Glycine residues in potassium channel-like selectivity filters determine potassium selectivity in four-loop-per-subunit HKT transporters from plants.
Plant HKT proteins comprise a family of cation transporters together with prokaryotic KtrB, TrkH, and KdpA transporter subunits and fungal Trk proteins. These transporters contain four loop domains in one polypeptide with a proposed distant homology to K(+) channel selectivity filters. Functional expression in yeast and Xenopus oocytes revealed that wheat HKT1 mediates Na(+)-coupled K(+) transport. Arabidopsis AtHKT1, however, transports only Na(+) in eukaryotic expression systems. To understand the molecular basis of this difference we constructed a series of AtHKT1/HKT1 chimeras and introduced point mutations to AtHKT1 and wheat HKT1 at positions predicted to be critical for K(+) selectivity. A single-point mutation, Ser-68 to glycine, was sufficient to restore K(+) permeability to AtHKT1. The reverse mutation in HKT1, Gly-91 to serine, abrogated K(+) permeability. This glycine in P-loop A of AtHKT1 and HKT1 can be modeled as the first glycine of the K(+) channel selectivity filter GYG motif. The importance of such filter glycines for K(+) selectivity was confirmed by interconversion of Ser-88 and Gly-88 in the rice paralogues OsHKT1 and OsHKT2. Surprisingly, all HKT homologues known from dicots have a serine at the filter position in P-loop A, suggesting that these proteins function mainly as Na(+) transporters in plants and that Na(+)/K(+) symport in HKT proteins is associated with a glycine in the filter residue. These data provide experimental evidence that the glycine residues in selectivity filters of HKT proteins are structurally related to those of K(+) channels.
Mäser
Pascal
P
Division of Biology, Cell and Developmental Biology Section, and Center for Molecular Genetics, University of California at San Diego, La Jolla, CA 92093-0116, USA.
Hosoo
Yoshihiro
Y
Goshima
Shinobu
S
Horie
Tomoaki
T
Eckelman
Brendan
B
Yamada
Katsuyuki
K
Yoshida
Kazuya
K
Bakker
Evert P
EP
Shinmyo
Atsuhiko
A
Oiki
Shigetoshi
S
Schroeder
Julian I
JI
Uozumi
Nobuyuki
N
D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D017360:Arabidopsis / Q000378:metabolism; D029681:Arabidopsis Proteins; D027682:Cation Transport Proteins / Q000378:metabolism*; D005998:Glycine / Q000737:chemistry*; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D009865:Oocytes / Q000378:metabolism; D010802:Phylogeny; D010940:Plant Proteins / Q000378:metabolism*; D010957:Plasmids / Q000378:metabolism; D017354:Point Mutation; D011188:Potassium / Q000378:metabolism; D015221:Potassium Channels / Q000737:chemistry*; D017434:Protein Structure, Tertiary; D012492:Salts / Q000494:pharmacology; D012694:Serine / Q000737:chemistry; D027981:Symporters / Q000378:metabolism*; D014981:Xenopus
D029681:Arabidopsis Proteins; D027682:Cation Transport Proteins; C490286:HKT1 protein, Arabidopsis; C088864:HKT1 protein, plant; D010940:Plant Proteins; D015221:Potassium Channels; D012492:Salts; D027981:Symporters; D012694:Serine; D011188:Potassium; D005998:Glycine