Characterization and Purification of a Na+/Ca2+ Exchanger from an Archaebacterium

摘要

The superfamily of cation/Ca²⁺ exchangers includes both Na⁺/Ca²⁺ exchangers (NCXs) and Na⁺/Ca²⁺,K⁺ exchangers (NCKX) as the families characterized in most detail. These Ca²⁺ transporters have prominent physiological roles. For example, NCX and NCKX are important in regulation of cardiac contractility and visual processes, respectively. The superfamily also has a large number of members of the YrbG family expressed in prokaryotes. However, no members of this family have been functionally expressed, and their transport properties are unknown. We have expressed, purified, and characterized a member of the YrbG family, MaX1 from Methanosarcina acetivorans. MaX1 catalyzes Ca²⁺ uptake into membrane vesicles. The Ca²⁺ uptake requires intravesicular Na⁺ and is stimulated by an inside positive membrane potential. Despite very limited sequence similarity, MaX1 is a Na⁺/Ca²⁺ exchanger with kinetic properties similar to those of NCX. The availability of a prokaryotic Na⁺/Ca²⁺ exchanger should facilitate structural and mechanistic investigations.