Human T-cell leukemia virus type 1 Tax oncoprotein prevents DNA damage-induced chromatin egress of hyperphosphorylated Chk2.

De novo expression of human T-cell leukemia virus type 1 Tax results in cellular genomic instability. We demonstrated previously that Tax associates with the cell cycle check point regulator Chk2 and proposed that the inappropriate activation of Chk2 provides a model for Tax-induced loss of genetic integrity (Haoudi, A., Daniels, R. C., Wong, E., Kupfer, G., and Semmes, O. J. (2003) J. Biol. Chem. 278, 37736-37744). Here we provide an explanation for how Tax induces some Chk2 activities but represses others. We show that Tax interaction with Chk2 generates two activation signals in Chk2, oligomerization and autophosphorylation. However, egress of Chk2 from chromatin, normally observed in response to ionizing radiation, was repressed in Tax-expressing cells. Analysis of chromatin-bound Chk2 from Tax-expressing cells revealed phosphorylation at Thr(378), Ser(379), Thr(383), Thr(387), and Thr(389). In contrast, chromatin-bound Chk2 in the absence of Tax was phosphorylated at Thr(383) and Thr(387) in response to ionizing radiation. We further establish that Tax binds to the kinase domain of Chk2. Confocal microscopy revealed a redistribution of Chk2 to colocalize with Tax in Tax speckled structures, which we have shown previously to coincide with interchromatin granules. We propose that Tax binding via the Chk2 kinase domain sequesters phosphorylated Chk2 within chromatin, thus hindering chromatin egress and appropriate response to DNA damage.