Potassium uptake systems of Candida krusei

Candida krusei is a pathogenic yeast species that is phylogenetically outside both of the well‐studied yeast groups, whole genome duplication and CUG. Like all other yeast species, it needs to accumulate high amounts of potassium cations, which are needed for proliferation and many other cell functions. A search in the sequenced genomes of nine C. krusei strains revealed the existence of two highly conserved genes encoding putative potassium uptake systems. Both of them belong to the TRK family, whose members have been found in all the sequenced genomes of species from the Saccharomycetales subclade. Analysis and comparison of the two C. krusei Trk sequences revealed all the typical features of yeast Trk proteins but also an unusual extension of the Ck Trk2 hydrophilic N‐terminus. The expression of both putative CkTRK genes in Saccharomyces cerevisiae lacking its own potassium importers showed that only Ck Trk1 is able to complement the absence of S. cerevisiae 's own transporters and provide cells with a sufficient amount of potassium. Interestingly, a portion of the Ck Trk1 molecules were localized to the vacuolar membrane. The presence of Ck Trk2 had no evident phenotype, due to the fact that this protein was not correctly targeted to the S. cerevisiae plasma membrane. Thus, Ck Trk2 is the first studied yeast Trk protein to date that was not properly recognized and targeted to the plasma membrane upon heterologous expression in S. cerevisiae .