Book of Abstracts: Albany 2011
June 14-18 2011
©Adenine Press (2010)
Protonation of Glutamate-208 Induces the Release of Agmatine in an Outward-Facing Conformation of Arginine/Agmatine Antiporter
Virulent enteric pathogens have developed several systems that maintain intracellular pH in order to survive extreme acidic conditions. One such mechanism is the exchange of arginine (Arg+) from the extracellular region with its intracellular decarboxylated form, agmatine (Agm2+). The net result of this process is the export of a virtual proton from the cytoplasm per antiport cycle (1). Crystal structures of the arginine/agmatine antiporter from E. coli, AdiC, have been recently resolved in both the apo and Arg+-bound outward-facing conformations (2;3), which permit us to assess for the first time the time-resolved mechanisms of interactions that enable the specific antiporter functionality of AdiC. Using data from approximately 1µs of molecular dynamics simulations, we show that the protonation of E208 selectively causes the dissociation and release of Agm2+, but not Arg+, to cell exterior. The impact of E208 protonation is transmitted to the substrate binding pocket via the reorientation of I205 carbonyl group at the irregular portion of transmembrane (TM) helix 6. This effect, which takes place only in the subunits where Agm2+ is released, invites attention to the functional role of the unwound portion of TM helices (TM6 W202-E208 in AdiC) in facilitating substrate translocation, reminiscent of the behavior observed in structurally similar Na+-coupled transporters.
This research was supported by NIH grants 1R01GM086238-01 and 1U54GM087519-01A1, and by the NSF through TeraGrid resources provided by Kraken (NICS) and Ranger (TACC) under grant number TG-MCB100108.
Department of Computational & Systems Biology, School of Medicine, University of Pittsburgh, 3064 BST3, 3501 Fifth Avenue, Pittsburgh, PA 15213.