Analysis of protein-tyrosine phosphatases involved in positive regulation of Src family protein-tyrosine kinases

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Protein-tyrosine kinases play important roles as signal transducers in the intracellular pathway evoked by extracellular stimuli. Src family protein-tyrosine kinases which belong to non-receptor type of protein-tyrosine kinases are also believed to function as signal transducers but the molecular mechanisms of their functions are yet to be elucidated because it is completely unknown how the Src family kinases are activated when the cells are stimulated by extracellular signals. It has been shown that the activities of Src family protein-tyrosine kinascs are positively regulated by tyrosine dephosphorylation at a regulatory site located in the carboxyl terminal region. To clarify their activation mechanism, we have attempted to identify a tyrosine phosphatase (s) that specifically acts on the regulatory site of the Src family protein-tyrosine kinases. Consequently, 1) we have succeeded in cDNA cloning of three receptor type of tyrosine phosphatases which are highly concentrated in the nervous system, and found that one of them (termed as BEM-1) had potential to dephosphorylate the regulatory site of Src family protein-tyrosine kinases in vitro. Its in vivo function is now under investigation. 2) Using phosphorylated Src protein as a substrate, we have purified a tyrosine phosphatase and determined its partial amino acid sequence. The sequence revealed that the purified enzyme was identical to SH-PTP2, a cytosolic tyrosine phosphatase having two SH2 domains. Ectopic overexpression of SH-PTP2 into fibroblast cells augmented the activation of Src family protein-tyrosine kinases when the cells were detached from the substrate and suppressed the dephosphorylation of p130^<cas> protein, which is known as one of the best targets of Src family kinases in vivo. These results suggested that SH-PTP2 can serve as a positive regulator for Src family protein-tyrosine kinases in vivo.