Regulation of Protein Kinase D by Multisite Phosphorylation

Activation of the serine/threonine kinase, protein kinase D (PKD/PKCμ) via a phorbol ester/PKC-dependent pathway involves phosphorylation events. The present study identifies five in vivophosphorylation sites by mass spectrometry, and the role of four of them was investigated by site-directed mutagenesis. Four sites are autophosphorylation sites, the first of which (Ser916) is located in the C terminus; its phosphorylation modifies the conformation of the kinase and influences duration of kinase activation but is not required for phorbol ester-mediated activation of PKD. The second autophosphorylation site (Ser203) lies in that region of the regulatory domain, which in PKCμ interacts with 14-3-3τ. The last two autophosphorylation sites (Ser744and Ser748) are located in the activation loop but are only phosphorylated in the isolated PKD-catalytic domain and not in the full-length PKD; they may affect enzyme catalysis but are not involved in the activation of wild-type PKD by phorbol ester. We also present evidence for proteolytic activation of PKD. The fifth site (Ser255) is transphosphorylated downstream of a PKC-dependent pathway after in vivo stimulation with phorbol ester. In vivo phorbol ester stimulation of an S255E mutant no longer requires PKC-mediated events. In conclusion, our results show that PKD is a multisite phosphorylated enzyme and suggest that its phosphorylation may be an intricate process that regulates its biological functions in very distinct ways.