title: Interaction of CTLA-4 with the clathrin-associated protein AP50 results in ligand-independent endocytosis that limits cell surface expression.
abstract: CTLA-4 is a lymphocyte cell surface receptor expressed by activated T cells that functions to down-regulate T cell responses induced by TCR and CD28 stimulation. Since CTLA-4 competes with CD28 for binding to the common ligands B7-1 and B7-2, the level of CTLA-4 surface expression is likely to play an important role in its ability to inhibit CD28-dependent T cell activation. The factors that regulate these levels are poorly understood. Recent studies have revealed that following T cell activation, the majority of CTLA-4 is localized intracellularly rather than on the cell surface, and surface CTLA-4 is rapidly reinternalized. In this study, we investigate the molecular mechanism underlying the rapid clearance of CTLA-4 from the cell surface. The data demonstrate that cell surface CTLA-4 is endocytosed into clathrin-coated vesicles even in the absence of ligand. The targeting of CTLA-4 to clathrin-coated vesicles is mediated by the clathrin-associated adaptor complex AP-2. The cytoplasmic domain of CTLA-4 was found to specifically bind to AP50, the medium chain subunit of AP-2 in both yeast two-hybrid and coimmunoprecipitation assays. The interaction requires the peptide sequence 199-GVYVKM-204 in the cytoplasmic tail of CTLA-4. Mutation of the CTLA-4 amino acid residue Y201 abrogates the interaction with AP50, resulting in the accumulation of CTLA-4 at the cell surface. Together these data suggest that the interaction of CTLA-4 with AP50 plays an important role in regulating the cell surface expression of CTLA-4.
authors.affiliation: Gwen Knapp Center for Lupus and Immunology Research, Department of Molecular Genetics and Cell Biology, The University of Chicago, IL 60637, USA.
authors.forename: E
authors.initials: E
authors.lastname: Chuang
authors.forename: M L
authors.initials: ML
authors.lastname: Alegre
authors.forename: C S
authors.initials: CS
authors.lastname: Duckett
authors.forename: P J
authors.initials: PJ
authors.lastname: Noel
authors.forename: M G
authors.initials: MG
authors.lastname: Vander Heiden
authors.forename: C B
authors.initials: CB
authors.lastname: Thompson
mesh_terms: D000069594:Abatacept; D033962:Adaptor Protein Complex 2; D033983:Adaptor Protein Complex mu Subunits; D033942:Adaptor Proteins, Vesicular Transport; D000595:Amino Acid Sequence; D015703:Antigens, CD; D000943:Antigens, Differentiation / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D060908:CTLA-4 Antigen; D004705:Endocytosis / Q000276:immunology*; D006801:Humans; D018796:Immunoconjugates; D019169:Jurkat Cells; D008213:Lymphocyte Activation / Q000276:immunology*; D008969:Molecular Sequence Data; D009154:Mutation; D009419:Nerve Tissue Proteins / Q000276:immunology* / Q000378:metabolism; D010750:Phosphoproteins / Q000276:immunology* / Q000378:metabolism; D011940:Receptor Aggregation; D015398:Signal Transduction / Q000276:immunology*; D013601:T-Lymphocytes / Q000276:immunology*
chemical_list: D033962:Adaptor Protein Complex 2; D033983:Adaptor Protein Complex mu Subunits; D033942:Adaptor Proteins, Vesicular Transport; D015703:Antigens, CD; D000943:Antigens, Differentiation; D060908:CTLA-4 Antigen; C556706:CTLA4 protein, human; D018796:Immunoconjugates; D009419:Nerve Tissue Proteins; D010750:Phosphoproteins; C438885:adaptor protein complex 2, mu 1 subunit; D000069594:Abatacept