Analysis of the twin-arginine motif of a haloarchaeal Tat substrate

The twin-arginine translocase (Tat) is a system specific to the transport of fully folded proteins. In contrast to most prokaryotes, the Tat pathway is the main route for export in halophilic archaea (haloarchaea). The haloarchaeal Tat system also seems to differ in a number of other aspects from the nonhalophilic counterparts, such as the constituents of the translocase and bioenergetic requirements. Therefore, it was important to test which features in haloarchaeal Tat substrates were important for transport, as these might also be different from those of nonhalophilic organisms. Here, we analysed residues in the so-called Tat motif, which is found in the amino-terminal signal peptide of all Tat substrates. Bioinformatics analysis showed that in haloarchaea, the consensus sequence of this motif is (S/T)RRx(F/L)L. Using the model protein AmyH, we found that both arginines and both hydrophobic residues were essential to translocation: either replacing one or both of the arginine residues with lysine, or replacing one of the hydrophobic residues with alanine, led to a block in translocation. Other residues in or around the motif were found not to be essential for transport.