In Escherichia coli, dimethyl sulfoxide reductase and formate dehydrogenase are cytoplasmic membrane proteins containing cofactors such as molybdenum and iron—sulfur clusters. They are embedded in the membrane in a mechanism called the twin-arginine translocation (TAT) pathway, which is different from GSP and ABC. The _ best known (or predicted) substrates of the TAT pathway are periplasmic proteins that bind a range of redox cofactors, including molybdopterin, Fe-S, Ni-Fe centres and others. These cofactors can be inserted into the peptide in the cytoplasm only and this requires substantial or complete folding of the mature protein, so it is believed that fully or largely folded structures are transported across the cytoplasmic membrane through this pathway. This system is also used for the translocation of other proteins that do not contain any cofactors, as these might fold too rapidly for the Sec system to handle.