3.A.1.3.25
Glutamine transporter, GlnQP. Takes up glutamine, asparagine and glutamate which compete for each other for binding to the substrate/transmembrane protein constituent of the system (Fulyani et al. 2015). Tandem substrate binding domains  differ in substrate specificity and affinity, allowing cells to efficiently accumulate different amino acids via a single ABC transporter. Analysis revealed the roles of individual residues in determining the substrate affinity (Fulyani et al. 2013). Protein-protein interactions and conformational states were studied simultaneously using PIFE-FRET (Ploetz et al. 2021). GlnPQ from L. lactis has two sequential covalently linked substrate-binding domains (SBDs), which capture the substrates and deliver them to the translocon. The two SBDs differ in their ligand specificities, binding affinities and the distance to the transmembrane domain, but both SBDs can bind their ligands simultaneously without affecting each other. Nemchinova et al. 2024 studied the binding of ligands to both SBDs; three high-resolution structures of SBD1, namely, the wild-type SBD1 with bound asparagine or arginine, and E184D SBD1 with glutamine bound were examined. Molecular dynamics (MD) simulations provided insight into the dynamics associated with open-closed transitions of the SBDs.