Implications for Cation Selectivity and Evolution by a Novel Cation Diffusion Facilitator Family Member From the Moderate Halophile Planococcus dechangensis

In the cation diffusion facilitator (CDF) family, the transported substrates are confined to divalent metal ions, such as Zn(2+), Fe(2+), and Mn(2+). However, this study identifies a novel CDF member designated MceT from the moderate halophile Planococcus dechangensis. MceT functions as a Na(+)(Li(+), K(+))/H(+) antiporter, together with its capability of facilitated Zn(2+) diffusion into cells, which have not been reported in any identified CDF transporters as yet. MceT is proposed to represent a novel CDF group, Na-CDF, which shares significantly distant phylogenetic relationship with three known CDF groups including Mn-CDF, Fe/Zn-CDF, and Zn-CDF. Variation of key function-related residues to “Y44-S48-Q150” in two structural motifs explains a significant discrimination in cation selectivity between Na-CDF group and three major known CDF groups. Functional analysis via site-directed mutagenesis confirms that MceT employs Q150, S158, and D184 for the function of MceT as a Na(+)(Li(+), K(+))/H(+) antiporter, and retains D41, D154, and D184 for its facilitated Zn(2+) diffusion into cells. These presented findings imply that MceT has evolved from its native CDF family function to a Na(+)/H(+) antiporter in an evolutionary strategy of the substitution of key conserved residues to “Q150-S158-D184” motif. More importantly, the discovery of MceT contributes to a typical transporter model of CDF family with the unique structural motifs, which will be utilized to explore the cation-selective mechanisms of secondary transporters.