AUTOPHOSPHORYLATION-ACTIVATED PROTEIN-KINASE INACTIVATES THE PROTEIN-TYROSINE-PHOSPHATASE ACTIVITY OF PROTEIN PHOSPHATASE 2A

Phosphorylation of the catalytic subunit of protein phosphatase 2A (PP2A) on threonines with a distinct autophosphorylation-activated protein kinase [Guo and Damuni (1993) Proc, Natl. Acad. Sci. USA 90, 2500-2504] inactivated the phosphatase with P-32-labelled myelin basic protein prepared by incubation with the kinase domain of the epidermal growth factor receptor, the src-family protein kinases p56(lck) and p60(c-src), myelin basic protein kinase-1, or protamine kinase. Phosphoamino acid analysis demonstrated that the kinase domain of the epidermal growth factor receptor, p56(lck) and p60(c-src) phosphorylated myelin basic protein on tyrosines, that the protamine kinase phosphorylated myelin basic protein on serines, and that myelin basic protein kinase-1 phosphorylated myelin basic protein on threonines. The results demonstrate that the autophosphorylation-activated protein kinase not only inactivates the protein serine/threonine phosphatase, but also the protein tyrosine phosphatase activity of PP2A. This autophosphorylation-activated protein kinase-mediated inactivation of PP2A may, in response to extracellular stimuli, not only contribute to the enhanced phosphorylation of cellular proteins on serines and threonines but also on tyrosines.