, 2009). Interestingly, CaTrk1 and CaTok1 have been proposed to be the effectors in killing C. albicans with the cationic protein Histatin 5, with Trk1 providing the essential pathway for the ATP loss observed during treatment with this toxic protein (Baev et al., 2004). Many nonconventional yeasts (Hansenula polymorpha, Debaryomyces, C. albicans) as Ion Channel Ligand Library in vitro well as mycelial fungi (Neurospora crassa; Haro et
al., 1999) contain, besides Trk, a second type of K+ transporter coded by HAK genes (High Affinity K+ transporter) (Table 1; Rodriguez-Navarro, 2000; Arino et al., 2010). Yeast HAK transporters are homologous to the Kup system of Escherichia coli. The role of Hak1 in potassium transport has been studied in two Debaryomyces species, D. occidentalis (Banuelos et al., 1995, 2000) and D. hansenii (Prista et al., 2007), containing both the TRK1 and HAK1 genes. Heterologous expression of DoHAK1 in S. cerevisiae mutants with defective K+ uptake improved both their growth at low K+- and potassium-transport capacity. It has been proposed that HAK transporters work as K+–H+ symporters with a high concentrative capacity and that they are expressed under K+ starvation. Under certain conditions, Na+ can substitute for H+ in D. hansenii and in this case, K+–Na+ symport would be the operating Ku-0059436 chemical structure mechanism of transport. The expression of DhHAK1
requires not only low external K+ but also low Na+, because in the absence of K+, the presence of Na+ prevents the expression of the gene. Further, the addition of millimolar concentrations of Astemizole either K+ or Na+ to D. hansenii cells provokes a fast decrease in HAK1 gene expression (Martínez et al., 2011). The existence of a third type of K+ uptake system, ACU ATPases (Alkali Cation Uptake), has been reported recently. This system is not widely distributed in nonconventional yeasts, but is present in some of them, such as Ustilago maydis or Pichia sorbitophila (Benito et al., 2004). The ACU ATPases form
a novel subfamily of P-type ATPases involved in high-affinity K+ or Na+ uptake. In U. maydis, two ACU genes have been identified and studied (Umacu1 and Umacu2). Deletion of the acu1 and acu2 genes and subsequent transport studies showed that they encode transporters mediating a high-affinity K+ and Na+ uptake. This finding was also confirmed by the heterologous expression of UmAcu2 ATPase in S. cerevisiae mutants. Besides P. sorbitophila, other yeasts have genes or pseudogenes whose translated sequences show high similarity to the Acu proteins of U. maydis (Benito et al., 2004), for example Pichia stipitis (Jeffries et al., 2007). Whereas a database search indicates that the genomes of most Candida, Zygosaccharomyces, Yarrowia or Pichia yeast species contain a gene orthologous to the S. cerevisiae TOK1 (coding for the only known yeast outward K+ rectifier), the best-known nonconventional yeasts S. pombe and D. hansenii seem to lack a similar system.