Abstract
Membrane transporters regulate multiple physiological processes including the pumping of ions, peptides and other metabolites such as neurotransmitters. Despite plethora of information available on transmembrane transporters, the possibility that transporters like all other enzymes studied to date display static and dynamic fluctuations in activity remains open. Importantly we currently ignore the impact such fluctuation have on the critical issue of pumping stoichiometry and therefore pumping efficiency. For example, several studies on P-type ATPases suggest substantially less amount of cations being transported to ATP hydrolyzed, possibly due to uncoupling of ATP-hydrolysis from cation transport during resting state.(1,2)Here, we present results from our ongoing investigation on time-resolved recording of single transporter events, using surface-based arrays of proteoliposomes.(3-5) Our single molecule studies have revealed fluctuations (on/off states) in the activity the P-type H+-ATPases-2 from Arabidopsis thaliana (AHA2). Insights from these on and off activity states for proton transporter will be used to estimate the true proton to ATP stoichiometry. Preliminary experiments were also performed to understand the role of the regulatory domain that is thought to be critical in coupling hydrolysis to proton coupling.(6)
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