Crossing the membrane in Archaea, the third domain of life

Many of the recent advancements in the field of protein translocation, particularly from the structural perspective, have relied on Archaea. For instance, the solved structures of the translocon from the methanoarchaeon Methanocaldococcus jannaschii of the ribosomal large subunit from the haloarchaeon Haloarcula marismortui and of components of the SRP pathway from several archaeal species have provided novel insight into various aspects of the translocation event. Given the major contribution that Archaea have made to our understanding of how proteins enter and traverse membranes, it is surprising that relatively little is known of protein translocation in Archaea in comparison to the well-defined translocation pathways of Eukarya and Bacteria. What is known, however, points to archaeal translocation as comprising a mosaic of eukaryal and bacterial traits together with aspects of the process seemingly unique to this, the third domain of life. Here, current understanding of archaeal protein translocation is considered. This article is part of a Special Issue entitled Protein translocation across or insertion into membranes.