Apoptin interacts with and regulates the activity of protein kinase C beta in cancer cells.

Apoptin, the VP3 protein from chicken anaemia virus (CAV), induces tumour cell-specific cell death and represents a potential future anti-cancer therapeutic. In tumour but not in normal cells, Apoptin is phosphorylated and translocates to the nucleus, enabling its cytotoxic activity. Recently, the β isozyme of protein kinase C (PKCβ) was shown to phosphorylate Apoptin in multiple myeloma cell lines. However, the exact mechanism and nature of interaction between PKCβ and Apoptin remain unclear. Here we investigated the physical and functional link between PKCβ and CAV-Apoptin as well as with the recently identified Apoptin homologue derived from human Gyrovirus (HGyV). In contrast to HCT116 colorectal cancer cells the normal colon mucosa cell lines expressed low levels of PKCβI and showed reduced Apoptin activation, as evident by cytoplasmic localisation, decreased phosphorylation and lack of cytotoxic activity. Co-immunoprecipitation and proximity ligation assay studies identified binding of both CAV- and HGyV-Apoptin to PKCβI in HCT116 cells. Using Apoptin deletion constructs the N-terminal domain of Apoptin was found to be required for interacting with PKCβI. FRET-based PKC activity reporter assays by fluorescence lifetime imaging microscopy showed that expression of Apoptin in cancer cells but not in normal cells triggers a significant increase in PKC activity. Collectively, the results demonstrate a novel cancer specific interplay between Apoptin and PKCβI. Direct interaction between the two proteins leads to Apoptin-induced activation of PKC and consequently activated PKCβI mediates phosphorylation of Apoptin to promote its tumour-specific nuclear translocation and cytotoxic function.