Model of the archaeal cell envelope showing different characterized secretion pathways. Proteins synthesized at the ribosome can follow several routes to the exterior of the cell. During co-translational translocation, the ribosome-nascent chain complex is targeted to the SecYE complex by the signal recognition particle. At the SecYE complex protein synthesis and translocation across the cytoplasm membrane occurs simultaneously. In the case of a preprotein with a class I signal peptide, the signal peptide is removed during translocation and the protein is released and folds at the external face of the membrane. Class III signal peptide containing proteins translocated via the SecYE complex are processed by PibD and subsequently assembled into a flagellum, pilus, bindosome or so far unknown cell surface structures. Alternatively, folded proteins are transported across the cytoplasmic membrane via the Twin arginine translocase pathway.
