Journal
FEBS JOURNAL
Volume 273, Issue 22, Pages 5121-5130Publisher
BLACKWELL PUBLISHING
DOI: 10.1111/j.1742-4658.2006.05509.x
Keywords
CITMHS family; citrate fermentation; citrate transport; Enterococcus faecalis; Me-citrate complex
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Secondary transporters of the bacterial CitMHS family transport citrate in complex with a metal ion. Different members of the family are specific for the metal ion in the complex and have been shown to transport Mg2+-citrate, Ca2+-citrate or Fe3+-citrate. The Fe3+-citrate transporter of Streptococcus mutans clusters on the phylogenetic tree on a separate branch with a group of transporters found in the phylum Firmicutes which are believed to be involved in anaerobic citrate degradation. We have cloned and characterized the transporter from Enterococcus faecalis EfCitH in this cluster. The gene was functionally expressed in Escherichia coli and studied using right-side-out membrane vesicles. The transporter catalyzes proton-motive-force-driven uptake of the Ca2+-citrate complex with an affinity constant of 3.5 mu M. Homologous exchange is catalyzed with a higher efficiency than efflux down a concentration gradient. Analysis of the metal ion specificity of EfCitH activity in right-side-out membrane vesicles revealed a specificity that was highly similar to that of the Bacillus subtilis Ca2+-citrate transporter in the same family. In spite of the high sequence identity with the S. mutans Fe3+-citrate transporter, no transport activity with Fe3+ (or Fe2+) could be detected. The transporter of E. faecalis catalyzes translocation of citrate in complex with Ca2+, Sr2+, Mn2+, Cd2+ and Pb2+ and not with Mg2+, Zn2+, Ni2+ and Co2+. The specificity appears to correlate with the size of the metal ion in the complex.
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