Cell cycle-dependent localization of the CDK2-cyclin E complex in Cajal (coiled) bodies.

We have found that CDK2 and cyclin E, but not cyclin A, accumulates within Cajal bodies (CBs) in a cell cycle-dependent fashion. In the absence of cyclin E, CDK2 is not enriched in the CB compartment, suggesting that the translocation of CDK2 to CBs is dependent on cyclin E. CDK2 and cyclin E could be recruited to CBs as a functional complex or CBs may serve as 'docking stations' for CDK2-cyclin E activation by CAKs during the G(1)/S transition. Notably, CDK7-cyclin H-Mat1 complexes are known to accumulate in CBs. Treatment of cells with inhibitors of either CDKs (olomoucine, 200 microM) or RNA polymerase I (actinomycin D, 0.05 microgram/ml), results in a striking reorganization of CDK2 and p80 coilin to the nucleolar periphery. Furthermore, we demonstrate that p80 coilin can be phosphorylated by purified CDK2-cyclin E complexes in vitro. Thus coilin and other CB proteins appear to be downstream targets of CDK2-cyclin E complex-mediated signaling pathways regulating cell cycle progression and controlling aspects of CB function. Possible roles for CDK2 and cyclin E in the well-documented association of CBs, histone gene clusters and RNA 3' end processing factors are discussed.