A. HAK transporters of groups I and II: Identification of amino acid involved in the affinity for K+

SENN, M. E.; RUBIO, F.; BAÑUELOS, M. A.; RODRIGUEZ-NAVARRO, A.

Madison. USA

Workshop; 12th International Workshop on Plant Membrane Biology; 2001

Potassium is an essential nutrient for plants that fulfills important functions related to osmoregulation, enzyme activity and movement. Plants take up K+ from the soil solution where the cation concentration is highly variable and several orders of magnitude lower than in the cytoplasm. Once K+ is taken up by the roots, it is redistributed through out the plant via xylem and phloem to make it available to all plant cells which accumulate much of it in the vacuole. Several K+ transport systems have been characterized that mediate the movement of K+ through the different membranes and recently genes encoding some of the K+ transporters have been identified. The HAK/KUP/KT genes comprise a large gene family present in many plant, fungi and bacterial species. A phylogenetic study of the family shows that the plant transporters cluster in four groups. In barley, two cDNAs HvHAK1 and HvHAK2 were isolated that encoded putative K+ transporters. HvHAK1 expressed in yeast mediated high-affinity K+ transport and it has been postulated that it is one of the major contributors to K+ absorption from low external K+ concentration. HvHAK2 did not produce functional complementation in yeast and its function could not studied. Several amino acids are conserved between HvHAK1 and HvHAK2 and others are exclusive of the phylogenetic groups to which both transporters belong. Here we report that when amino acid residues characteristic of HvHAK2 are substituted for the corresponding HvHAK1 residues, HvHAK1 affinity for K+ decreases. Moreover, expression of HvHAK2 in bacteria suggest a low affinity of the transporter for K+. Physiological implications of these results will be presented.