Four CD45/P56lck‐associated phosphorproteins (pp29‐pp32) undergo alterations in human T cell activation

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Abstract
In human T lymphocytes a functional complex is formed between the protein tyrosine phosphatase CD45, the protein tyrosine kinase p56lck and a phospho‐protein, pp32, a possible common substrate. Here we demonstrate that the previously described pp32 protein is composed of two distinct molecules (pp29 and pp32) in both resting human T lymphocytes and continuously proliferating T lymphoma lines. Importantly, T lymphocyte activation employing CD2 monoclonal antibodies (mAb), CD3 mAb or phorbol 12, 13 dibutyrate results in loss of pp29 and pp32 from the CD45/p56lck molecular complex and concomitant association of two distinct phosphoproteins with different molecular weights (pp30 and pp31). These events appear to be unrelated to clonal T cell growth but rather depend on receptor‐mediated differentiation signals. Reprecipitation experiments employing an antiserum directed at a consensus sequence of GTP‐binding proteins suggest that all four pp29‐pp32 molecules might represent proteins with GTP‐binding properties. Biochemical analysis of pp29‐pp32 employing V8‐protease digestion indicates that they differ in low‐molecular weight fragments of 8, 5, 4.5, 4 and 3 kDa, respectively.