Two stages of substrate discrimination dictate selectivity in the MetNI-Q ABC transporter system.

The MetNI-Q importer, an ATP-binding cassette (ABC) transporter, mediates the uptake of both L- and D- enantiomers of methionine. Original uptake studies show a strong preference for L-Met over D-Met, but the molecular basis of this selectivity is unclear. In this work, we systematically examine substrate discrimination by the MetNI transporter and MetQ substrate binding protein using an array of biophysical and biochemical techniques.