Substrate binding domain of putative ABC transporters involved in cystine import; the type 2 periplasmic binding protein fold
This group contains uncharacterized periplasmic cystine-binding domain of ATP-binding cassette (ABC) transporters. Cystine is an oxidized dimeric form of cysteine that is required for optimal bacterial growth. In Bacillus subtilis, three ABC transporters, TcyJKLMN (YtmJKLMN), TcyABC (YckKJI), and YxeMNO are involved in uptake of cystine. Likewise, three uptake systems were identified in Salmonella enterica serovar Typhimurium, while in Escherichia coli, two transport systems seem to be involved in cystine uptake. Moreover, L-cystine limitation was shown to prevent virulence of Neisseria gonorrhoeae; thus, its L-cystine solute receptor (Ngo0372) may be suited as target for an antimicrobial vaccine. The cystine receptor belongs to the type 2 periplasmic binding fold protein superfamily (PBP2). The PBP2 proteins are typically comprised of two globular subdomains connected by a flexible hinge and bind their ligand in the cleft between these domains in a manner resembling a Venus flytrap. After binding their specific ligand with high affinity, they can interact with a cognate membrane transport complex comprised of two integral membrane domains and two receptor cytoplasmically-located ATPase domains. This interaction triggers the ligand translocation across the cytoplasmic membrane energized by ATP hydrolysis.
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