Phosphorylation and activation of human tyrosine hydroxylase in vitro by mitogen‐activated protein (MAP) kinase and MAP‐kinase‐activated kinases 1 and 2

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Abstract
Mitogen‐activated protein‐kinase (MAP) kinase‐activated protein kinases 1 and 2 (MAPKAP kinase‐1, MAPKAP kinase‐2), were found to phosphorylate bacterially expressed human tyrosine hydroxylase in vitro at comparable rates to other proteins thought to be physiological substrates of these protein kinases. The phosphorylation of all four alternatively spliced forms of human tyrosine hydroxylase by MAPKAP kinases‐1 and ‐2 reached plateau values at 1 mol/mol subunit and 2 mol/mol subunit, respectively; the sites of phosphorylation were identified as Ser40 (MAPKAP kinase‐1) and Ser19 and Ser40 (MAPKAP kinase‐2). In contrast to calmodulin‐dependent protein kinase‐II, which phosphorylates Ser19 faster than Ser40, MAPKAP kinase‐2 phosphorylated Ser40 about twice as fast as Ser19. The maximal activation of tyrosine hydroxylase by MAPKAP kinase‐1 or‐2 was about 3‐fold, and activation by MAPKAP kinases‐1 and ‐2 or calmodulin‐dependent protein kinase‐II correlated with the extent of phosphorylation of Ser40. The four alternatively spliced forms of human tyrosine hydroxylase were phosphorylated at Ser31 by MAP kinase, but at markedly different rates (3=4 > 1 » 2). Forms 3 and 4 were phosphorylated rapidly and stoichiometrically by MAP kinase doubling the activity, while phosphorylation of form 1 by MAP kinase to 0.4 mol/mol subunit increased activity by 40%. The effect on activity of phosphorylating both Ser31 and Ser40 was not additive. The possible roles of MAPKAP kinase‐1, MAPKAP kinase‐2 and MAP kinase in the regulation of tyrosine hydroxylase in vivo are discussed.

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