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Unformatted text preview: The origin of the eukaryotic cell: A genomic investigation Hyman Hartman and Alexei Fedorov Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139; and Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138 Communicated by Carl R. Woese, University of Illinois at UrbanaChampaign, Urbana, IL, December 10, 2001 (received for review October 25, 2001) We have collected a set of 347 proteins that are found in eukaryotic cells but have no significant homology to proteins in Archaea and Bacteria. We call these proteins eukaryotic signature proteins (ESPs). The dominant hypothesis for the formation of the eukaryotic cell is that it is a fusion of an archaeon with a bacterium. If this hypothesis is accepted then the three cellular domains, Eukarya, Archaea, and Bacteria, would collapse into two cellular domains. We have used the existence of this set of ESPs to test this hypothesis. The evidence of the ESPs implicates a third cell (chronocyte) in the formation of the eukaryotic cell. The chronocyte had a cytoskeleton that enabled it to engulf prokaryotic cells and a complex internal membrane system where lipids and proteins were synthesized. It also had a complex internal signaling system involving calcium ions, calmodulin, inositol phosphates, ubiquitin, cyclin, and GTP-binding proteins. The nucleus was formed when a number of archaea and bacteria were engulfed by a chronocyte. This formation of the nucleus would restore the three cellular domains as the Chronocyte was not a cell that belonged to the Archaea or to the Bacteria. R ecently, Horiike et al. (1) proposed that the eukaryotic nucleus was derived from the symbiosis of Archaea in Bacteria. Their results were based on a search for homologies between proteins found in the yeast genome and those found in the genomes of Archaea and Bacteria. However, the use of homologies to determine relationships between the three main cellular domains is misleading because of the extensive horizon- tal transfer of genes between the Archaea and the Bacteria (2) as well as between Archaea, Bacteria, and the Eukarya. An alternative to searching for protein homologies in studying the evolution of cellular domains is to search for proteins unique to one domain with no significant homology to proteins in the other domains. This approach has been used by Woeses group to search for signature proteins in Archaea that are unique to the Archaea and that are absent from the Bacteria and Eukarya (3). In this paper we set out to find the signature proteins that would delineate the Eukarya from the Archaea and Bacteria. We char- acterize the set of eukaryotic signature proteins (ESPs) as those proteins that have homologs in all main branches of eukaryotes (animals, plants, fungi, and protozoa), but do not have any ho- mologs in the Archaea and Bacteria. Our ESP set was derived from the completely sequenced genomes of Saccharomyces cerevisiae , Drosophila melanogaster...
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This note was uploaded on 12/18/2010 for the course GENETICS 486 taught by Professor Hey during the Fall '10 term at Rutgers.
- Fall '10