The Twin-Arginine Translocation Pathway
Sascha Panahandeh, Eva Holzapfel and Matthias Müller
from: Bacterial Secreted Proteins: Secretory Mechanisms and Role in Pathogenesis (Edited by: Karl Wooldridge). Caister Academic Press, U.K. (2009)
The twin-arginine translocation (Tat) pathway is a protein transport system in bacteria, archaea and chloroplasts with the ability to export proteins in a fully folded conformation. Proteins are targeted to the Tat pathway by an N-terminal signal peptide containing an almost invariant twin-arginine sequence motif. Pretranslocational folding is necessitated by the incorporation of metallo-cofactors, assembly into oligomeric complexes, and presumably rapid folding kinetics. Many Tat systems comprise three functionally individual membrane proteins, termed TatA, TatB, and TatC, whereas especially Gram-positive bacteria possess minimal TatAC translocases, in which TatA functionally replaces TatB. TatC and TatB form a complex that is involved in recognition of Tat signal sequences and their insertion into the membrane. TatA mediates the actual translocation event, but it is unclear whether it does so by forming the pore-like structures that it displays when purified to homogeneity. Energy is derived from either component of the proton-motive force, ΔpH or ΔΨ, and is required only for late steps following signal sequence cleavage. Substrates that either lack the twin-arginine pair or are in a malfolded conformation in general are not translocated. The mechanisms by which non-functional substrates are rejected are not understood. For cofactor-containing substrates, proof-reading seems to depend on the activity of specific cytosolic chaperones read more ...