KLHL12 promotes non-lysine ubiquitination of the dopamine receptors D-4.2 and D-4.4, but not of the ADHD-associated D-4.7 variant

Dopamine D-4 Receptor Polymorphism : The dopamine D-4 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 D-4 Receptor on Non-Lysine Residues : In previous studies we have shown that KLHL12, a BTB-Kelch protein, specifically interacts with the polymorphic repeats of the dopamine D-4 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 D-4 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 D-4 Receptor Polymorphic Variants : Additionally, we show that the dopamine D-4.7 receptor variant, which is associated with a predisposition to develop attention deficient hyperactivity disorder, is differentially ubiquitinated compared to the other common receptor variants D-4.2 and D-4.4. Together, our study suggests that GPCR ubiquitination is a complex and variable process

Regulation of G protein-coupled receptors by ubiquitination

G protein-coupled receptors (GPCRs) comprise the largest family of membrane receptors that control many cellular processes and consequently often serve as drug targets. These receptors undergo a strict regulation by mechanisms such as internalization and desensitization, which are strongly influenced by posttranslational modifications. Ubiquitination is a posttranslational modification with a broad range of functions that is currently gaining increased appreciation as a regulator of GPCR activity. The role of ubiquitination in directing GPCRs for lysosomal degradation has already been well-established. Furthermore, this modification can also play a role in targeting membrane and endoplasmic reticulum-associated receptors to the proteasome. Most recently, ubiquitination was also shown to be involved in GPCR signaling. In this review, we present current knowledge on the molecular basis of GPCR regulation by ubiquitination, and highlight the importance of E3 ubiquitin ligases, deubiquitinating enzymes and β-arrestins. Finally, we discuss classical and newly-discovered functions of ubiquitination in controlling GPCR activity

The G protein-coupled receptor heterodimer network (GPCR-HetNet) and its hub components

G protein-coupled receptors (GPCRs) oligomerization has emerged as a vital characteristic of receptor structure. Substantial experimental evidence supports the existence of GPCR-GPCR interactions in a coordinated and cooperative manner. However, despite the current development of experimental techniques for large-scale detection of GPCR heteromers, in order to understand their connectivity it is necessary to develop novel tools to study the global heteroreceptor networks. To provide insight into the overall topology of the GPCR heteromers and identify key players, a collective interaction network was constructed. Experimental interaction data for each of the individual human GPCR protomers was obtained manually from the STRING and SCOPUS databases. The interaction data were used to build and analyze the network using Cytoscape software. The network was treated as undirected throughout the study. It is comprised of 156 nodes, 260 edges and has a scale-free topology. Connectivity analysis reveals a significant dominance of intrafamily versus interfamily connections. Most of the receptors within the network are linked to each other by a small number of edges. DRD2, OPRM, ADRB2, AA2AR, AA1R, OPRK, OPRD and GHSR are identified as hubs. In a network representation 10 modules/clusters also appear as a highly interconnected group of nodes. Information on this GPCR network can improve our understanding of molecular integration. GPCR-HetNet has been implemented in Java and is freely available at http://www.iiia.csic.es/similar to ismel/GPCR-Nets/index.html

The G protein-coupled receptor heterodimer network (GPCR-HetNet) and its hub components

G protein-coupled receptors (GPCRs) oligomerization has emerged as a vital characteristic of receptor structure. Substantial experimental evidence supports the existence of GPCR-GPCR interactions in a coordinated and cooperative manner. However, despite the current development of experimental techniques for large-scale detection of GPCR heteromers, in order to understand their connectivity it is necessary to develop novel tools to study the global heteroreceptor networks. To provide insight into the overall topology of the GPCR heteromers and identify key players, a collective interaction network was constructed. Experimental interaction data for each of the individual human GPCR protomers was obtained manually from the STRING and SCOPUS databases. The interaction data were used to build and analyze the network using Cytoscape software. The network was treated as undirected throughout the study. It is comprised of 156 nodes, 260 edges and has a scale-free topology. Connectivity analysis reveals a significant dominance of intrafamily versus interfamily connections. Most of the receptors within the network are linked to each other by a small number of edges. DRD2, OPRM, ADRB2, AA2AR, AA1R, OPRK, OPRD and GHSR are identified as hubs. In a network representation 10 modules/clusters also appear as a highly interconnected group of nodes. Information on this GPCR network can improve our understanding of molecular integration. GPCR-HetNet has been implemented in Java and is freely available at http://www.iiia.csic.es/~ismel/GPCR-Nets/index.html. © 2014 by the authors; licensee MDPI, Basel, Switzerland.This work has been supported by the Swedish Royal Academy of Sciences (Stiftelsen B. von Beskows Fond and Stiftelsen Hierta-Retzius stipendiefond) and Karolinska Institutets Forskningsstiftelser 2011 and 2012 to D.O.B.-E., by grants from the Swedish Medical Research Council (04X-715), Telethon TV3’s La Marató Foundation 2008 and Hjärnfonden to K.F., D.O.B.-E., I.B. and W.R.-F. belong to the “Academia de Biólogos Cubanos” group. Feliciano Calvo and Carmelo Million are acknowledged for their support during the GPCR heterodimer list preparation.Peer Reviewe

Treatment with NaOH decreases ubiquitination of D_4.2R WT and D_{4.2 4KR}R mutant.

<p>A) HEK293T cells were transiently transfected as indicated. 48 h post-transfection, cells were harvested and lysed. 5% of the lysates were used for IB to visualize (Flag Ub)n-proteins, HA D₄R, and Etag KLHL12, respectively (left panels). The rest of the lysates were subjected to IP with anti-HA (16B12). After IP proteins were eluted in 0.5% SDS at 95°C. Next, lysates were incubated with or without addition of 50 mM NaOH for 1 h at 32°C, mock treated samples were incubated with PBS. Additionally, samples were treated with a highly reducing agent (5 min, 95°C, 100 mM DTT). After dilution with RIPA lysis buffer a second round of IP was performed with anti-HA antibody. Specific purification of the receptor after the second IP was confirmed upon IB with rat anti-HA (1:2000), whereas receptor ubiquitination was revealed upon IB with anti-Flag-HRP (right panels, 1:2000). N: samples treated with 50 mM NaOH, D: samples treated with 100 mM DTT. Experiment with two different chemical treatments was performed three times. Additionally, two more experiments were performed with NaOH treatment alone. B-E) Quantification of the ubiquitination signal detected in four to seven independent experiments was performed using Fiji software. Values were normalized to the receptor level in the corresponding lanes. Graph bars show percentage of maximal ubiquitination signal (mean ± SEM) where maximum is equal to 100% and represents the signal detected in samples which did not undergo any chemical treatment (NT- non-treated) in each independent experiment. Compared with NT: ***, P < 0.001; **, P < 0.01; *, P < 0.05. F) Quantification of the ubiquitination signal detected in five to seven independent experiments was performed using Fiji software. Values were normalized to the receptor level in the corresponding lanes. Graph bars show percentage of ubiquitination signal detected for WT D_4.2R (mean ± SEM). Compared with the ubiquitination signal detected for WT D_4.2R without or with overexpression of KLHL12, respectively: ***, P < 0.001; **, P < 0.01.</p

Role of dimerization in dopamine D-4 receptor biogenesis

Dopamine receptors are G protein-coupled receptors critically involved in locomotion, reward, and cognitive processes. Export of dopamine receptors to the plasma membrane is thought to follow the default secretory pathway, whereby proteins travel from the endoplasmatic reticulum (ER), through the Golgi apparatus, to arrive at the cell surface. Several observations indicate that trafficking from the ER to the plasma membrane is tightly regulated, and that correct folding in the ER acts as a bottle neck to the maturation of the dopamine D-4 receptors. The dopamine D-4 receptor is an interesting receptor since it has a polymorphic region in its third intracellular loop, resulting in receptor isoforms of varying length and amino acid composition. Correct folding is enhanced by: (1) interaction with specific proteins, such as ER resident chaperones, (2) interaction with pharmacological chaperones, for example, ligands that are membrane permeable and can bind to the receptor in the ER, and (3) receptor dimerization; the assembly of multisubunit proteins into a quaternary structure is started in the ER before cell surface delivery, which helps in correct folding and subsequent expression. These interactions help the process of GPCR folding, but more importantly they ensure that only properly folded proteins proceed from the ER to the trans-Golgi network. In this review we will mainly focus on the role of receptor dimerization in dopamine D-4 receptor maturation