A search for c-Abl interacting proteins resulted in the recovery of PSTPIP1, originally identified as a binding protein of the PEST-type protein tyrosine phosphatases (PTP). PSTPIP1 was phosphorylated by c-Abl, and growth factor-induced PSTPIP1 phosphorylation was diminished in Abl null fibroblasts. PSTPIP1 was able to bridge c-Abl to the PEST-type PTPs. Several experiments suggest that the PEST-type PTPs negatively regulate c-Abl activity: c-Abl was hyperphosphorylated in PTP-PEST-deficient cells; disruption of the c-Abl-PSTPIP1-PEST-type PTP ternary complex by overexpression of PSTPIP1 mutants increased c-Abl phosphotyrosine content; and PDGF-induced c-Abl kinase activation was prolonged in PTP-PEST-deficient cells. Dephosphorylation of c-Abl by PEST-type PTP represents a novel mechanism by which c-Abl activity is regulated.
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