Identification of a dominant epitope in human vaccinia-related kinase 1 (VRK1) and detection of different intracellular subpopulations.

The human VRK1 is a new ser-thr kinase expressed in many cell types. VRK1 is a regulator of p53 and other transcription factors related with cellular responses to stress. The human VRK1 protein has a dominant epitope located in its C-terminal region, between residues 333 and 396, which is detected by different antibodies. All the antibodies detect the same protein in immunoblots and immunoprecipitations. But the antibodies have a different reactivity when a single aminoacid substitution in T355, mimicking phosphorylation, is introduced next to the nuclear localization signal. These differences in reactivity permit the identification of different intracellular subpopulations. Most of the intracellular VRK1 protein is nuclear, but in some cells it is also detected in the cytosol, depending on the type of tissue. These different locations are detected by immunohistochemistry of human biopsies and immunofluorescence of cell lines. Some antibodies identify a subpopulation within the vesicular system, particularly in the Golgi apparatus. The different reactivity of the VRK1 protein indicates that this protein has a subcellular localization that can be regulated, thus adding an additional level of regulatory complexity to the VRK1 protein.