Functional interactions between herpesvirus oncoprotein MEQ and cell cycleregulator CDK2

Marek's disease virus, an avian alphaherpesvirus, has been used as an excel lent model to study herpesvirus oncogenesis, One of its potential oncogenes , MEQ, has been demonstrated to transform a rodent fibroblast cell line, Ra t-2, in vitro by inducing morphological transformation and anchorage- and s erum-independent growth and by protecting cells from apoptosis induced by t umor necrosis factor alpha, CZ-ceramide, UV irradiation, or serum deprivati on. In this report, we show that there is a cell cycle-dependent colocaliza tion of MEQ protein and cyclin-dependent kinase 2 (CDK2) in coiled bodies a nd the nucleolar periphery during the G(1)/S boundary and early S phase. To our knowledge, this is the first demonstration that CDK2 is found to local ize to coiled bodies. Such an in vivo association and possibly subsequent p hosphorylation may result in the cytoplasmic translocation of MEQ protein. Indeed, MEQ is expressed in both the nucleus and the cytoplasm during the G (1)/S boundary and early S phase. In addition, we were able to show in vitr o phosphorylation of MEQ by CDKs. We have mapped the CDK phosphorylation si te of MEQ to be serine 42, a residue in the proximity of the bZIP domain. A n indirect-immunofluorescence study of the MEQ S42D mutant, in which the CD K; phosphorylation site was mutated to a charged residue, reveals more prom inent cytoplasmic localization. This lends further support to the notion th at the translocation of MEQ is regulated by phosphorylation, Furthermore, p hosphorylation of MEQ by CDKs drastically reduces the DNA binding activity of MEQ, which may in part account for the lack of retention of MEQ oncoprot ein in the nucleus. Interestingly, the localization of CDK2 in coiled bodie s and the nucleolar periphery is observed only in MEQ-transformed Rat-2 cel ls, implicating MEQ in modifying the subcellular localization of CDK2. Take n together, our data suggest that there is a novel reciprocal modulation be tween the herpesvirus oncoprotein MEQ and CDK2.