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Zhou Y, Zhang Q, Zhao Z, Hu X, You Q, Jiang Z. Targeting kelch-like (KLHL) proteins: achievements, challenges and perspectives. Eur J Med Chem 2024; 269:116270. [PMID: 38490062 DOI: 10.1016/j.ejmech.2024.116270] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/02/2024] [Revised: 02/07/2024] [Accepted: 02/19/2024] [Indexed: 03/17/2024]
Abstract
Kelch-like proteins (KLHLs) are a large family of BTB-containing proteins. KLHLs function as the substrate adaptor of Cullin 3-RING ligases (CRL3) to recognize substrates. KLHLs play pivotal roles in regulating various physiological and pathological processes by modulating the ubiquitination of their respective substrates. Mounting evidence indicates that mutations or abnormal expression of KLHLs are associated with various human diseases. Targeting KLHLs is a viable strategy for deciphering the KLHLs-related pathways and devising therapies for associated diseases. Here, we comprehensively review the known KLHLs inhibitors to date and the brilliant ideas underlying their development.
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Affiliation(s)
- Yangguo Zhou
- Jiang Su Key Laboratory of Drug Design and Optimization and State Key Laboratory of Natural Medicines, China Pharmaceutical University, Nanjing, 210009, China; Department of Medicinal Chemistry, School of Pharmacy, China Pharmaceutical University, Nanjing, 210009, China
| | - Qiong Zhang
- Jiang Su Key Laboratory of Drug Design and Optimization and State Key Laboratory of Natural Medicines, China Pharmaceutical University, Nanjing, 210009, China; Department of Medicinal Chemistry, School of Pharmacy, China Pharmaceutical University, Nanjing, 210009, China
| | - Ziquan Zhao
- Jiang Su Key Laboratory of Drug Design and Optimization and State Key Laboratory of Natural Medicines, China Pharmaceutical University, Nanjing, 210009, China; Department of Medicinal Chemistry, School of Pharmacy, China Pharmaceutical University, Nanjing, 210009, China
| | - Xiuqi Hu
- Jiang Su Key Laboratory of Drug Design and Optimization and State Key Laboratory of Natural Medicines, China Pharmaceutical University, Nanjing, 210009, China; Department of Medicinal Chemistry, School of Pharmacy, China Pharmaceutical University, Nanjing, 210009, China
| | - Qidong You
- Jiang Su Key Laboratory of Drug Design and Optimization and State Key Laboratory of Natural Medicines, China Pharmaceutical University, Nanjing, 210009, China; Department of Medicinal Chemistry, School of Pharmacy, China Pharmaceutical University, Nanjing, 210009, China.
| | - Zhengyu Jiang
- Jiang Su Key Laboratory of Drug Design and Optimization and State Key Laboratory of Natural Medicines, China Pharmaceutical University, Nanjing, 210009, China; Department of Medicinal Chemistry, School of Pharmacy, China Pharmaceutical University, Nanjing, 210009, China.
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2
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van Overbeek NK, Aguirre T, van der Heden van Noort GJ, Blagoev B, Vertegaal ACO. Deciphering non-canonical ubiquitin signaling: biology and methodology. Front Mol Biosci 2024; 10:1332872. [PMID: 38414868 PMCID: PMC10897730 DOI: 10.3389/fmolb.2023.1332872] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/2023] [Accepted: 12/20/2023] [Indexed: 02/29/2024] Open
Abstract
Ubiquitination is a dynamic post-translational modification that regulates virtually all cellular processes by modulating function, localization, interactions and turnover of thousands of substrates. Canonical ubiquitination involves the enzymatic cascade of E1, E2 and E3 enzymes that conjugate ubiquitin to lysine residues giving rise to monomeric ubiquitination and polymeric ubiquitination. Emerging research has established expansion of the ubiquitin code by non-canonical ubiquitination of N-termini and cysteine, serine and threonine residues. Generic methods for identifying ubiquitin substrates using mass spectrometry based proteomics often overlook non-canonical ubiquitinated substrates, suggesting that numerous undiscovered substrates of this modification exist. Moreover, there is a knowledge gap between in vitro studies and comprehensive understanding of the functional consequence of non-canonical ubiquitination in vivo. Here, we discuss the current knowledge about non-lysine ubiquitination, strategies to map the ubiquitinome and their applicability for studying non-canonical ubiquitination substrates and sites. Furthermore, we elucidate the available chemical biology toolbox and elaborate on missing links required to further unravel this less explored subsection of the ubiquitin system.
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Affiliation(s)
- Nila K. van Overbeek
- Department of Cell and Chemical Biology, Leiden University Medical Center, Leiden, Netherlands
| | - Tim Aguirre
- Department of Cell and Chemical Biology, Leiden University Medical Center, Leiden, Netherlands
| | | | - Blagoy Blagoev
- Department of Biochemistry and Molecular Biology, University of Southern Denmark, Odense, Denmark
| | - Alfred C. O. Vertegaal
- Department of Cell and Chemical Biology, Leiden University Medical Center, Leiden, Netherlands
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3
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Abstract
The traditional textbook describes ubiquitylation as the conjugation of ubiquitin to a target by forming a covalent bond connecting ubiquitin's carboxy-terminal glycine residue with an acceptor amino acid like lysine or amino-terminal methionine in the substrate protein. While this adequately depicts a significant fraction of cellular ubiquitylation processes, a growing number of ubiquitin modifications do not follow this rule. Recent data demonstrate that ubiquitin can also be efficiently attached to other amino acids, such as cysteine, serine, and threonine, via ester bonding. Initially observed for a virus-encoded ubiquitin ligase, which targets a cysteine residue in a host protein to initiate its degradation, ester-linked ubiquitylation is now shown to also drive regular cellular processes. These ubiquitylation events expand the complexity and diversity of ubiquitin signaling and broaden the capability of cellular messages in the so-called ubiquitin code. Still, questions on the prevalence, relevance, and involvement in physiological and cellular functions await clearing. In this review, we aim to summarize our knowledge on ester-linked ubiquitylation and introduce experimental strategies to circumvent technical issues that complicate analysis of this uncommon posttranslational modification.
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Affiliation(s)
- Alba Ferri-Blazquez
- Max-Delbrück-Center for Molecular Medicine in the Helmholz Association, Berlin Buch, Germany.
- Institute for Biology, Humboldt-Universität zu Berlin, Berlin, Germany.
| | - Ernst Jarosch
- Max-Delbrück-Center for Molecular Medicine in the Helmholz Association, Berlin Buch, Germany
| | - Thomas Sommer
- Max-Delbrück-Center for Molecular Medicine in the Helmholz Association, Berlin Buch, Germany
- Institute for Biology, Humboldt-Universität zu Berlin, Berlin, Germany
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4
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Ye G, Wang J, Yang W, Li J, Ye M, Jin X. The roles of KLHL family members in human cancers. Am J Cancer Res 2022; 12:5105-5139. [PMID: 36504893 PMCID: PMC9729911] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2022] [Accepted: 11/08/2022] [Indexed: 12/15/2022] Open
Abstract
The Kelch-like (KLHL) family members consist of three domains: bric-a-brac, tramtrack, broad complex/poxvirus and zinc finger domain, BACK domain and Kelch domain, which combine and interact with Cullin3 to form an E3 ubiquitin ligase. Research has indicated that KLHL family members ubiquitinate target substrates to regulate physiological and pathological processes, including tumorigenesis and progression. KLHL19, a member of the KLHL family, is associated with tumorigenesis and drug resistance. However, the regulation and cross talks of other KLHL family members, which also play roles in cancer, are still unclear. Our review mainly explores studies concerning the roles of other KLHL family members in tumor-related regulation to provide novel insights into KLHL family members.
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Affiliation(s)
- Ganghui Ye
- The Affiliated Hospital of Medical School, Ningbo UniversityNingbo 315020, Zhejiang, P. R. China,Department of Biochemistry and Molecular Biology, Zhejiang Key Laboratory of Pathophysiology, Medical School of Ningbo UniversityNingbo 315211, Zhejiang, P. R. China
| | - Jie Wang
- The Affiliated Hospital of Medical School, Ningbo UniversityNingbo 315020, Zhejiang, P. R. China,Department of Biochemistry and Molecular Biology, Zhejiang Key Laboratory of Pathophysiology, Medical School of Ningbo UniversityNingbo 315211, Zhejiang, P. R. China
| | - Weili Yang
- Yinzhou People’s Hospital of Medical School, Ningbo UniversityNingbo 315040, Zhejiang, P. R. China
| | - Jinyun Li
- The Affiliated Hospital of Medical School, Ningbo UniversityNingbo 315020, Zhejiang, P. R. China,Department of Biochemistry and Molecular Biology, Zhejiang Key Laboratory of Pathophysiology, Medical School of Ningbo UniversityNingbo 315211, Zhejiang, P. R. China
| | - Meng Ye
- The Affiliated Hospital of Medical School, Ningbo UniversityNingbo 315020, Zhejiang, P. R. China,Department of Biochemistry and Molecular Biology, Zhejiang Key Laboratory of Pathophysiology, Medical School of Ningbo UniversityNingbo 315211, Zhejiang, P. R. China
| | - Xiaofeng Jin
- The Affiliated Hospital of Medical School, Ningbo UniversityNingbo 315020, Zhejiang, P. R. China,Department of Biochemistry and Molecular Biology, Zhejiang Key Laboratory of Pathophysiology, Medical School of Ningbo UniversityNingbo 315211, Zhejiang, P. R. China
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Patwardhan A, Cheng N, Trejo J. Post-Translational Modifications of G Protein-Coupled Receptors Control Cellular Signaling Dynamics in Space and Time. Pharmacol Rev 2020; 73:120-151. [PMID: 33268549 DOI: 10.1124/pharmrev.120.000082] [Citation(s) in RCA: 75] [Impact Index Per Article: 18.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
G protein-coupled receptors (GPCRs) are a large family comprising >800 signaling receptors that regulate numerous cellular and physiologic responses. GPCRs have been implicated in numerous diseases and represent the largest class of drug targets. Although advances in GPCR structure and pharmacology have improved drug discovery, the regulation of GPCR function by diverse post-translational modifications (PTMs) has received minimal attention. Over 200 PTMs are known to exist in mammalian cells, yet only a few have been reported for GPCRs. Early studies revealed phosphorylation as a major regulator of GPCR signaling, whereas later reports implicated a function for ubiquitination, glycosylation, and palmitoylation in GPCR biology. Although our knowledge of GPCR phosphorylation is extensive, our knowledge of the modifying enzymes, regulation, and function of other GPCR PTMs is limited. In this review we provide a comprehensive overview of GPCR post-translational modifications with a greater focus on new discoveries. We discuss the subcellular location and regulatory mechanisms that control post-translational modifications of GPCRs. The functional implications of newly discovered GPCR PTMs on receptor folding, biosynthesis, endocytic trafficking, dimerization, compartmentalized signaling, and biased signaling are also provided. Methods to detect and study GPCR PTMs as well as PTM crosstalk are further highlighted. Finally, we conclude with a discussion of the implications of GPCR PTMs in human disease and their importance for drug discovery. SIGNIFICANCE STATEMENT: Post-translational modification of G protein-coupled receptors (GPCRs) controls all aspects of receptor function; however, the detection and study of diverse types of GPCR modifications are limited. A thorough understanding of the role and mechanisms by which diverse post-translational modifications regulate GPCR signaling and trafficking is essential for understanding dysregulated mechanisms in disease and for improving and refining drug development for GPCRs.
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Affiliation(s)
- Anand Patwardhan
- Department of Pharmacology and the Biomedical Sciences Graduate Program, School of Medicine, University of California, San Diego, La Jolla, California
| | - Norton Cheng
- Department of Pharmacology and the Biomedical Sciences Graduate Program, School of Medicine, University of California, San Diego, La Jolla, California
| | - JoAnn Trejo
- Department of Pharmacology and the Biomedical Sciences Graduate Program, School of Medicine, University of California, San Diego, La Jolla, California
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Chen Z, Wasney GA, Picaud S, Filippakopoulos P, Vedadi M, D'Angiolella V, Bullock AN. Identification of a PGXPP degron motif in dishevelled and structural basis for its binding to the E3 ligase KLHL12. Open Biol 2020; 10:200041. [PMID: 32574548 PMCID: PMC7333892 DOI: 10.1098/rsob.200041] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
Wnt signalling is dependent on dishevelled proteins (DVL1-3), which assemble an intracellular Wnt signalosome at the plasma membrane. The levels of DVL1-3 are regulated by multiple Cullin-RING E3 ligases that mediate their ubiquitination and degradation. The BTB-Kelch protein KLHL12 was the first E3 ubiquitin ligase to be identified for DVL1-3, but the molecular mechanisms determining its substrate interactions have remained unknown. Here, we mapped the interaction of DVL1-3 to a ‘PGXPP' motif that is conserved in other known partners and substrates of KLHL12, including PLEKHA4, PEF1, SEC31 and DRD4. To determine the binding mechanism, we solved a 2.4 Å crystal structure of the Kelch domain of KLHL12 in complex with a DVL1 peptide that bound with low micromolar affinity. The DVL1 substrate adopted a U-shaped turn conformation that enabled hydrophobic interactions with all six blades of the Kelch domain β-propeller. In cells, the mutation or deletion of this motif reduced the binding and ubiquitination of DVL1 and increased its stability confirming this sequence as a degron motif for KLHL12 recruitment. These results define the molecular mechanisms determining DVL regulation by KLHL12 and establish the KLHL12 Kelch domain as a new protein interaction module for a novel proline-rich motif.
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Affiliation(s)
- Zhuoyao Chen
- Structural Genomics Consortium, University of Oxford, Old Road Campus Research Building, Roosevelt Drive, Oxford OX3 7DQ, UK
| | - Gregory A Wasney
- Structural Genomics Consortium, University of Toronto, MaRS Centre, South Tower, 101 College Street, Toronto, M5G 1L7, Canada
| | - Sarah Picaud
- Structural Genomics Consortium, University of Oxford, Old Road Campus Research Building, Roosevelt Drive, Oxford OX3 7DQ, UK
| | - Panagis Filippakopoulos
- Structural Genomics Consortium, University of Oxford, Old Road Campus Research Building, Roosevelt Drive, Oxford OX3 7DQ, UK
| | - Masoud Vedadi
- Structural Genomics Consortium, University of Toronto, MaRS Centre, South Tower, 101 College Street, Toronto, M5G 1L7, Canada
| | - Vincenzo D'Angiolella
- Department of Oncology, Cancer Research UK and Medical Research Council Institute for Radiation Oncology, University of Oxford, Oxford OX3 7DQ, UK
| | - Alex N Bullock
- Structural Genomics Consortium, University of Oxford, Old Road Campus Research Building, Roosevelt Drive, Oxford OX3 7DQ, UK
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7
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Targeted protein degradation: expanding the toolbox. Nat Rev Drug Discov 2019; 18:949-963. [PMID: 31666732 DOI: 10.1038/s41573-019-0047-y] [Citation(s) in RCA: 475] [Impact Index Per Article: 95.0] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 09/23/2019] [Indexed: 12/19/2022]
Abstract
Proteolysis-targeting chimeras (PROTACs) and related molecules that induce targeted protein degradation by the ubiquitin-proteasome system represent a new therapeutic modality and are the focus of great interest, owing to potential advantages over traditional occupancy-based inhibitors with respect to dosing, side effects, drug resistance and modulating 'undruggable' targets. However, the technology is still maturing, and the design elements for successful PROTAC-based drugs are currently being elucidated. Importantly, fewer than 10 of the more than 600 E3 ubiquitin ligases have so far been exploited for targeted protein degradation, and expansion of knowledge in this area is a key opportunity. Here, we briefly discuss lessons learned about targeted protein degradation in chemical biology and drug discovery and systematically review the expression profile, domain architecture and chemical tractability of human E3 ligases that could expand the toolbox for PROTAC discovery.
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Jerabkova K, Sumara I. Cullin 3, a cellular scripter of the non-proteolytic ubiquitin code. Semin Cell Dev Biol 2019; 93:100-110. [DOI: 10.1016/j.semcdb.2018.12.007] [Citation(s) in RCA: 17] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2018] [Revised: 12/20/2018] [Accepted: 12/20/2018] [Indexed: 11/29/2022]
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9
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Qian M, Vasudevan L, Huysentruyt J, Risseeuw MDP, Stove C, Vanderheyden PML, Van Craenenbroeck K, Van Calenbergh S. Design, Synthesis, and Biological Evaluation of Bivalent Ligands Targeting Dopamine D 2 -Like Receptors and the μ-Opioid Receptor. ChemMedChem 2018; 13:944-956. [PMID: 29451744 DOI: 10.1002/cmdc.201700787] [Citation(s) in RCA: 20] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/15/2017] [Indexed: 12/13/2022]
Abstract
Currently, there is mounting evidence that intermolecular receptor-receptor interactions may result in altered receptor recognition, pharmacology and signaling. Heterobivalent ligands have been proven useful as molecular probes for confirming and targeting heteromeric receptors. This report describes the design and synthesis of novel heterobivalent ligands for dopamine D2 -like receptors (D2 -likeR) and the μ-opioid receptor (μOR) and their evaluation using ligand binding and functional assays. Interestingly, we identified a potent bivalent ligand that contains a short 18-atom linker and combines good potency with high efficacy both in β-arrestin 2 recruitment for μOR and MAPK-P for D4 R. Furthermore, this compound was characterized by a biphasic competition binding curve for the D4 R-μOR heterodimer, indicative of a bivalent binding mode. As this compound possibly bridges the D4 R-μOR heterodimer, it could be used as a pharmacological tool to further investigate the interactions of D4 R and μOR.
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Affiliation(s)
- Mingcheng Qian
- Laboratory for Medicinal Chemistry, Ghent University, Ottergemsesteenweg 460, 9000, Ghent, Belgium.,Laboratory of Toxicology, Ghent University, Ottergemsesteenweg 460, 9000, Ghent, Belgium
| | - Lakshmi Vasudevan
- Laboratory of Toxicology, Ghent University, Ottergemsesteenweg 460, 9000, Ghent, Belgium
| | - Jelle Huysentruyt
- Laboratory of Toxicology, Ghent University, Ottergemsesteenweg 460, 9000, Ghent, Belgium
| | - Martijn D P Risseeuw
- Laboratory for Medicinal Chemistry, Ghent University, Ottergemsesteenweg 460, 9000, Ghent, Belgium
| | - Christophe Stove
- Laboratory of Toxicology, Ghent University, Ottergemsesteenweg 460, 9000, Ghent, Belgium
| | - Patrick M L Vanderheyden
- Department Research Group of Molecular and Biochemical Pharmacology, Faculty of Sciences and Bioengineering Sciences, Vrije Universiteit Brussel, VUB-MBFA, Pleinlaan 2, 1050, Brussels, Belgium
| | | | - Serge Van Calenbergh
- Laboratory for Medicinal Chemistry, Ghent University, Ottergemsesteenweg 460, 9000, Ghent, Belgium
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Peeler JC, Schedin-Weiss S, Soula M, Kazmi MA, Sakmar TP. Isopeptide and ester bond ubiquitination both regulate degradation of the human dopamine receptor 4. J Biol Chem 2017; 292:21623-21630. [PMID: 29101232 PMCID: PMC5766964 DOI: 10.1074/jbc.m116.758961] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/16/2016] [Revised: 10/25/2017] [Indexed: 12/30/2022] Open
Abstract
How an optimal level of human dopamine D4 receptor (hD4R) is maintained in synaptic membranes is not known. We show here that hD4R is ubiquitinated in primary neurons. We go on to show that ubiquitin is attached to hD4R through isopeptide and ester bonds. When lysine (Lys) residues of the hD4R are substituted with arginine (Arg) residues, cellular hD4R protein levels increase. A synergistic effect on hD4R levels is noted when cytoplasmic serine (Ser) and threonine (Thr) residues are mutated. Chloroquine, an inhibitor of lysosomal degradation, did not have an effect on hD4R protein levels. However, treatment with bortezomib, an inhibitor of the 20S proteasome, caused a dose-dependent increase in hD4R protein levels. The effect of bortezomib was attenuated in the receptor variants that lacked Lys or Ser/Thr residues, and the hD4R mutant that lacked 17 cytoplasmic Lys, Ser, and Thr residues was nearly insensitive to bortezomib treatment. We conclude that both isopeptide and ester bond ubiquitination regulate proteasomal degradation of hD4R.
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Affiliation(s)
- Jennifer C Peeler
- From the Laboratory of Chemical Biology and Signal Transduction, The Rockefeller University, New York, New York 10065 and
| | - Sophia Schedin-Weiss
- Department of Neurobiology, Care Sciences and Society, Division of Neurogeriatrics, Center for Alzheimer Research, Karolinska Institutet, 141 57 Huddinge, Sweden
| | - Mariluz Soula
- From the Laboratory of Chemical Biology and Signal Transduction, The Rockefeller University, New York, New York 10065 and
| | - Manija A Kazmi
- From the Laboratory of Chemical Biology and Signal Transduction, The Rockefeller University, New York, New York 10065 and
| | - Thomas P Sakmar
- From the Laboratory of Chemical Biology and Signal Transduction, The Rockefeller University, New York, New York 10065 and .,Department of Neurobiology, Care Sciences and Society, Division of Neurogeriatrics, Center for Alzheimer Research, Karolinska Institutet, 141 57 Huddinge, Sweden
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11
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Regulation of G Protein-Coupled Receptors by Ubiquitination. Int J Mol Sci 2017; 18:ijms18050923. [PMID: 28448471 PMCID: PMC5454836 DOI: 10.3390/ijms18050923] [Citation(s) in RCA: 35] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2017] [Revised: 04/20/2017] [Accepted: 04/23/2017] [Indexed: 02/07/2023] Open
Abstract
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.
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12
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Abstract
Ubiquitination is a post-translational modification that targets proteins for degradation but can also regulate other cellular processes such as endocytosis, trafficking and DNA repair. We investigate ubiquitination of the dopamine D4receptor (D4R) which belongs to the superfamily of G protein-coupled receptors (GPCR). Several polymorphic variants of the D4R exist, which differ in the number of 16-amino acid repeats in the third intracellular loop (IC3) of the receptor. The functional role of this polymorphic region is not known but persons with the seven-repeat allele show a predisposition to develop attention deficit hyperactivity disorder (ADHD). We identified a protein, KLHL12, which specifically interacts with this polymorphic region and enhances ubiquitination of the D4R. We have tested the influence of KLHL12 on the ubiquitination of the most common D4R polymorphic variants and found that KLHL12 strongly promotes ubiquitination of the two- and four-repeat variant but has hardly any effect on ubiquitination of the seven-repeat D4R. This suggests that differential ubiquitination of the D4R may have functional implications. Moreover, we were able to demonstrate that KLHL12-mediated D4R ubiquitination does not lead to receptor degradation. Next, we aimed to identify specific residues in the sequence of D4R which undergo ubiquitination and observed that the lysine-less receptor mutant is still ubiquitinated. Subsequently, we have tested the hypothesis whether KLHL12 could promote ubiquitination on non-lysine residues of the D4R. The importance of the cysteine and serine/threonine residues in the ubiquitination process of the receptor was examined and the obtained results confirmed that D4R can be ubiquitinated on non-lysine residues. In this review we summarize our data on D4R ubiquitination and put this in the light of other GPCR ubiquitination studies.
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