The catalytic activity of the CD45 membrane‐proximal phosphatase domain is required for TCR signaling and regulation.

Cell surface expression of CD45, a receptor‐like protein tyrosine phosphatase (PTPase), is required for T cell antigen receptor (TCR)‐mediated signal transduction. Like the majority of transmembrane PTPases, CD45 contains two cytoplasmic phosphatase domains, whose relative in vivo function is not known. Site‐directed mutagenesis of the individual catalytic residues of the two CD45 phosphatase domains indicates that the catalytic activity of the membrane‐proximal domain is both necessary and sufficient for restoration of TCR signal transduction in a CD45‐deficient cell. The putative catalytic activity of the distal phosphatase domain is not required for proximal TCR‐mediated signaling events. Moreover, in the context of a chimeric PTPase receptor, the putative catalytic activity of the distal phosphatase domain is not required for ligand‐induced negative regulation of PTPase function. We also demonstrate that the phosphorylation of the C‐terminal tyrosine of Lck, a site of negative regulation, is reduced only when CD45 mutants with demonstrable in vitro phosphatase activity are introduced into the CD45‐deficient cells. These results demonstrate that the phosphatase activity of CD45 is critical for TCR signaling, and for regulating the levels of C‐terminal phosphorylated Lck molecules.