An Arginine-Agmatine Antiporter Optimized for Extreme Acid Resistance in Enteric Bacteria

Abstract

Enteric bacteria such as E. coli and Salmonella species can maintain cytoplasmic pH in the extremely acidic gastric environments by consuming intracellular protons for arginine decarboxylation. This reaction is kept energetically favorable by an APC (amino-acid-polyamine-organocation) family antiporter AdiC, which imports arginine while pumping out the decarboxylated product agmatine in a 1:1 exchange stoichiometry. To characterize AdiC's transport behavior in extreme acid conditions, we developed an oriented reconstituted liposome system able to hold a 3-unit transmembrane pH gradient. Our work reveals two unique functional properties by which AdiC maximizes its effectiveness in conferring acid resistance. First, AdiC possesses a pH-sensing mechanism that permits maximal transport activity only under intra- and extracellular pH conditions of bacteria experiencing strong acid stress (i.e., outside pH 2-3; inside pH 4.5-5.5). Second, we demonstrate that AdiC mainly catalyzes electropositive exchange between extracellular arginine (50% Arg+, 50% Arg2+, pH2.2) and cytoplasmic agmatine (Agm2+, pH5.5) - the exchange is strongly inhibited by the imposition of negative membrane potentials. Thus, at gastric pH, AdiC selectively imports the less common Arg+ against the useless α-carboxyl protonated Arg2+, whose deprotonation upon entry into the cytosol would cancel out the proton-removing effect of arginine decarboxylation. We are currently investigating the underlying mechanisms for this second feature of AdiC. Preliminary results from testing various arginine analogues suggest that AdiC differentiates Arg+ and Arg2+ based on their difference in valence number rather than the protonation status in the α-carboxyl.