Cell Cycle-dependent Subcellular Translocation of the Human DNA Licensing Inhibitor Geminin

Once per cell cycle replication is crucial for maintaining genome integrity. Geminin interacts with the licensing factor Cdt1 to prevent untimely replication and is controlled by APC/C-dependent cell cycle specific proteolysis during mitosis and in G₁. We show here that human geminin, when expressed in human cells in culture under a constitutive promoter, is excluded from the nucleus during part of the G₁ phase and at the transition from G₀ to G₁. The N-terminal 30 amino acids of geminin, which contain its destruction box, are essential for nuclear exclusion. In addition, 30 amino acids within the central domain of geminin are required for both nuclear exclusion and nuclear accumulation. Cdt1 overexpression targets geminin to the nucleus, while reducing Cdt1 levels by RNAi leads to the appearance of endogenous geminin in the cytoplasm. Our data propose a novel means of regulating the balance of Cdt1/geminin in human cells, at the level of the subcellular localization of geminin.