Molecular basis for redox control by the human cystine/glutamate antiporter System xc^-

Abstract

Cysteine plays an essential role in cellular redox homeostasis as a key constituent of the tripeptide glutathione (GSH). A rate limiting step in cellular GSH synthesis is the availability of cysteine. However, circulating cysteine exists in the blood as the oxidised di-peptide cystine, requiring specialised transport systems for its import into the cell. System xc- is a dedicated cystine transporter, importing cystine in exchange for intracellular glutamate. To counteract elevated levels of reactive oxygen species in cancerous cells system xc- is frequently upregulated, making it an attractive target for anticancer therapies. However, the molecular basis for ligand recognition and transport remains elusive, hampering efforts to specifically target this system. Here we present the cryo-EM structure of system xc- in both the apo and glutamate bound states. Structural comparisons reveal an allosteric mechanism for ligand discrimination, supported by molecular dynamics and cell-based assays, establishing a mechanism for cystine recognition and transport in human cells.

Integrated AlphaFold2 and DEER investigation of the conformational dynamics of a pH-dependent APC antiporter

“…Nevertheless, despite these unique adaptations and their functional divergence from human transporters, both exchangers have served as model systems for eukaryotic homologs since their initial structural characterization over a decade ago [29][30][31][32] . The crystal structures of GadC and AdiC, which were determined in inward-facing and outward-facing conformations, respectively, superimpose over recent cryo-EM structures of human orthologs and have been used, for example, for numerous drug docking studies [33][34][35][36][37][38] . Nevertheless, an absence of detailed studies of their energy landscapes and conformational dynamics has hindered framing these structures in the broader context of antiport.Insights in this regard may be obtained from more distant homologs of APC transporters that have been subjects of extensive study [39][40][41][42][43][44][45][46][47][48][49][50][51] .…”