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

In human T lymphocytes a functional complex is formed between the protein tyrosine phosphatase CD45, the protein tyrosine kinase p56lck and a phosphoprotein, 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.