Overexpression of p21cip1 induces cell cycle arrest. Although this ability has been correlated with its nuclear localization, the evidence is not conclusive. The mutants that were used to inhibit its nuclear translocation could no longer bind to several proteins known to interact with the last 25 amino acids of p21cip1. Here we used point mutation analysis and fusion of the proteins to DsRed to identify which amino acids are essential for the nuclear localization of p21cip1. We conclude that amino acids RKR140–142 are essential for nuclear translocation of p21cip1. While wild-type DsRed-p21 induces cell cycle arrest in 95% of transfected cells, overexpression of cytoplasmatic p21AAA140–142 arrested only 20% of transfected cells. We conclude that cytoplasmatic p21, with no deletion in the C-terminal region, had a much lower capacity to arrest the cell cycle.
