title: The E3 ubiquitin ligase atrophin interacting protein 4 binds directly to the chemokine receptor CXCR4 via a novel WW domain-mediated interaction.
abstract: The E3 ubiquitin ligase atrophin interacting protein 4 (AIP4) mediates ubiquitination and down-regulation of the chemokine receptor CXCR4. AIP4 belongs to the Nedd4-like homologous to E6-AP carboxy terminus domain family of E3 ubiquitin ligases, which typically bind proline-rich motifs within target proteins via the WW domains. The intracellular domains of CXCR4 lack canonical WW domain binding motifs; thus, whether AIP4 is targeted to CXCR4 directly or indirectly via an adaptor protein remains unknown. Here, we show that AIP4 can interact directly with CXCR4 via a novel noncanonical WW domain-mediated interaction involving serine residues 324 and 325 within the carboxy-terminal tail of CXCR4. These serine residues are critical for mediating agonist-promoted binding of AIP4 and subsequent ubiquitination and degradation of CXCR4. These residues are phosphorylated upon agonist activation and phosphomimetic mutants show enhanced binding to AIP4, suggesting a mechanism whereby phosphorylation mediates the interaction between CXCR4 and AIP4. Our data reveal a novel noncanonical WW domain-mediated interaction involving phosphorylated serine residues in the absence of any proline residues and suggest a novel mechanism whereby an E3 ubiquitin ligase is targeted directly to an activated G protein-coupled receptor.
authors.affiliation: Program in Molecular Biology, Stritch School of Medicine, Loyola University Chicago, Maywood, IL 60153, USA.
authors.forename: Deepali
authors.initials: D
authors.lastname: Bhandari
authors.forename: Seth L
authors.initials: SL
authors.lastname: Robia
authors.forename: Adriano
authors.initials: A
authors.lastname: Marchese
mesh_terms: D001665:Binding Sites; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D006801:Humans; D010766:Phosphorylation; D025941:Protein Interaction Mapping; D017434:Protein Structure, Tertiary; D019718:Receptors, CXCR4 / Q000737:chemistry / Q000378:metabolism*; D043562:Receptors, G-Protein-Coupled / Q000378:metabolism; D012097:Repressor Proteins / Q000032:analysis / Q000737:chemistry / Q000378:metabolism*; D044767:Ubiquitin-Protein Ligases / Q000032:analysis / Q000737:chemistry / Q000378:metabolism*
chemical_list: C527276:CXCR4 protein, human; D019718:Receptors, CXCR4; D043562:Receptors, G-Protein-Coupled; D012097:Repressor Proteins; C432966:ITCH protein, human; D044767:Ubiquitin-Protein Ligases