Protein Translocation in the Three Domains of Life: Variations on a Theme

translocation components of organisms from all three domains of life. The Sec61/SecYEG Complex In all organisms, the core of the protein translocation apparatus is thought to be a heterotrimeric complex of integral membrane proteins, known as the SecYEG Boston, Massachusetts 02115 complex in bacteria (Brundage et al., 1990), and the ‡ Max Delbrü ck Center for Molecular Medicine Sec61 complex in eukaryotes (Gö rlich and Rapoport, Robert-Rö ssle-Stra␤e 10 1993). These complexes probably form a pore, through D-13122 Berlin Buch which proteins pass on their way out of the cytoplasm Germany (Hanein et al., 1996). Two of the subunits of these complexes are conserved in the domains—the SecY/Sec61␣ and the SecE/Sec61␥ proteins (Hartmann et al., 1994) in bacteria and eukaryotes, respectively. These proteins In all cells, extracytoplasmic proteins must be translo-are essential for protein translocation in vivo and in vitro. cated across lipid bilayers to reach their final destina-Without exception, archaeal homologs of SecY/ tion. Most protein translocation across hydrophobic Sec61␣ and SecE/Sec61␥ are more closely related to membranes occurs through an evolutionarily conserved the eukaryotic than to the bacterial members of these proteinaceous complex. In addition to this complex, the families (Figure 2). protein translocation machinery in bacteria and eukary-Analyses of SecY/Sec61␣ and SecE/Sec61␥ homo-otes employs a number of other proteins that do not logs from a number of organisms have led to the identifi-appear to be shared between these two domains of cation of regions that are conserved in all cells and living organisms (Murphy and Beckwith, 1996; Rapoport regions of conservation that are limited to only one or et al., 1996; Schatz and Dobberstein, 1996). The func-two domains. Such information could provide clues tions of many of these proteins and the mechanism of about which portions of SecY/Sec61␣ and SecE/Sec61␥ protein translocation remain largely unknown. interact with each other or with additional components Genome sequencing projects are providing the basis of the protein translocation apparatus. For example, as for a novel approach to learning more about protein will be discussed below, homologs of SecD and SecF, translocation in all cells. Protein translocation has been two components that are thought to interact with the studied only in bacteria and eukaryotes, two of the three bacterial heterotrimeric complex, have been identified domains of life. The completion of the genome se-in archaea but not in yeast. Regions of bacterial SecY quences of organisms in the third domain, the archaea, that …