InterPro domain: IPR006312

Translocation of proteins across the two membranes of Gram-negative bacteriacan be carried out via a number of routes. Most proteins marked for export carry a secretion signal at their N terminus, and are secreted by the general secretory pathway. The signal peptide is cleaved as they pass through the outer membrane. Other secretion systems include the type III system found in a select group of Gram-negative plant and animal pathogens, and the CagA system of Helicobacter pylori [ 1 ].

In some bacterial species, however, there exists a system that operates independently of the Sec pathway [ 2 ]. It selectively translocates periplasmic-bound molecules that are synthesised with, or are in close association with, "partner" proteins bearing an (S/T)RRXFLK twin arginine motif at the N terminus. The pathway is therefore termed the Twin-Arginine Translocation or TAT system. Surprisingly, the four components that make up the TAT system are structurally and mechanistically related to a pH-dependent import system in plant chloroplast thylakoid membranes [ 3 ]. Thegene products responsible for the Sec-independent pathway are called TatA,TatB, TatC and TatE.

TatA and TatE are highly related proteins and appear to overlap in functionality [ 3 ]. Translocation occurred in single mutants of either TatA or TatE, though much less efficiently, but double mutants showed no detectable translocation.

1. Overlapping functions of components of a bacterial Sec-independent protein export pathway. EMBO J. 17, 3640-50
2. The Tat protein export pathway. Mol. Microbiol. 35, 260-74