Structural basis of isethionate transport by a TRAP transporter from a sulfate-reducing bacterium

Sulfate-reducing bacteria import organosulfur compounds from the environment for anaerobic respiration. They contribute to human disease and are problematic in industrial settings because they produce hydrogen sulfide. Here, we demonstrate how the sulfate-reducing bacterium Oleidesulfovibrio alaskensis imports isethionate, a common organosulfonate, using a tripartite ATP-independent periplasmic (TRAP) transporter ( Oa IsePQM). The cryo-EM structure of isethionate-bound Oa IseQM to 2.98 Å resolution defines the substrate-binding site, two Na+-binding sites, and a distinct fusion helix. Key residues within the Oa IseQM substrate-binding site are identified using substitution and proteoliposome assays. Functional studies demonstrate that Oa IseQM requires the substrate-binding protein ( Oa IseP) and a Na+ gradient to drive transport. Modeling of the Oa IsePQM complex supports that elevator-type conformational changes are involved in this unique coupled transport process. This work expands our knowledge of the transport of organosulfur compounds in bacteria and establishes Oa IsePQM as a new model system for exploring the mechanism of TRAP transporters.