title: Structure-Function analysis of the CTLA-4 interaction with PP2A.
abstract: BACKGROUND
  CTLA-4 functions primarily as an inhibitor of T cell activation. There are several candidate explanations as to how CTLA-4 modulates T cell responses, but the exact mechanism remains undefined. The tail of CTLA-4 does not have any intrinsic enzymatic activity but is able to associate with several signaling molecules including the serine/threonine phosphatase PP2A. PP2A is a heterotrimeric molecule comprised of a regulatory B subunit associated with a core dimer of a scaffolding (A) and a catalytic (C) subunit.
  
  
  RESULTS
  Here, we performed an analysis of the human CTLA-4 interface interacting with PP2A. We show that PP2A interacts with the cytoplasmic tail of CTLA-4 in two different sites, one on the lysine rich motif, and the other on the tyrosine residue located at position 182 (but not the tyrosine 165 of the YVKM motif). Although the interaction between CTLA-4 and PP2A was not required for inhibition of T cell responses, it was important for T cell activation by inverse agonists of CTLA-4. Such an interaction was functionally relevant because the inverse agonists induced IL-2 production in an okadaic acid-dependent manner.
  
  
  CONCLUSIONS
  Our studies demonstrate that PP2A interacts with the cytoplasmic tail of human CTLA-4 through two motifs, the lysine rich motif centered at lysine 155 and the tyrosine residue 182. This interaction and the phosphatase activity of PP2A are important for CTLA-4-mediated T cell activation.
authors.affiliation: FOCIS Centre for Clinical Immunology and Immunotherapeutics, Robarts Research Institute, The University of Western Ontario, London, Ontario, Canada. wateft@uwo.ca
authors.forename: Wendy A
authors.initials: WA
authors.lastname: Teft
authors.forename: Thu A
authors.initials: TA
authors.lastname: Chau
authors.forename: Joaquín
authors.initials: J
authors.lastname: Madrenas
mesh_terms: D015703:Antigens, CD / Q000737:chemistry* / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D060908:CTLA-4 Antigen; D004789:Enzyme Activation / Q000187:drug effects / Q000276:immunology; D006801:Humans; D019169:Jurkat Cells; D008213:Lymphocyte Activation / Q000187:drug effects; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D019319:Okadaic Acid / Q000494:pharmacology; D011485:Protein Binding / Q000187:drug effects / Q000276:immunology; D054730:Protein Interaction Domains and Motifs; D054648:Protein Phosphatase 2 / Q000737:chemistry* / Q000276:immunology / Q000378:metabolism*; D017434:Protein Structure, Tertiary / Q000235:genetics; D013329:Structure-Activity Relationship; D013601:T-Lymphocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D019076:Transgenes
chemical_list: D015703:Antigens, CD; D060908:CTLA-4 Antigen; C556706:CTLA4 protein, human; D019319:Okadaic Acid; C517189:PPP2CA protein, human; D054648:Protein Phosphatase 2