Tat transport of linker-containing proteins in Escherichia coli

The twin-arginine translocation (Tat) system serves to translocate folded and often cofactor-containing proteins across biological membranes. The mechanistic limits of the Tat system can be explored by addressing the transport of specifically designed Tat substrates. It thus could be recently shown that unstructured proteins are also accepted by the Tat system, but only if they are polar on their surface. Using the iron-sulfur cofactor-containing model Tat-substrate high potential iron-sulfur protein (HiPIP), we now demonstrate that the bacterial Tat system can translocate small globular proteins even when a long unstructured linker peptide of 110 residues is sandwiched between the signal peptide and the N-terminus of the mature domain. The iron-sulfur cofactor was fully assembled in the transported protein, which demonstrates that HiPIP was folded during translocation. Linker lengths of 148 and 205 residues almost blocked or completely abolished Tat transport, respectively. The tolerance for long unfolded linker peptides challenges our current understanding of the Tat mechanism.