Structures and ion transport mechanisms of plant high-affinity potassium transporters

Plant high-affinity K^(+) transporters(HKTs)mediate Na^(+) and K^(+) uptake,maintain Na^(+)/K^(+) homeostasis,and therefore play crucial roles in plant salt tolerance.In this study,we present cryoelectron microscopy structures of HKTs from two classes,classI HKT1;1 from Arabidopsis thaliana(AtHKT1;1)and class II HKT2;1 from Triticum aestivum(TaHKT2;1),in both Na^(+) -and K^(+) -bound states at 2.6-to 3.0-A resolutions.BothAtHKT1;1and TaHKT2;1function ashomodimers.Each HKT subunit consists of four tan-dem domain units(D1-D4)with a repeated K^(+) -channel-like M-P-M topology.In each subunit,D1-D4 assemble into an ion conduction pore with a pseudo-four-fold symmetry.Although both TaHKT2;1and AtHKT1;1 have only one putative Na^(+) ion bound in the selectivity filter with a similar coordination pattern,the two HKTs display different K^(+) binding modes in the filter.TaHKT2;1 has three K^(+) ions bound in the selec-tivity filter,but AtHKT1;1 has only two K^(+) ions bound in the filter,which has a narrowed external entrance due to the presence of a Ser residue in the first filter motif.These structures,along with compu-tational,mutational,and electrophysiological analyses,enable us to pinpoint key residues that are critical for the ion selectivity of HKTs.The findings provide new insights into the ion selectivity and ion transport mechanisms of plant HKTs and improve our understanding about how HKTs mediate plant salt tolerance and enhance crop growth.