Relaxed Specificity of the Bacillus subtilis TatAdCd Translocase in Tat-Dependent Protein Secretion

Protein translocation via the twin arginine translocation (TAT) pathway is characterized by the transloca-tion of prefolded proteins across the hydrophobic lipid bilayer of the membrane. In Bacillus subtilis, two different Tat translocases are involved in this process, and both display different substrate specificities: PhoD is secreted via TatAdCd, whereas YwbN is secreted via TatAyCy. It was previously assumed that both TatAy and TatCy are essential for the translocation of the YwbN precursor. Through complementation studies, we now show that TatAy can be functionally replaced by TatAd when the latter is offered to the cells in excess amounts. Moreover, under conditions of overproduction, TatAdCd, in contrast to TatAyCy, shows an increased tolerance toward the acceptance of various Tat-dependent proteins. The translocation of proteins across the hydrophobic membranes of cells is an essential mode of survival for all organisms. Several mechanisms have evolved to ensure this transport. Among these mechanisms, the twin arginine translocation (TAT) pathway has been shown to be capable of translocating fully folded proteins across the membrane. Proteins that depend on this translocase for their secretio