Assessing the efficacy of vesicle fusion with planar membrane arrays using a mitochondrial porin as reporter

The gene for a putative cation calcium exchanger (CCX) from Arabidopsis thaliana, AtCCX5, was cloned and its function was analyzed in yeast. Green fluorescent protein-tagged AtCCX5 expressed in yeast was localized in the plasma membrane and nuclear periphery. The yeast transformants expressing AtCCX5 were created and their growth in the presence of various cations (K⁺, Na⁺, Ca²⁺, Mg²⁺, Fe²⁺, Cu²⁺, Co²⁺, Cd²⁺, Mn²⁺, Ba²⁺, Ni²⁺, Zn²⁺, and Li⁺) were analyzed. AtCCX5 expression was found to affect the response to K⁺ and Na⁺ in yeast. The AtCCX5 transformant also showed a little better growth to Zn²⁺. The yeast mutant 9.3 expressing AtCCX5 restored growth of the mutant on medium with low K⁺ (0.5 mM), and also suppressed its Na⁺ sensitivity. Ion uptake experiments showed that AtCCX5 mediated relatively high-affinity K⁺ uptake and was also involved in Na⁺ transport in yeast. Taken together, these findings suggest that the AtCCX5 is a novel transport protein involves in mediating high-affinity K⁺ uptake and Na⁺ transport in yeast.