Fig. 1

Two major bacterial export pathways: Sec and Tat. a The general secretion (Sec) protein export pathway. In the cotranslational mode (1), Sec substrates possessing highly hydrophobic signal peptides (SP) are recognized at the ribosome by the signal recognition particle (SRP). Subsequently, the ribosome-nascent chain (RNC)-SRP complex docks to the SRP-receptor FtsY and the RNC is then further transferred to the SecYEG translocation pore such that ribosomal exit site is in close proximity to SecYEG. The energy for translocation in the cotranslational export mode is provided by further elongation of the substrate at the ribosome. In the posttranslational mode (2), Sec-dependent precursor proteins are kept in an export-competent state by posttranslationally interacting proteins (PIP’s) such as SecB, the general chaperones GroELS/DnaK–DnaJ–GrpE or the soluble form of SecA. The signal peptide (SP) is recognized by the SecA protein which pushes the protein through the SecYEG protein conducting channel in a stepwise and ATP-dependent manner. In addition, SecDF exerts a proton motive force (pmf)-dependent pulling force on the substrate from the trans-side of the cytoplasmic membrane (CM). During or shortly after translocation, the signal peptide is removed by signal peptidase (SPase) and the mature protein is released on the trans-side of the CM. b The twin-arginine translocation (Tat) protein export pathway. After folding and, if required (as shown here), cofactor insertion, preproteins containing a signal peptide with a twin-arginine motif (RR) are recognized by a receptor complex consisting of TatC and TatB. Subsequently, homooligomeric complexes of TatA are recruited to the substrate-loaded receptor complex in a proton motive force (pmf)-dependent manner, followed by the translocation of the substrate across the cytoplasmic membrane (CM). Following to substrate translocation, the signal peptide is cleaved by signal peptidase (SPase) and the mature protein is released on the trans-side of the membrane