Structures And Mechanisms Of E. Coli Transporters Uraa And Adic

Secondary active transporters, which can be roughly divided into symporters andantiporters, are responsible for the concentrative transport of substrate across themembrane. UraA is in charge of the symport of proton and uracil into E. coli, whileAdiC is an arginine/agmatine antiporter.NAT/NCS2family are responsible for the permeation of nucleobases in allkingdoms of life and the transport of vitamin C in mammals. In this thesis wedetermined the crystal structure of a representative NAT protein, the E. coli uracil:H+symporter UraA, in complex with uracil at2.9resolution. UraA exhibits a novelstructural fold, with14transmembrane segments (TMs) divided into two invertedrepeats. A pair of anti-parallelβ-strands is located in the middle of TM3and TM10andplays an important role in the structural organization and substrate recognition. Thestructure is spatially arranged into a core domain and a gate domain. Uracil, located atthe interface between the two domains, is coordinated mainly by residues from the coredomain. Structural analysis suggests that alternating-access of the substrate may beachieved through conformational changes of the gate domain.Besides, we also determined the crystal structure of AdiC, the arginine:agmatineantiporter from E. coli O157:H7and a member of the aminoacid/polyamine/organocation (APC) superfamily of transporters at3.6resolution. Theoverall fold is similar to that of several LeuT fold transporters. AdiC contains12transmembrane segments, forms a homodimer, and exists in an outward-facing, openconformation in the crystals. A conserved, acidic pocket opens to the periplasm.Combined with the arginine bound occluded AdiC structure at3.0resolution, weproposed that threepotential gates, involving four aromatic residues and Glu208,maywork in concert to differentially regulate the uploadand release of Arg and Agm. Furtherstructural and biochemical analysis reveals the essential ligand-binding residues, definesthe transport route, and suggests a conserved mechanism for the antiporter activity.