The Hydrophilic C-terminus of Yeast Plasma-membrane Na/H Antiporters Impacts Their Ability to Transport K.

Yeast plasma-membrane Na/H antiporters (Nha/Sod) ensure the optimal intracellular level of alkali-metal cations and protons in cells. They are predicted to consist of 13 transmembrane segments (TMSs) and a large hydrophilic C-terminal cytoplasmic part with seven conserved domains. The substrate specificity, specifically the ability to recognize and transport K cations in addition to Na and Li, differs among homologs. In this work, we reveal that the composition of the C-terminus impacts the ability of antiporters to transport particular cations. In the osmotolerant yeast Zygosaccharomyces rouxii, the Sod2-22 antiporter only efficiently exports Na and Li, but not K. The introduction of a negative charge or removal of a positive charge in one of the C-terminal conserved regions (C3) enabled ZrSod2-22 to transport K. The same mutations rescued the low level of activity and purely Li specificity of ZrSod2-22 with the A179T mutation in TMS6, suggesting a possible interaction between this TMS and the C-terminus. The truncation or replacement of the C-terminal part of ZrSod2-22 with the C-terminus of a K-transporting Nha/Sod antiporter (Saccharomyces cerevisiae Nha1 or Z. rouxii Nha1) also resulted in an antiporter with the capacity to export K. In addition, in ScNha1, the replacement of three positively charged arginine residues 539-541 in the C3 region with alanine caused its inability to provide cells with tolerance to Li. All our results demonstrate that the physiological functions of yeast Nha/Sod antiporters, either in salt tolerance or in K homeostasis, depend on the composition of their C-terminal parts.