PML enhances the regulation of p53 by CK1 in response to DNA damage

A central role for CK1 in catalyzing phosphorylation of the p53 transactivation domain at serine 20 after HHV-6B viral infection

The tumor suppressor protein p53 is activated by distinct cellular stresses including radiation, hypoxia, type I interferon, and DNA/RNA virus infection. The transactivation domain of p53 contains a phosphorylation site at Ser20 whose modification stabilizes the binding of the transcriptional co-activator p300 and whose mutation in murine transgenics induces B-cell lymphoma. Although the checkpoint kinase CHK2 is implicated in promoting Ser20 site phosphorylation after irradiation, the enzyme that triggers this phosphorylation after DNA viral infection is undefined. Using human herpesvirus 6B (HHV-6B) as a virus that induces Ser20 site phosphorylation of p53 in T-cells, we sought to identify the kinase responsible for this virus-induced p53 modification. The p53 Ser20 kinase was fractionated and purified using cation, anion, and dye-ligand exchange chromatography. Mass spectrometry identified casein kinase 1 (CK1) and vaccinia-related kinase 1 (VRK1) as enzymes that coeluted with virus-induced Ser20 site kinase activity. Immunodepletion of CK1 but not VRK1 removed the kinase activity from the peak fraction, and bacterially expressed CK1 exhibited Ser20 site kinase activity equivalent to that of the virus-induced native CK1. CK1 modified p53 in a docking-dependent manner, which is similar to other known Ser20 site p53 kinases. Low levels of the CK1 inhibitor D4476 selectively inhibited HHV-6B-induced Ser20 site phosphorylation of p53. However, x-ray-induced Ser20 site phosphorylation of p53 was not blocked by D4476. These data highlight a central role for CK1 as the Ser20 site kinase for p53 in DNA virus-infected cells but also suggest that distinct stresses may selectively trigger different protein kinases to modify the transactivation domain of p53 at Ser20.

Role of nuclear bodies in apoptosis signalling

Promyelocytic leukemia nuclear bodies (PML NBs) are dynamic macromolecular multiprotein complexes that recruit and release a plethora of proteins. A considerable number of PML NB components play vital roles in apoptosis, senescence regulation and tumour suppression. The molecular basis by which PML NBs control these cellular responses is still just beginning to be understood. In addition to PML itself, numerous further tumour suppressors including transcriptional regulator p53, acetyl transferase CBP (CREB binding protein) and protein kinase HIPK2 (homeodomain interacting protein kinase 2) are recruited to PML NBs in response to genotoxic stress or oncogenic transformation and drive the senescence and apoptosis response by regulating p53 activity. Moreover, in response to death-receptor activation, PML NBs may act as nuclear depots that release apoptotic factors, such as the FLASH (FLICE-associated huge) protein, to amplify the death signal. PML NBs are also associated with other nuclear domains including Cajal bodies and nucleoli and share apoptotic regulators with these domains, implying crosstalk between NBs in apoptosis regulation. In conclusion, PML NBs appear to regulate cell death decisions through different, pathway-specific molecular mechanisms.