Potassium transport by amino acid permeases in Saccharomyces cerevisiae

Deletion of the potassium transporter genes TRK1 and TRK2 impairs potassium uptake in Saccharomyces cerevisiae, resulting in a greatly increased requirement for the ion and the inability to grow on low pH medium. Selection for mutations that restored growth of trk1Δ trk2Δ cells on low pH (3.0) medium led to the isolation of a dominant suppressor that also partially suppressed the increased K⁺ requirement of these cells. Molecular analysis revealed the suppressor to be an allele of BAP2 that encodes a permease for branched chain amino acids. The suppressor mutation (BAP2-1) converts a phenylalanine codon, highly conserved among the amino acid permease genes, to a serine codon in a region predicted to lie within the sixth membrane-spanning domain. Generation of the analogous mutation in the histidine permease produced an allele, HIP1-293, that similarly suppressed the low pH sensitivity of trk1Δ trk2Δ cells. Suppression of trk1Δ trk2Δ phenotypes by BAP2-1 or HIP1-293 was correlated with increased Rb⁺ uptake. The presence of the substrate amino acids enhanced but was not essential for suppression of trk1Δ trk2Δ phenotypes and increased Rb⁺ uptake. The conserved site altered by the suppressor mutations appears to be important; his4 HIP1-293 cells show an increased requirement for histidine compared with his4 HIP1 cells.