The current model of active transport via ABC importers is mostly based on structural, biochemical and genetic data. We here establish single-molecule F€ orster resonance energy transfer (smFRET) assays to monitor the conformational states and heterogeneity of the osmoregulatory type I ABC importer OpuA from Lactococcus lactis. We present data probing both intradomain distances that elucidate conformational changes within the substrate-binding domain (SBD) OpuAC, and interdomain distances between SBDs or transmembrane domains. Using this methodology, we studied ligand-binding mechanisms, as well as ATP and glycine betaine dependences of conformational changes. Our work expands the scope of smFRET investigations towards a class of so far unstudied ABC importers, and paves the way for a full understanding of their transport cycle in the future.

The current model of active transport via ABC importers is mostly based on structural, biochemical and genetic data. We here establish single-molecule Förster-resonance energy transfer (smFRET) assays to monitor the conformational states and heterogeneity of the type-I ABC importer OpuA from Lactococcus lactis. Our studies include intradomain assays that elucidate conformational changes within the substrate-binding domain (SBD) OpuAC and interdomain assays between SBDs or transmembrane domains. Using the methodology, we studied ligand-binding mechanisms as well as ATP and glycine betaine dependences of conformational changes. Our study expands the scope of smFRET investigations towards a class of so far unstudied ABC importers, and paves the way for a full understanding of their transport cycle in the future.