A functional complex is formed in human T lymphocytes between the protein tyrosine phosphatase CD45, the protein tyrosine kinase p56lck and pp32, a possible common substrate.

In vitro protein kinase assays of CD45 immunoprecipitates prepared from digitonin lysates of resting human T lymphocytes resulted in exclusive tyrosine phosphorylation of a 32-kDa protein (pp32). Reprecipitation of the in vitro phosphorylated proteins and Western blot analysis of whole CD45 immunoprecipitates employing antisera specifically directed at different protein tyrosine kinases demonstrated that the p56lck protein tyrosine kinase was responsible for in vitro phosphorylation of pp32. Since in vitro kinase assays of p56lck immunoprecipitates also resulted in phosphorylation of pp32, the present data strongly suggest that a functional complex is formed between CD45, p56lck and pp32. Such a notion is supported by the findings that phosphorylation of pp32 by p56lck correlated with expression of the CD45 molecules and that in vitro phosphorylated pp32 was completely dephosphorylated by purified CD45.