Coclustering CD45 with CD4 or CD8 alters the phosphorylation and kinase activity of p56lck.

Antibody-mediated CD4 crosslinking results in increased tyrosine phosphorylation and tyrosine kinase activity of the associated p56lck. Treatment with anti-CD4 and anti-Ig also induced the phosphorylation of p56lck in a CD45- mutant cell line, indicating that the increase in phosphotyrosine content of p56lck is not the result of being sequestered from CD45 protein tyrosine phosphatase (PTPase). Antibody-mediated coclustering of CD45 with CD4 inhibited the anti-CD4-induced phosphorylation of p56lck on tyrosine and the concomitant increase in in vitro kinase activity. Similar results were obtained when CD45 was coclustered with CD8 on cytotoxic T cell lines. These observations provide strong evidence that p56lck is a substrate for CD45 in vivo and provide an assay to study the regulation and specificity of CD45 PTPase activity.