Understanding Archaeal Protein Translocation: Haloferax volcanii as a Model System

The biogenesis of extra-cytoplasmic proteins requires negotiation of the hydrophobic barrier presented by lipid-based membranes. Unlike the well-defined eukaryal and bacterial protein translocation systems, little is known about how proteins cross into and/or across the plasma membrane of Archaea. In Eukarya and Bacteria, protein translocation occurs at membrane sites composed of evolutionarily conserved core proteins acting together with other domain-specific components. Analysis of archaeal genomes and individual genes from other archaeal strains for which no complete genome sequences are available reveals the existence of archaeal homologues of certain elements of the bacterial or eukaryotic systems, as well as the apparent absence of other components of these two systems (Eichler 2000). Thus, while archaeal translocation represents a hybrid of the bacterial and eukaryotic models, closer examination also reveals the existence of archaeal-specific properties. These could be related to the unique chemical composition of the archaeal membrane or to the extreme conditions in which Archaea can exist, including highly saline environments.