Abstract
Various hypotheses have been proposed on the evolutionary origin of eukaryotic nucleus. Because one of the major cargoes in the nucleocytoplasmic export in the eukaryotic cell is the ribosome, its stimulating proteins called Ribosome Export Factors (REFs) might have an evolutionary history of inscribing the origin of eukaryotic nucleus. With the aim of understanding the evolutionary origin of the nucleus, here we employed the yeast REFs and searched for their evolutionary origin in more than 500 genomes of archaea and eubacteria by the PSI-BLAST search. Our results showed that the non-membranous REFs (non-mREFs) originated exclusively from eubacterial proteins, whereas the membranous REFs (mREFs) are from both archaeal and eubacterial proteins. Since the non-mREFs just work inside the nucleus while the mREFs shuttle between the nucleus and the cytoplasm, these results suggest that the extant REFs working inside the nucleus have derived exclusively from eubacterial proteins, implying that the nucleus arose in a cell that contained chromosomes possessing a substantial fraction of eubacterial genes, in line with the predictions of several models entailing endosymbiosis at eukaryote origins.