Summary Maintenance of intracellular K(+) homeostasis is one of the crucial requisites for the survival of yeast cells. In Saccharomyces cerevisiae, the high K(+) content corresponds to a steady state between simultaneous influx and efflux across the plasma membrane. One of the transporters formerly believed to extrude K(+) from the yeast cells (besides Ena1-4p and Nha1p) was named Kha1p and presumed as a putative plasma membrane K(+)/H(+) antiporter. We prepared kha1 and tok1-kha1 deletion strains in the B31 and MAB 2d background. Both the strains contain the ena1-4 and nha1 deletions; that means they lack the main active sodium and potassium efflux systems. MAB 2d has additional trk1 and trk2 deletions, i.e. is impaired in active K(+) uptake as well. We performed a large physiological study with these strains to specify the phenotype of kha1 deletion. In our experiments, no difference in K(+) content or efflux was observed in strains lacking the KHA1 gene compared with control strains. Two main phenotype manifestations of the kha1 deletion were growth defect on high external pH and hygromycin sensitivity. The correlation between these phenotypes and the kha1 deletion was confirmed by plasmid complementation. Fluorescence microscopy of green fluorescent protein (GFP)-tagged Kha1p showed that this antiporter is localized preferentially intracellularly (in contrast to the plasma membrane Na(+)/H(+) antiporter Nha1p). Based on these findings, Kha1p is probably not localized in plasma membrane and does not mediate efflux of alkali metal cations from cells, but is important for the regulation of intracelular cation homeostasis and optimal pH control, similarly as the Nhx1p.
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