Abstract
Sodium-calcium exchangers (NCX) play a critical role in a variety of physiological processes involved in calcium signaling. NCXs are ubiquitous secondary-active transporters that couple the downhill movement of Na+ to the translocation of Ca2+ in the opposite direction. Mammalian NCXs have been widely studied functionally; however, structural insight into their transport mechanism comes from the crystal structure of the prokaryotic homolog NCX_Mj. This prokaryotic exchanger has been proposed to be a good model for the NCX family; yet, functional understanding of this protein remains incomplete. Here, we study purified NCX_Mj reconstituted into liposomes and demonstrate, under well-controlled experimental conditions, that this homolog in fact shares key functional features of the NCX family. Transport assays and reversal-potential measurements enable us to delineate the essential characteristics of this antiporter, and to establish that its ion-exchange stoichiometry is 3Na+:1Ca2+. Together with previous studies, this work reaffirms the notion that NCX_Mj is a functional representative of the NCX family and thus a valid model system to investigate the mechanism of ion recognition and membrane transport in sodium-calcium exchangers.
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