Human ARF protein interacts with Topoisomerase I and stimulates its activity

The ARF gene (p19(ARF) in mouse and p14(ARF) in man) has become a central actor of the cell cycle regulation process as it participates to the ARF-MDM2-p53 pathway and the Rh-E2F-1 pathway. By use of immunoprecipitation and Western blotting (IP/WB), we now show that ARF physically associates with Topoisomerase I(Topo I), ARF-Topo I immune complexes were detected in SF9 insect cells infected with recombinant baculoviruses encoding the two genes as well as in 293 cells that express endogeneously these proteins. Preparations of a GST-ARF recombinant protein stimulated the DNA relaxation activity of Topo I but, in contrast, had no effect an the decatenation activity of Topo TT, The Topo I stimulation was also detected in cell extracts of SF9 cells expressing both proteins, A confocal microscopy study indicated that part of ARF and Topo I colocalized in the granular component structure of the nucleolus. As a whole, our data indicate that Topo I is a new partner of ARF and suggest that ARF is involved in cell reactions that require Topo I.