Archaeal protein translocation: Crossing membranes in the third domain of life

Proper cell function relies on correct protein localization. As a first step in the delivery of extracytoplasmic proteins to their ultimate destinations, the hydrophobic barrier presented by lipid-based membranes must be overcome. In contrast to the well-defined bacterial and eukaryotic protein translocation systems, little is known about how proteins cross the membranes of archaea, the third and most recently described domain of life. In bacteria and eukaryotes, protein translocation occurs at proteinaceous sites comprised of evolutionarily conserved core components acting in concert with other, domain-specific elements. Examination of available archaeal genomes as well as cloning of individual genes from other archaeal strains reveals the presence of homologues to selected elements of the bacterial or eukaryotic translocation machines. Archaeal genomic searches, however, also reveal an apparent absence of other, important components of these two systems. Archaeal translocation may therefore represent a hybrid of the bacterial and eukaryotic models yet may also rely on components or themes particular to this domain of life. Indeed, considering the unique chemical composition of the archaeal membrane as well as the extreme conditions in which archaea thrive, the involvement of archaeal-specific translocation elements could be expected. Thus, understanding archaeal protein translocation could reveal the universal nature of certain features of protein translocation which, in some cases, may not be readily obvious from current comparisons of bacterial and eukaryotic systems. Alternatively, elucidation of archaeal translocation could uncover facets of the translocation process either not yet identified in bacteria or eukaryotes, or which are unique to archaea. In the following, the current status of our understanding of protein translocation in archaea is reviewed.