Tatpathways

Tatpathways, or twin-arginine translocation pathways, are protein export systems found in many bacteria, archaea, and the chloroplasts of plants. They transport fully folded proteins across the cytoplasmic membrane, typically into the periplasm in Gram-negative bacteria or into the thylakoid lumen in chloroplasts. In some Gram-positive bacteria, export occurs across the cytoplasmic membrane to the surrounding cell envelope.

Core components include TatA, TatB, and TatC. TatC is a multi-pass membrane protein that acts as the main receptor; TatB assists in substrate recognition; and TatA forms the dynamic oligomeric translocon. In many bacteria, an additional TatA-like paralog called TatE serves as a minor component. Substrates are directed to the system by N-terminal signal peptides containing a twin-arginine motif, typically in the consensus SRRxFLK.

Export is powered by the proton motive force across the membrane rather than ATP hydrolysis. Substrates are believed to be folded in the cytoplasm and, once engaged, the Tat complex assembles to transport them in their native, cofactor-containing folded state. Two leading models describe the mechanism: one posits a pore formed by TatA oligomers through which the substrate passes; the other suggests a membrane-weakening or alternative conduit mechanism that permits translocation without a traditional channel.

Substrates include cofactors-containing enzymes such as hydrogenases, nitrogenase components, and other redox enzymes. The Tat pathway is important for microbial energy metabolism and, in plants, for plastid function. Variability in gene organization and component presence across taxa reflects evolutionary adaptation and specialization of the export system.