Protein kinase C‐δ interacts with and phosphorylates ARD1

Abstract Protein kinase C‐δ (PKCδ) is a diacylglycerol‐dependent, calcium‐independent novel PKC isoform that is engaged in various cell signaling pathways, such as cell proliferation, apoptosis, inflammation, and oxidative stress. In this study, we searched for proteins that bind PKCδ using a yeast two‐hybrid assay and identified murine arrest‐defective 1 (mARD1) as a binding partner. The interaction between PKCδ and mARD1 was confirmed by glutathione S ‐transferase pull‐down and co‐immunoprecipitation assays. Furthermore, recombinant PKCδ phosphorylated full‐length mARD1 protein. The NetPhos online prediction tool suggested PKCδ phosphorylates Ser 80 , Ser 108 , and Ser 114 residues of mARD1 with the highest probability. Based on these results, we synthesized peptides containing these sites and examined their phosphorylations using recombinant PKCδ. Autoradiography confirmed these sites were efficiently phosphorylated. Consequent mass spectrometry and peptide sequencing in combination with MALDI‐TOF MS/MS confirmed that Ser 80 and Ser 108 were major phosphorylation sites. The alanine mutations of Ser 80 and Ser 108 abolished the phosphorylation of mARD1 by PKCδ in 293T cells supporting these observations. In addition, kinase assays using various PKC isotypes showed that Ser 80 of ARD1 was phosphorylated by PKCβI and PKCζ isotypes with the highest selectivity, while Ser 108 and/or Ser 114 were phosphorylated by PKCγ with activities comparable to that of the PKCδ isoform. Overall, these results suggest the possibility that PKCδ transduces signals by regulating phosphorylation of ARD1.