T cell receptor (TCR) is a complex multi-protein cell surface receptor composed of the ligand-sensing TCR heterodimer and three signaling subunits. These signaling subunits contain cytoplasmic immunoreceptor tyrosine-based activation motifs (ITAMs) with a consensus sequence of YxxL/Ix6-12YxxL/I. The two tyrosines are phosphorylated by a Src kinase, Lck or Fyn. Binding of the dually phosphorylated ITAM peptide reorients the two SH2 domains relative to each other, destabilizing inhibitory interactions made by the linker connecting the SH2 and kinase domains [1].

1: Regulation of T cell receptor activation by dynamic membrane binding of the CD3epsilon cytoplasmic tyrosine-based motif. Xu C, Gagnon E, Call ME, Schnell JR, Schwieters CD, Carman CV, Chou JJ, Wucherpfennig KW; Cell. 2008;135:702-713. PMID:19013279