Molecular analysis of plant Na and K transporter involved in response to environmental stress

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1 The Ktr-system in Synechocystis sp.PCC 6803 was found to be Na-activated K transporter. The K transport activity was inhibited by CCCP and was not found in the mutant strain which has a mutation of the respiratory chain system. These indicate that proton motive force is involved in the Ktr-mediated K transport. Hyperosmotic stress led initially to loss of net K from the cells. The ktrB mutant shocked with sorbitol failed to reaccumulate K up to its original level. The K uptake via KtrABE palys a crucial role in the early phase of cell turgor regulation after hyperosmotic shock.2 Bacterial Ktr system and plant HKT-system has eight transmembrane topology. The positive residue in the eighth transmembrane segment was replaced with other amino acids. The conversion from Arg to Lys sustained the K uptake activity but that to negative or non-polar amino acids gave loss of the function. The positive residue is important for the activity.3 KUP-type transporter was not inserted into the rough microsomal membrane. Then the transporter was expressed in E.coli and their membrane structure was determined.4 Some of plant K channels has the putative Ca binding sites. The mutation in the region inhibited the K uptake activity. The region is important for the transport activity.5 Arabidopsis AtHKT1 transporter was expressed in the vascular tissuels in the aerial part of the plant. The null mutant showed that Na accumulated in the xylem tissue and Na was not supplied into the phloem tissue. AtHKT1 is one of the components for Na recirculation of plant.