Self-non-self discrimination is central to T cell-mediated immunity. The kinetic proofreading model can explain T cell antigen receptor (TCR) ligand discrimination; however, the rate-limiting steps have not been identified. Here, we show that tyrosine phosphorylation of the T cell adapter protein LAT at position Y132 is a critical kinetic bottleneck for ligand discrimination. LAT phosphorylation at Y132, mediated by the kinase ZAP-70, leads to the recruitment and activation of phospholipase C-γ1 (PLC-γ1), an important effector molecule for T cell activation. The slow phosphorylation of Y132, relative to other phosphosites on LAT, is governed by a preceding glycine residue (G131) but can be accelerated by substituting this glycine with aspartate or glutamate. Acceleration of Y132 phosphorylation increases the speed and magnitude of PLC-γ1 activation and enhances T cell sensitivity to weaker stimuli, including weak agonists and self-peptides. These observations suggest that the slow phosphorylation of Y132 acts as a proofreading step to facilitate T cell ligand discrimination.

Department of Immunology Duke University Medical Center Durham NC USA

Department of Molecular and Cell Biology University of California Berkeley Berkeley CA USA

Department of Molecular and Cell Biology University of California Berkeley Berkeley CA USA Department of Chemistry Columbia University New York NY USA

Department of Molecular and Cell Biology University of California Berkeley Berkeley CA USA Howard Hughes Medical Institute University of California Berkeley Berkeley CA USA

Division of Rheumatology Rosalind Russell and Ephraim P Engleman Arthritis Research Center Department of Medicine University of California San Francisco San Francisco CA USA

Division of Rheumatology Rosalind Russell and Ephraim P Engleman Arthritis Research Center Department of Medicine University of California San Francisco San Francisco CA USA Howard Hughes Medical Institute University of California San Francisco San Francisco CA USA

Harvard Stem Cell Institute Harvard University Cambridge MA USA Stem Cell Program and Division of Hematology Oncology Boston Children's Hospital and Dana Farber Cancer Institute

Institute of Molecular Genetics of the Czech Academy of Sciences Prague Czech Republic

Program in Immunology Harvard Medical School Boston MA USA

Program in Immunology Harvard Medical School Boston MA USA Howard Hughes Medical Institute Boston Children's Hospital and Harvard University Boston MA USA

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