Membrane translocation and periplasmic folding in E. coli. Most polypeptides cross the cytoplasmic membrane in an unfolded conformation using the Sec translocase (1), following delivery to SecA at the inner surface of the membrane by DnaK or SecB chaperones. Polypeptides with highly hydrophobic signal sequences or transmembrane domains may, however, be recognised by Ffh which, together with its FtsY receptor, can target the polypeptide to either the Sec machinery or to the YidC translocase (2). Alternatively, the twin-arginine translocation (Tat) machinery is responsible for the translocation of already folded proteins (3), typically with bound metal cofactors. After cleavage of the leader peptide upon crossing the membrane, partially folded proteins may (4) aggregate, (5) be degraded by periplasmic proteases, or fold into their native state, often with the assistance of periplasmic chaperones (6) and/or folding catalysts such as disulfide bond metabolising enzymes (7) or peptidyl-prolyl cis-trans isomerases (8).