title: Evidence that a kinase distinct from protein kinase C and phosphatidylinositol 3-kinase mediates ligation-dependent serine/threonine phosphorylation of the T-lymphocyte co-stimulatory molecule CD28.
abstract: The CD28 cytoplasmic tail contains several potential phosphorylation sites for the serine/threonine kinase protein kinase C (PKC) and/or proline-directed serine/threonine kinases, such as extracellular signal-regulated kinases. We demonstrate that ligation of CD28 by B7.1 results in strong serine/threonine phosphorylation of CD28. It is unlikely that ligation-stimulated phosphorylation of CD28 is mediated via activation of PKC, since it was not prevented by pre-treatment of Jurkat cells with inhibitors of PKC, and it was not mimicked by treatment with PKC activators such as PMA. Nevertheless, despite for lack of detectable effects of PMA treatment on CD28 phosphorylation, PMA did partially inhibit the association of CD28 with the putative signalling molecule phosphatidylinositol 3-kinase (PI 3-kinase) and the subsequent accumulation of PtdIns(3,4,5)P3. PI 3-kinase exhibits dual specificity as both a lipid kinase and a protein serine kinase, and site-specific mutagenesis of the Tyr173 residue in the CD28 cytoplasmic tail, which abolishes CD28 coupling to PI 3-kinase [Pages, Ragueneau, Rottapel, Truneh, Nunes, Imbert and Olive (1994) Nature (London) 369, 327-329], also prevents ligation-stimulated phosphorylation of CD28. However, the two PI 3-kinase inhibitors wortmannin and LY294002 had no effect on phosphorylation of CD28 after ligation by B7.1. This study therefore demonstrates that (1) a CD28-activated serine/threonine kinase distinct from both PKC and PI 3-kinase mediates ligation-stimulated CD28 phosphorylation, and (2) the PMA-stimulated down-regulation of the coupling of CD28 to PI 3-kinase is not due to PMA-stimulated phosphorylation of CD28.
authors.affiliation: Pharmacology Group, School of Pharmacy and Pharmacology, Bath University, U.K.
authors.forename: R V
authors.initials: RV
authors.lastname: Parry
authors.forename: D
authors.initials: D
authors.lastname: Olive
authors.forename: J
authors.initials: J
authors.lastname: Westwick
authors.forename: D M
authors.initials: DM
authors.lastname: Sansom
authors.forename: S G
authors.initials: SG
authors.lastname: Ward
mesh_terms: D018122:B7-1 Antigen / Q000378:metabolism / Q000502:physiology; D018106:CD28 Antigens / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D004789:Enzyme Activation / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D019169:Jurkat Cells; D008024:Ligands; D016297:Mutagenesis, Site-Directed; D019869:Phosphatidylinositol 3-Kinases / Q000502:physiology*; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010766:Phosphorylation / Q000187:drug effects; D010768:Phosphoserine / Q000378:metabolism; D010769:Phosphothreonine / Q000378:metabolism; D019000:Phosphotyrosine / Q000235:genetics; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000502:physiology*; D017346:Protein Serine-Threonine Kinases / Q000502:physiology*; D011505:Protein-Tyrosine Kinases / Q000502:physiology*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
chemical_list: D018122:B7-1 Antigen; D018106:CD28 Antigens; D004791:Enzyme Inhibitors; D008024:Ligands; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010768:Phosphoserine; D010769:Phosphothreonine; D019000:Phosphotyrosine; D011493:Protein Kinase C; D017346:Protein Serine-Threonine Kinases; D011505:Protein-Tyrosine Kinases; D013755:Tetradecanoylphorbol Acetate