The twin-arginine translocation (Tat) core complex has an asymmetric TatA₁B₂C₃ composition that recognizes substrate signal peptides through specific interactions between the twin-arginine consensus motif and conserved TatC residues, while the complex's highly tilted transmembrane helices induce extreme membrane thinning that facilitates subsequent TatA recruitment and substrate transport on the periphery rather than through the interior of the complex.

The twin-arginine translocation (Tat) system is a mechanistically unique protein transport pathway moving folded proteins across membranes. It is found in all domains of life and is essential for plant photosynthesis and bacterial virulence. A core complex composed of membrane proteins TatA, TatB, and TatC binds substrate proteins and then recruits additional TatA protomers to form the transport site. Here we present structures of resting and substrate-bound states of the Tat core complex together with biochemical and functional analysis. Substrate proteins associate with the core complex solely through their N-terminal signal peptides with their Tat targeting sequences making specific contacts with TatC and the peptide body clamped by TatB. The core complex contains highly tilted transmembrane helices that drive extreme local membrane thinning. Based on our results we propose a model for the early steps in Tat transport.