Novel P-Type ATPases Mediate High-Affinity Potassium or Sodium Uptake in Fungi

Abstract

Fungi have an absolute requirement for K ⁺ , but K ⁺ may be partially replaced by Na ⁺ . Na ⁺ uptake in Ustilago maydis and Pichia sorbitophila was found to exhibit a fast rate, low K _m , and apparent independence of the membrane potential. Searches of sequences with similarity to P-type ATPases in databases allowed us to identify three genes in these species, Umacu1 , Umacu2 , and PsACU1 , that could encode P-type ATPases of a novel type. Deletion of the acu1 and acu2 genes proved that they encoded the transporters that mediated the high-affinity Na ⁺ uptake of U. maydis . Heterologous expressions of the Umacu2 gene in K ⁺ transport mutants of Saccharomyces cerevisiae and transport studies in the single and double Δ acu1 and Δ acu2 mutants of U. maydis revealed that the acu1 and acu2 genes encode transporters that mediated high-affinity K ⁺ uptake in addition to Na ⁺ uptake. Other fungi also have genes or pseudogenes whose translated sequences show high similarity to the ACU proteins of U. maydis and P. sorbitophila . In the phylogenetic tree of P-type ATPases all the identified ACU ATPases define a new cluster, which shows the lowest divergence with type IIC, animal Na ⁺ ,K ⁺ -ATPases. The fungal high-affinity Na ⁺ uptake mediated by ACU ATPases is functionally identical to the uptake that is mediated by some plant HKT transporters.