Protein kinase D2 (DKD2) can be activated in response to the peptide hormone gastrin. Blume et al. clarify how phosphorylation at multiple sites regulates PKD2’s actions in the cell. In gastric cancer cells that overexpress the gastrin receptors, PKD2 with a fluorescent tag was seen to move to the nucleus when cells were treated with gastrin. Protein kinase C η (PKCη) phosphorylated PKD2 on two sites that must be phosphorylated to activate kinase activity of PKD2, but overexpression of PKCη did not cause nuclear accumulation of PKD2. The authors therefore searched for another phosphorylation site and used mass spectrometry to identify Ser244 as a site phosphorylated in response to gastrin. Expression of mutant forms of PKD indicated that phosphorylation at the Ser244 site was critical for nuclear localization and that optimal nuclear localization also required catalytic activity of PKD2. Fluorescence loss in photobleaching studies showed that phosphorylation at Ser244 primarily prevented nuclear export of the kinase. Inhibitor studies identified casein kinase 1 (CK1) delta and epsilon isoforms as potential mediators of Ser244 phosphorylation, and the role of CK1 enzymes was confirmed in studies in which the enzymes were depleted with siRNA and others showing gastrin activation of the CKs. Other PKD enzymes regulate transcription by phosphorylating class IIa histone deacetylases, and the authors demonstrated interaction of PKD2 with HDAC7 that was enhanced in gastrin-treated cells. siRNA-mediated depletion of PKD2 showed that the kinase was required for nuclear export of HDAC7, which led to increased transcription of HDAC7 target genes. The authors thus conclude that regulation of gene expression through PKD2 is controlled by one kinase (PKCη) that controls catalytic activity of PKD2 and another (CK1δ) that controls its subcellular localization.
J. von Blume, U. Knippschild, F. Dequiedt, G. Giamas, A. Beck, A. Auer, J. Van Lint, G. Adler, T. Seufferlein, Phosphorylation at Ser244 by CK1 determines nuclear localization and substrate targeting of PKD2. EMBO J. 26, 4619-4633 (2007). [PubMed]