotes ubiquitination of the dopamine D4 receptor on non-lysine residues. By using lysine-deficient receptor mutants and chemical approaches we concluded that ubiquitination on cysteine, serine and/or threonine is possible.Differential Ubiquitination of the Dopamine D4 Receptor Polymorphic VariantsAdditionally, we show that the dopamine D4.7 receptor variant, which is associated with a predisposition to develop attention deficient hyperactivity disorder, is differentially ubiquitinated compared to the other common receptor variants D4.2 and D4.4. Together, our study suggests that GPCR ubiquitination is a complex and variable process.