KLHL12-mediated ubiquitination of the dopamine D4 receptor does not target the receptor for degradation

In previous studies, we identified KLHL12 as a novel interaction partner of the dopamine D4 receptor that functions as an adaptor in a Cul3-based E3 ubiquitin ligase complex to target the receptor for ubiquitination. In this study, we show that KLHL12 promotes poly-ubiquitination of the receptor by performing ubiquitination assays in eukaryotic cells. Furthermore, we demonstrate that KLHL12 not only interacts with both immature, ER-associated and mature, plasma membrane-associated D4 receptors, but also promotes ubiquitination of both receptor subpools. However, KLHL12 does not promote proteasomal degradation of newly synthesized receptors through the ER-associated degradation pathway or lysosomal degradation of mature receptors. Moreover, our data reveal that D4 receptors do not undergo agonist-promoted ubiquitination or degradation, in contrast to many other G-protein-coupled receptors (GPCRs). Together, our data show that KLHL12-mediated ubiquitination of the D4 receptor does not target the receptor for degradation, suggesting that D4 receptor ubiquitination, and maybe ubiquitination of GPCRs in general, might have alternative functions.