How Translocons Select Transmembrane Helices

Like all cellular proteins, membrane proteins are synthesized by ribosomes. But unlike their soluble counterparts, highly hydrophobic membrane proteins require auxiliary machineries to prevent aggregation in aqueous cellular compartments. The principal machine is the translocon, which works in concert with ribosomes to manage the orderly insertion of alpha-helical membrane proteins directly into the endoplasmic reticulum membrane of eukaryotes or into the plasma membrane of bacteria. In the course of insertion, membrane proteins come into thermodynamic equilibrium with the lipid membrane, where physicochemical interactions determine the final three-dimensional structure. Much progress has been made during the past several years toward understanding the physical chemistry of membrane protein stability, the structure of the translocon machine, and the mechanisms by which the translocon selects and inserts transmembrane helices. We review this progress and consider the connection between the physical principles of membrane protein stability and translocon selection of transmembrane helices.