Potassium and sodium transport in non-animal cells: the Trk/Ktr/HKT transporter family

Abstract

Bacterial Trk and Ktr, fungal Trk and plant HKT form a family of membrane transporters permeable to K(+) and/or Na(+) and characterized by a common structure probably derived from an ancestral K(+) channel subunit. This transporter family, specific of non-animal cells, displays a large diversity in terms of ionic permeability, affinity and energetic coupling (H(+)-K(+) or Na(+)-K(+) symport, K(+) or Na(+) uniport), which might reflect a high need for adaptation in organisms living in fluctuating or dilute environments. Trk/Ktr/HKT transporters are involved in diverse functions, from K(+) or Na(+) uptake to membrane potential control, adaptation to osmotic or salt stress, or Na(+) recirculation from shoots to roots in plants. Structural analyses of bacterial Ktr point to multimeric structures physically interacting with regulatory subunits. Elucidation of Trk/Ktr/HKT protein structures along with characterization of mutated transporters could highlight functional and evolutionary relationships between ion channels and transporters displaying channel-like features.

Fig. 1

Proposed evolution of the Trk/Ktr/HKT family. Fungal Trk and plant HKT are single subunit systems whereas bacterial Trk and Ktr systems are multimeric complexes, associating an ion-conducting transmembrane subunit (TrkG, H or I, and KtrB or D) to at least one peripheral regulatory subunit (TrkA and KtrA or C). Bacterial Trk and Ktr ion-conducting subunits, fungal Trk, and plant HKT could have originated from a common K⁺ channel ancestor subunit with one MPM domain by a process of gene duplication and fusion [54]. Bacterial TrkA- and KtrA-type peripheral regulatory subunits could have been derived from the cleavage of the cytoplasmic C-terminal domain of this K⁺ channel ancestor subunit, thus forming a distinct regulatory subunit [54]

Fig. 2

Phylogenetic tree of the Trk/Ktr/HKT transporter family. Four branches, corresponding to fungal Trk and plant HKT transporters, and to bacterial Trk and Ktr ion-conducting subunits, are distinguished within the phylogenetic tree. The bacterial KcsA K⁺ channel displays a closer phylogenetic relationship to bacterial Trk and Ktr transporters than to the eukaryotic fungal Trk and plant HKT. Inset Phylogenetic tree of the bacterial TrkA and KtrA-type regulatory subunits. Multiple alignments of Trk/Ktr/HKT full polypeptides were performed using CLUSTALW 1.8. Phylogenetic trees were generated with the distance-based neighbor joining program NEIGHBOR from Phylip (version 3.573c, http://www.phylogeny.fr), and visualized using Phylodendron (http://iubio.bio.indiana.edu/treeapp/treeprint-form.html). One hundred bootstrap replicates were performed for the ion-conducting proteins and 1,000 for the regulatory proteins. Accession numbers of ion-conducting proteins: AaKtrB NP_214039; AtHKT1;1 NP_567354.1 (At4g10310); BsKtrB NP_390988; BsKtrD NP_389233; EcTrkG NP_415881; EcTrkH AAA67646; EhNtpJ BAA04279; HcTrk1 CAL36606.1; HeTrkH AAR91792; HeTrkI AAR91793; LbTrk1 XP_001879952.1; LbTrk2 XP_001880417.1; NcTrk1 XP_957340.1; NcTrk2 XP_959511.1; OsHKT1;1 Q7XPF8.2; OsHKT1;3 Q6H501.1; OsHKT1;4 Q7XPF7.2; OsHKT1;5 Q0JNB6.1; OsHKT2;1 Q0D9S3.1; OsHKT2;2 BAB61791.1; OsHKT2;3 Q8L481.1; OsHKT2;4 Q8L4K5.1; PaTrk1 XP_001905383.1; PaTrk2 XP_001908358.1; PpHKT XP_001763760.1; PtHKT1;1 XP_002325229.1; ScTrk1 NP_012406.1; ScTrk2 NP_012976.1; SIKcsA CAA86025.1; SpTrk1 NP_593934.1; SpTrk2 XP_001713037.1; SspNtpJ NP_441336.1; VaKtrB ZP_01261970.1; VaKtrD ZP_01258579.1; VaTrkG ZP_01262256.1; VaTrkH ZP_01261138.1. Accession numbers of regulatory subunits: BsKtrA NP_390541.1; BsKtrC NP_389334; EcTrkA NP_417748.1; HeTrkA AAR91791; SspKtrA NP_442775; VaKtrA ZP_01261971.1; VaTrkA ZP_01261140.1. Aa, Aquifex aeolicus VF5; At, Arabidopsis thaliana; Bs, Bacillus subtilis; Ec, Escherichia coli; Eh, Enterococcus hirae; Hc, Hebeloma cylindrosporum; He, Halomonas elongata; Lb, Laccaria bicolor; Nc, Neurospora crassa; Os, Oryza sativa; Pa, Podospora anserina; Pp, Physcomitrella patens; Pt, Populus trichocarpa; Sc, Saccharomyces cerevisiae; Sl, Streptomyces lividans; Sp, Schizosaccharomyces pombe; Ssp, Synechocystis sp. PCC6803; Va, Vibrio alginolyticus

Fig. 3

Functions of HKT transporters in planta. The schematic diagram presents three HKT transporters for which a role in planta has been obtained through complementary experimental approaches including localization at the tissue/cell level, functional characterization in heterologous systems and analysis of mutant plant phenotype. The rice OsHKT2;1 transporter plays a role in root Na⁺ uptake [14, 40]. AtHKT1;1, the only HKT member in Arabidopsis thaliana, and OsHKT1;5 from rice are involved in protecting plant leaves from Na⁺ over-accumulation during salinity stress [30, 34, 124, 128]. Ep epidermis, Cp cortex parenchyma, En endodermis, Pe pericycle, Xp xylem parenchyma cell, Bs bundle sheath cell, Mc mesophyll cell, Cc companion cell

Fig. 4

Structure–function relationship of Trk/Ktr/HKT transporters: compilation of data from mutagenesis studies using the four-MPM structural model. a Data from site-directed mutagenesis studies (open symbols; circle single-point mutation; triangle deletion) and from random mutagenesis (filled circles) are tentatively compiled along a diagrammatic representation of the four-MPM model, independently of the nature of the studied transporters (bacterial Ktr, fungal Trk, or plant HKT). Mutations are designated with numbers (facing the corresponding symbols in the diagrammatic representation). For each mutation (number), information regarding the corresponding organism and transporter, the mutated residue(s) and the literature reference is provided in the list below the diagram. b Transporters frequently used in site-directed mutagenesis analyses are the bacterial KtrB from V. alginolyticus and Synechocystis, the fungal Trk from S. cerevisiae and the plant transporters AtHKT1 from A. thaliana and TaHKT1 from T. aestivum. Sequence alignments of the four P regions (P_A to P_D) and of the last M2 transmembrane segment (M2_D) from these four transporter proteins are shown. Mutated residues are boxed in red