Stimulation of the steady state ATPase kinetics of the ABC exporter NaAtm1 by a transported substrate GSSG.

We report the steady state ATPase activities of the ATP Binding Cassette (ABC) exporter NaAtm1 in the absence and presence of a transported substrate, oxidized glutathione (GSSG), in detergent, nanodiscs and proteoliposomes. The steady state kinetic data were fit to the "non-essential activator model" where the basal ATPase rate of the transporter is stimulated by the transported substrate, GSSG. The detailed kinetic parameters varied between the different reconstitution conditions, highlighting the importance of the lipid environment for NaAtm1 function. The increased ATPase rates in the presence of GSSG more than compensate for the modest negative cooperativity observed between MgATP and GSSG in lipid environments. These studies highlight the central role of the elusive ternary complex in accelerating the ATPase rate that is at the heart of coupling mechanism between substrate transport and ATP hydrolysis. This article is protected by copyright. All rights reserved.