Evidence in support of a four transmembrane-pore-transmembrane topology model for the
 Arabidopsis thaliana
 Na
 +
 /K
 +
 translocating AtHKT1 protein, a member of the superfamily of K
 +
 transporters

Kato, Yasuhiro; Sakaguchi, Masao; Mori, Yasuo; Saito, Kumiko; Nakamura, Tatsunosuke; Bakker, Evert P.; Sato, Yoko; Goshima, Shinobu; Uozumi, Nobuyuki

doi:10.1073/pnas.101556598

Cited by 127 publications

(110 citation statements)

References 49 publications

(76 reference statements)

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“…Each MPM domain of HKT/Trk/Ktr transporters harbors sequence and structure similarities with K + channel subunits that display a single MPM domain and associate into tetramers to form functional channels . The eight transmembrane segments of the core region of HKT/Trk/Ktr transporters are organized with a 4-fold symmetry around a central permeation pathway (pore) lined by the four pore loop domains Kato et al, 2001). This model has recently been confirmed by the elucidation of the crystal structure of a bacterial member from the HKT/Trk/Ktr superfamily, VpTrkH from Vibrio parahaemolyticus (Cao et al, 2011).…”

mentioning

confidence: 57%

The Rice Monovalent Cation Transporter OsHKT2;4: Revisited Ionic Selectivity

et al. 2012

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The family of plant membrane transporters named HKT (for high-affinity K+ transporters) can be subdivided into subfamilies 1 and 2, which, respectively, comprise Na+-selective transporters and transporters able to function as Na+-K+ symporters, at least when expressed in yeast (Saccharomyces cerevisiae) or Xenopus oocytes. Surprisingly, a subfamily 2 member from rice (Oryza sativa), OsHKT2;4, has been proposed to form cation/K+ channels or transporters permeable to Ca2+ when expressed in Xenopus oocytes. Here, OsHKT2;4 functional properties were reassessed in Xenopus oocytes. A Ca2+ permeability through OsHKT2;4 was not detected, even at very low external K+ concentration, as shown by highly negative OsHKT2;4 zero-current potential in high Ca2+ conditions and lack of sensitivity of OsHKT2;4 zero-current potential and conductance to external Ca2+. The Ca2+ permeability previously attributed to OsHKT2;4 probably resulted from activation of an endogenous oocyte conductance. OsHKT2;4 displayed a high permeability to K+ compared with that to Na+ (permeability sequence: K+ > Rb+ ≈ Cs+ > Na+ ≈ Li+ ≈ NH4 +). Examination of OsHKT2;4 current sensitivity to external pH suggested that H+ is not significantly permeant through OsHKT2;4 in most physiological ionic conditions. Further analyses in media containing both Na+ and K+ indicated that OsHKT2;4 functions as K+-selective transporter at low external Na+, but transports also Na+ at high (>10 mm) Na+ concentrations. These data identify OsHKT2;4 as a new functional type in the K+ and Na+-permeable HKT transporter subfamily. Furthermore, the high permeability to K+ in OsHKT2;4 supports the hypothesis that this system is dedicated to K+ transport in the plant.

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confidence: 57%

The Rice Monovalent Cation Transporter OsHKT2;4: Revisited Ionic Selectivity

et al. 2012

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“…The structure of K ϩ channel pore is well characterized, and the M1-P-M2 structure found in K ϩ channels is also seen in Na ϩ and Ca 2ϩ channels and Na ϩ /K ϩ transporters from bacteria and plants (10,12,13,40). The pore-forming structure is therefore ubiquitous in cat- ion transport proteins.…”

Section: Discussionmentioning

confidence: 86%

Topogenesis of Two Transmembrane Type K+ Channels, Kir 2.1 and KcsA

Umigai

Sato

Mizutani

et al. 2003

Journal of Biological Chemistry

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Potassium channels, which control the passage of K ؉ across cell membranes, have two transmembrane segments, M1 and M2, separated by a hydrophobic P region containing a highly conserved signature sequence. Here we analyzed the membrane topogenesis characteristics of the M1, M2, and P regions in two animal and bacterial two-transmembrane segment-type K ؉ channels, Kir 2.1 and KcsA, using an in vitro translation and translocation system. In contrast to the equivalent transmembrane segment, S5, in the voltage-dependent K ؉ channel, KAT1, the M1 segment in KcsA, was found to have a strong type II signal-anchor function, which favors the N cyt /C exo topology. The N-terminal cytoplasmic region was required for efficient, correctly orientated integration of M1 in Kir 2.1. Analysis of N-terminal modification by in vitro metabolic labeling showed that the N terminus in Kir 2.1 was acetylated. The hydrophobic P region showed no topogenic function, allowing it to form a loop, but not a transmembrane structure in the membrane; this region was transiently exposed in the endoplasmic reticulum lumen during the membrane integration process. M2 was found to possess a stop-transfer function and a type I signalanchor function, enabling it to span the membrane. The C-terminal cytoplasmic region in KcsA was found to affect the efficiency with which the M2 achieved their final structure. Comparative topogenesis studies of Kir 2.1 and KcsA allowed quantification of the relative contributions of each segment and the cytoplasmic regions to the membrane topology of these two proteins. The membrane topogenesis of the pore-forming structure is discussed using results for Kir 2.1, KcsA, and KAT1.

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“…HKT1 and AtHKT1 have sequence similarity to the yeast Trk1-2 and bacterial KtrAB transporters (33,35,36) that are hypothesized to have evolved from bacterial KcsA-like K ϩ channels (35). The defining structure is composed of two transmembrane helices (M1 and M2) and an intervening hairpin segment (MPM) that forms the outer portion of the pore and mediates ion selectivity.…”

Section: K ؉ Deficiency Phenotype Of Sos3-1 Mutant Is Suppressed By Hkt1mentioning

confidence: 99%

AtHKT1 is a salt tolerance determinant that controls Na ⁺ entry into plant roots

Rus

Yokoi

Sharkhuu

et al. 2001

Proc. Natl. Acad. Sci. U.S.A.

446

313

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Two Arabidopsis thaliana extragenic mutations that suppress NaCl hypersensitivity of the sos3-1 mutant were identified in a screen of a T-DNA insertion population in the genetic background of Col-0 gl1 sos3-1. Analysis of the genome sequence in the region flanking the T-DNA left border indicated that sos3-1 hkt1-1 and sos3-1 hkt1-2 plants have allelic mutations in AtHKT1. AtHKT1 mRNA is more abundant in roots than shoots of wild-type plants but is not detected in plants of either mutant, indicating that this gene is inactivated by the mutations. hkt1-1 and hkt1-2 mutations can suppress to an equivalent extent the Na ؉ sensitivity of sos3-1 seedlings and reduce the intracellular accumulation of this cytotoxic ion. Moreover, sos3-1 hkt1-1 and sos3-1 hkt1-2 seedlings are able to maintain [K ؉ ]int in medium supplemented with NaCl and exhibit a substantially higher intracellular ratio of K ؉ ͞Na ؉ than the sos3-1 mutant. Furthermore, the hkt1 mutations abrogate the growth inhibition of the sos3-1 mutant that is caused by K ؉ deficiency on culture medium with low Ca 2؉ (0.15 mM) and <200 M K ؉ . Interestingly, the capacity of hkt1 mutations to suppress the Na ؉ hypersensitivity of the sos3-1 mutant is reduced substantially when seedlings are grown in medium with low Ca 2؉ (0.15 mM). These results indicate that AtHKT1 is a salt tolerance determinant that controls Na ؉ entry and high affinity K ؉ uptake. The hkt1 mutations have revealed the existence of another Na ؉ influx system(s) whose activity is reduced by high [Ca 2؉ ]ext. H igh [NaCl] ext disturbs intracellular ion homeostasis of plants, which leads to membrane dysfunction, attenuation of metabolic activity, and secondary effects that cause growth inhibition and lead ultimately to cell death (1). Both glycophytes and halophytes use a similar strategy that involves regulation of net Na ϩ flux across the plasma membrane and vacuolar compartmentalization of the internalized cation to mediate intracellular Na ϩ homeostasis. This strategy requires the coordinated function of numerous ion transport determinants and effectively partitions the toxic ion away from critical cytosolic and organellar machinery. Under conditions of high [Na ϩ ] ext , the functioning of these determinants also facilitates the use of Na ϩ as an osmolyte to mediate osmotic adjustment that is necessary for cell expansion (1-3). Because vacuolar expansion is the primary mechanism of plant cell enlargement, this strategy is likely to be an essential adaptation to saline environments.Recently, putative plasma membrane and tonoplast localized Na ϩ ͞H ϩ transporters were identified in plants that are presumed to mediate energized transport of Na ϩ outward from the cytosol to the apoplast or into the vacuole (4-7). These transporters are apparently the molecular effectors of Na ϩ ͞H ϩ antiporter activities associated with plasma membrane and tonoplast vesicles that were described more than a decade ago (1,3,8,9). The plasma membrane Na ϩ ͞H ϩ

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Evidence in support of a four transmembrane-pore-transmembrane topology model for the Arabidopsis thaliana Na ⁺ /K ⁺ translocating AtHKT1 protein, a member of the superfamily of K ⁺ transporters

Cited by 127 publications

References 49 publications

The Rice Monovalent Cation Transporter OsHKT2;4: Revisited Ionic Selectivity

The Rice Monovalent Cation Transporter OsHKT2;4: Revisited Ionic Selectivity

Topogenesis of Two Transmembrane Type K+ Channels, Kir 2.1 and KcsA

AtHKT1 is a salt tolerance determinant that controls Na ⁺ entry into plant roots

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Evidence in support of a four transmembrane-pore-transmembrane topology model for the Arabidopsis thaliana Na + /K + translocating AtHKT1 protein, a member of the superfamily of K + transporters

Cited by 127 publications

References 49 publications

The Rice Monovalent Cation Transporter OsHKT2;4: Revisited Ionic Selectivity

The Rice Monovalent Cation Transporter OsHKT2;4: Revisited Ionic Selectivity

Topogenesis of Two Transmembrane Type K+ Channels, Kir 2.1 and KcsA

AtHKT1 is a salt tolerance determinant that controls Na + entry into plant roots

Contact Info

Product

Resources

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Evidence in support of a four transmembrane-pore-transmembrane topology model for the Arabidopsis thaliana Na ⁺ /K ⁺ translocating AtHKT1 protein, a member of the superfamily of K ⁺ transporters

AtHKT1 is a salt tolerance determinant that controls Na ⁺ entry into plant roots