We have identified the two main K(+) transporters in the yeast Hansenula polymorpha. So far this is the only yeast with these transporters amenable to molecular genetic analysis. Two ORF-encoding permeases with high similarity to Trk1 and Hak1 are present in the genome of this yeast. Deletion of either of these genes led to defective growth in low K(+). The K(+) and Rb(+) uptake rates showed high affinity of Hak1 for K(+), while the affinity estimated for Trk1 was two orders of magnitude lower. TRK1 was not transcriptionally regulated and HAK1 was strongly induced in response to very low K(+) and down-regulated by the presence of K(+). This process is clearly dependent on calcineurin. The use of a set of strains carrying mutations affecting intracellular protein trafficking revealed that in response to K(+), Hak1 is endocytosed and degraded in the vacuole, this depending on the ubiquitin ligase Rsp5. This is a first insight into the transcriptional and post-translational mechanisms regulating a high-affinity K(+) transporter (HAK-type transporter) that allows cells to respond and adapt to K(+) availability.
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