The molecular mechanism of translocation through the nuclear pore complex is highly conserved

In this report we investigated the activity of vertebrate nuclear transport factors in a primitive organism, Amoeba proteus, to better understand evolutionary changes in the transport mechanisms of organisms expected to have different requirements for nucleocytoplasmic exchange. It was initially determined that FxFG-containing nucleoporins and Ran, both of which are essential for nuclear import in vertebrates, as well as yeast, are also present and functional in amoebae. This suggests that there are fundamental similarities in the transport process; however, there are also significant differences. Transport substrates containing either the hnRNP A1 M9 shuttling signal (a GST/GFP/M9 fusion protein) or the classical bipartite NLS (colloidal gold coated with BSA-bipartite NLS conjugates), both of which are effectively transported in vertebrate cells, are excluded from the nucleus when microinjected into amoebae. However, when these substrates are injected along with transportin or importin alpha/beta, respectively, the vertebrate receptors for these signals, they readily accumulate in the nucleoplasm. These results indicate that although the molecular recognition of substrates is not well conserved between vertebrates and amoebae, vertebrate transport receptors are functional in A. proteus, showing that the translocation machinery is highly conserved. Since selected nuclear import pathways can be investigated in the absence of competing endogenous transport, A. proteus might provide a useful in vivo system for investigating specific molecular interactions involved in trafficking.