The subcellular localization of vaccinia‐related kinase‐2 (VRK2) isoforms determines their different effect on p53 stability in tumour cell lines

Abstract

VRK is a new kinase family of unknown function. Endogenous human vacinia‐related kinase 2 (VRK2) protein is present in both the nucleus and the cytosol, which is a consequence of alternative splicing of two VRK2 messages coding for proteins of 508 and 397 amino acids, respectively. VRK2A has a C‐terminal hydrophobic region that anchors the protein to membranes in the endoplasmic reticulum (ER) and mitochondria, and it colocalizes with calreticulin, calnexin and mitotracker; whereas VRK2B is detected in both the cytoplasm and the nucleus. VRK2A is expressed in all cell types, whereas VRK2B is expressed in cell lines in which VRK1 is cytoplasmic. Both VRK2 isoforms have an identical catalytic N‐terminal domain and phosphorylate p53 in vitro uniquely in Thr18. Phosphorylation of the p53 protein in response to cellular stresses results in its stabilization by modulating its binding to other proteins. However, p53 phosphorylation also occurs in the absence of stress. Only overexpression of the nuclear VRK2B isoform induces p53 stabilization by post‐translational modification, largely due to Thr18 phosphorylation. VRK2B may play a role in controlling the binding specificity of the N‐terminal transactivation domain of p53. Indeed, the p53 phosphorylated by VRK2B shows a reduction in ubiquitination by Mdm2 and an increase in acetylation by p300. Endogenous p53 is also phosphorylated in Thr18 by VRK2B, promoting its stabilization and transcriptional activation in A549 cells. The relative phosphorylation of Thr18 by VRK2B is similar in magnitude to that induced by taxol, which might use a different signalling pathway. In this context, VRK2B kinase might functionally replace nuclear VRK1. Therefore, these kinases might be components of a new signalling pathway that is likely to play a role in normal cell proliferation.

Plk3 Interacts with and Specifically Phosphorylates VRK1 in Ser342, a Downstream Target in a Pathway That Induces Golgi Fragmentation

“…2D), although their appearance was slightly different from that in HEK293T cells. Endogenous VRK1 is mostly located within the nucleus (24,40), but in some cell types a subpopulation is also detected in the cytosol (7,37) or in cytosolic vesicles and the Golgi apparatus (38). These data suggested that at least a subpopulation of endogenous VRK1 and Plk3 proteins might form an intracellular particulate complex.…”

VRK2 (vaccinia related kinase 2)

“…The subcellular localization of protein codified from this gene is dependent on two alternatively spliced variants (Blanco et al, 2006). Other alternative splicing messages have been detected but no proteins have been identified expressed from these alternative messages in humans.…”

“…VRK2 protein has two isoforms: VRK2A isoform is a 508 amino acid 58.141 KDa protein and VRK2B isoform, a 397 amino acid 45.030 KDa protein (Blanco et al, 2006). Both isoforms present a serine-threonine kinase domain (residues 29-266) with an ATP-binding site (residues 36-61) and three active kinase motifs (residues 164-168, 184-188 and 218-220).…”

“…VRK2B is expressed only in some cell lines (Blanco et al, 2006;Fernandez et al, 2010). VRK2A expression positively correlated with estrogen and progesterone receptors and inversely with ErbB2 in breast carcinomas (Fernandez et al, 2010).…”

“…VRK2B lacks this region and it is a soluble protein localized in cytosol and nucleus (Blanco et al, 2006).…”