The Twin-Arginine Pathway for Protein Secretion

About 20 to 30% of proteins synthesized in the bacterial cytoplasm are destined for extracytoplasmic locations ( 1 ). They pass the cytoplasmic membrane using specialized transport systems, involving gated pores, energy, and signal peptides to direct protein export. Two major protein export systems are known, namely, the general secretory (Sec) pathway and the twin-arginine translocation (Tat) pathway ( Fig. 1 ). Most proteins use the Sec pathway, common to all domains of life. The Tat pathway, the focus of this review, is more exclusive. For example, it has only ∼30 native substrates in the Gram-negative bacterium Escherichia coli, and it is not universally conserved ( 2 ).