MUTANT FORMS OF THE PROTEIN-TYROSINE-PHOSPHATASE-ALPHA SHOW DIFFERENTIAL ACTIVITIES TOWARDS INTRACELLULAR SUBSTRATES

BHK cells overexpressing five million IR (BHK-IR) respond to insulin w ith reduced growth and detachment from the dish surface, We have recen tly identified protein tyrosine phosphatase (PTP) alpha as a negative regulator of the insulin receptor (IR) tyrosine kinase that is able to rescue BHK-IR cells from the insulin effect. In this report we descri be the effect of several point mutations in PTP alpha on the phosphata se activity and regulation of insulin signaling in BHK-IR cells. Analy sis of total cellular phosphotyrosine protein revealed several molecul es that were dephosphorylated when PTP alpha or a phosphatase active m utant was overexpressed, By contrast, some proteins were tyrosine phos phorylated as strong or to an even higher extent as in the parental li ne when PTP alpha Y798F was present. We conclude that mutation of the carboxyterminal tyrosine in PTP alpha uncovers a dual function of this phosphatase in BHK cells: reduction of the IR signal and activation o f an endogenous kinase. (C) 1998 Academic Press.