Protein transport into the nucleus: characterization of nuclear localization signals in the protozoan ciliate Euplotes

In E. raikovi, a nuclear protein kinase, designated Er-MAPK1, appears to be phosphorylated in association with the mechanism of signal transduction which promotes cell proliferation through autocrine interactions between cell type-specific signaling protein pheromones and their membrane receptors. This kinase shows significant structural matching to mammalian kinases that are localized in the nucleus of specialized cell types, such as the "Male germ cell-Associated Kinases" and the "Intestinal Cell Kinases". Two Arg/Lys-rich motifs were identified in the Er-MAPK1 C-terminal domain as putative “nuclear localization signals” and their effective function in directing this protein into the nucleus was studied by expressing GFP-tagged protein constructs in mammalian fibroblasts. The results obtained provide evidence that distant related organisms such as ciliates and mammals use the same molecular language for the nuclear translocation and localization of proteins, suggesting that this language arose early in the evolution of the eukaryotic cell.