Evolution of the intracellular transport mechanisms in eukaryotes: ciliates and mammals use the same translocation and nuclear localization signals

In the ciliate E. raikovi, self/non-self recognition phenomena are controlled by cell type-specific, water-borne signal proteins (pheromones) by their binding to target cell-surface receptors. The downstream signal transduction pathway activated by the pheromone-receptor interactions of self type (that promote the vegetative, mitogenic cell growth) involves the phosphorylation of a nuclear protein kinase, designated Er-MAPK1, which is structurally similar to the "intestinal-cell kinase" and "male germ cell-associated kinase" described in mammals. To identify the sequence segments responsible for Er-MAPK1 nuclear localization, mouse fibroblasts were transfected with plasmids containing the reporter gene for the "Green-Fluorescent Protein" (GFP) associated to different fragments of the Er-MAPK1 coding sequence. By expressing GFP-tagged protein constructs in mammalian cells, in the C-terminal domain of Er-MAPK1 it was effectively possible to identify an Arg/Lys-rich motif that is required for the nuclear entry of GFP-fused constructs. These results provide evidence that distant related organisms such as ciliates and mammals use the same molecular language for the nuclear translocation and localization of proteins, thus suggesting that this language arose early in the evolution of the eukaryotic cell.