E2F1 transcription factor regulates the expression of genes involved in cell proliferation, differentiation, DNA repair and apoptosis. E2F1 proteins are accumulated at G1/S phase transition and are destructed in late S phase, and are tightly regulated by proteasome-dependent pathway. Because E2F1 degradation is caused by its ubiquitination by ubiquitin ligase $p45^{SKP2}$, the regulation of E2F1 ubiquitination is important for cell cycle progression. In this study, Isopeptidase T (IsoT) was found to interact with E2F1 in yeast two-hybrid system. IsoT is deubiquitinating enzyme (DUB), which belongs to ubiquitin specific protease (USP). IsoT is known to disassemble isopeptide bond of branched polyubiquitin chains. However, none of its functions in mammalian cells have not yet been clarified. The present study has shown that IsoT interacts with E2F1 in HeLa cells. N terminus of E2F1, which has p45$^{SKP2}$ binding site, was required for the interaction. The interaction between IsoT and E2F1 decreases E2F1 ubiquitination and stabilizes E2F1. Increased E2F1 protein levels induce the expression of target genes such as p107 and cyclin E which regulate G1/S phase transition. These results imply that IsoT regulates E2F1 stability and target genes expression through ubiquitin-mediated pathway.
