Purified components of the Escherichia coli Tat protein transport system form a double-layered ring structure

The Escherichia coli twin arginine translocation (Tat) system mediates Sec- independent export of protein precursors bearing twin arginine signal pepti des. The genes tatA, tatB, tatC and tatE code for integral membrane protein s that are components of the Tat pathway. Cells co-overexpressing tatABCDE show an increased rate of export of a signal peptide-defective Tat precurso r protein and a complex containing the TatA and TatB proteins can be purifi ed from the membranes of such cells. The purified TatAB complex has an appa rent molecular mass of 600 kDa as measured by gel permeation chromatography and, like the membranes of wild-type cells, contains a large molar excess of TatA over TatB. Negative stain electron microscopy of the complex reveal s cylindrical structures that may correspond to the Tat protein transport c hannel.