Stimulation of mature T cells activates a downstream signaling cascade involving temporally
and spatially regulated phosphorylation and dephosphorylation events mediated by protein-
tyrosine kinases and phosphatases, respectively. PTPN22 (Lyp), a non-receptor protein-
tyrosine phosphatase, is expressed exclusively in cells of hematopoietic origin, notably in T
cells where it represses signaling through the T cell receptor. We used substrate trapping
coupled with mass spectrometry-based peptide identification in an unbiased approach to …