Transmembrane insertion of twin-arginine signalpeptides is driven by TatC and regulated by TatB

摘要

The twin-arginine translocation (Tat) pathway of bacteria and plant chloroplasts mediates thetransmembrane transport of folded proteins, which harbour signal sequences with aconserved twin-arginine motif. Many Tat translocases comprise the three membraneproteins TatA, TatB and TatC. TatC was previously shown to be involved in recognizingtwin-arginine signal peptides. Here we show that beyond recognition, TatC mediates thetransmembrane insertion of a twin-arginine signal sequence, thereby translocating the signalsequence cleavage site across the bilayer. In the absence of TatB, this can lead to the removalof the signal sequence even from a translocation-incompetent substrate. Hence interaction oftwin-arginine signal peptides with TatB counteracts their premature cleavage uncoupled fromtranslocation. This capacity of TatB is not shared by the homologous TatA protein. Collectivelyour results suggest that TatC is an insertase for twin-arginine signal peptides and thattranslocation-proficient signal sequence recognition requires the concerted action of TatCand TatB.