A NEW FAMILY OF KC TRANSPORTERS FROM ARABIDOPSIS THAT ARE CONSERVED ACROSS PHYLA

Transport of K+ in higher plants, as in bacteria and fungi, is mediate d by two broad classes of transport proteins that operate in the milli molar and micromolar K+ concentration ranges. A search of the Expresse d Sequence Tag database using amino acid consensus sequences for the K + transporters HAK1 from Schwanniomyces and Kup of Escherichia coli yi elded two homologous sequences for Arabidopsis. Cloning and sequencing of these genes gave single open reading frames for the putative trans porters, AtKT1 and AtKT2, with predicted molecular weights of 79 and 8 8 kDa. The predicted gene products showed a high degree of homology at the amino acid level (56% identity) and exhibited significant hydroph obic stretches in their N-terminal halves, consistent with 12 membrane -spanning, alpha-helical domains. Database searches using AtKT1 and At KT2 identified 10 additional sequences in Arabidopsis as well as addit ional homologous sequences in the plant species Oryza and Allium, the bacterium Lactococcus lactis, and in Homo sapiens. Expression of AtKT2 rescued growth on low millimolar [K+] in Saccharomyces cerevisiae car rying deletions for the genes encoding the K+ transporters TRK1 and TR K2. Rescue was associated with a 2-fold stimulation of Rb+ uptake and was sensitive to competition with external Na+ but not to extracellula r pH, indicating that the gene encodes a low-affinity K+ transporter. These and additional results suggest that AtKT1 and AtKT2 belong to a superfamily of cation transporters that have been conserved through ev olution. (C) 1997 Federation of European Biochemical Societies.