The evolution of protein targeting and translocation systems.

Cells have evolved increasingly complex membrane systems for compartmentalization and thereby for the regulation of multiple cellular pathways. The existence of such membranes required the evolution of molecular machines that allow and regulate the exchange of material between intracellular compartments or with the exterior. Here, we have summarized the current concepts for the origin and evolution of the targeting and translocation systems required for the specific insertion of transmembrane proteins into their target membranes and for the transport of protein cargos across membranes. The basic pathways developed in prokaryotes were modified and extended to suffice for the much more complex membrane systems found in eukaryotes, allowing not only the identification of basic mechanistic principles, but also phylogenetic studies to elucidate evolutionary relations.