Contribution of kinases and the CD45 phosphatase to the generation of tyrosine phosphorylation patterns in the T-cell receptor complex ζ chain

The ζ subunit of the T-cell receptor complex plays a crucial role in coupling the antigen binding αβ and γδ heterodimers to the downstream activation pathways. Three tandem amino acid sequence motifs containing pairs of exactly spaced Tyr-X-X-Leu/Ile sequences, designated as Immunoreceptor Tyrosine-based Activation Motifs (ITAMs), control this function. The phosphorylated forms of ITAMs serve as docking sites for several src homology 2 (SH2) domain containing signaling proteins. The composition of the assembled signaling complex and the outcome of cell activation depends on the tyrosine phosphorylation pattern of the ζ polypeptide. The mechanism that conducts the generation of various phosphorylated forms has not yet been well established. In this study we have analyzed the ability of src family tyrosine kinases and the CD45 tyrosine phosphatase in determining the phosphorylation state of the different ITAMs and the individual tyrosine residues of the TCR ζ chain. The intracellular part of the ζ chain was phosphorylated by src family tyrosine kinases, p56(lck) and p59(fyn) in vitro. Synthetic oligopeptides representing full-length or half-sized ITAMs with a single tyrosine residue were also phosphorylated by both p56(lck) and p59(fyn). In contrast, an additional membrane proximal tyrosine residue in the human ζ chain, located outside of the ITAMs, was not phosphorylated. We also examined the activity of the CD45 phosphatase, using a panel of ITAM derivatives, in which one or both tyrosines were phosphorylated. The efficiency of ITAM dephosphorylation by CD45 was dependent on the primary sequence of the oligopeptides and the position of the phosphotyrosine residues. The in vitro data suggest that the CD45 phosphatase rather than the tyrosine kinase(s) may control the generation of specific phosphorylation patterns of the ζ chain during cell activation.