2.A.38 The K+ Transporter (Trk) Family

The proteins of the Trk family are derived from Gram-negative and Gram-positive bacteria, yeast and plants. The proteins of E. coli K12 (TrkH and TrkG) as well as several yeast and plant proteins have been functionally and topologically characterized. While TrkH is generally present in E. coli and other enteric bacteria, TrkG is encoded by a foreign gene located within the prophage rac region of the E. coli K12 chromosome. TrkG is not present in several other E. coli strains. The sizes of the Trk family members vary from 423 residues to 1235 residues. The bacterial proteins are of 423-558 residues, the Triticum aestivum protein is 533 residues, and the yeast proteins vary between 841 and 1241 residues. These proteins possess 8 putative transmembrane α-helical spanners. An 8 TMS topology with N- and C-termini on the inside, has been established for AtHKT1 of A. thaliana (Kato et al., 2001) and Trk2 of S. cerevisiae (Zeng et al., 2004). This folding pattern resembles quadruplicated primitive K+ channels of the VIC superfamily (TC #1.A.1) instead of typical 12 TMS carriers (Matsuda et al., 2004). As homology has been established between Trk carriers and VIC family channels, the latter were presumably the precursors of the former.

The phylogenetic tree reveals that the proteins cluster according to phylogeny of the source organism with (1) the Gram-negative bacterial Trk proteins, (2) the Gram-negative and Gram-positive bacterial Ktr proteins, (3) the yeast proteins and (4) the plant proteins comprising four distinct clusters (Saier et al., 1999). S. cerevisiae possesses two paralogues, high- and low-affinity K+ transporters.

The KtrAB system of Vibrio alginolyticus consists of a cytoplasmic ATP-binding regulatory subunit, KtrA (Kroning et al., 2007), shared by several other K+ transporters and channels including an integral membrane protein, KtrB, which is very similar to NtpJ of E. hirae and distantly related to TrkH of E. coli (Nakamura et al., 1998b). KtrA binds ATP to a β-α-β-Rossman fold. Other nucleotides bound, but only ATP bound with high affinity and changed the conformation of KtrA so that KtrA and KtrB associate (Kroning et al., 2007) in vivo, both ATP and ΔΨ were required for transport activity. All of these systems may be multicomponent. However, the integral membrane constituents have been proposed to exhibit a basic structure of four consecutive M1-P-loop-M2 motifs analogous to the KcsA K+ channel of Streptomyces lividans (see TC #1.A.1). Supporting this model, modification of the four putative selectivity filter glycine residues (one in each P-loop) in KtrB of V. alginolyticus altered the K+ affinity (100 fold) and selectivity, altering the K+/Na+ selectivity (Tholema et al., 2005). Two homologues of the V. alginolyticus KtrAB are found in B. subtilis. Both systems catalyze K+ uptake but with differing Kms: 1 mM for KtrAB and 10 mM for KtrCD (Holtmann et al., 2003).

The E. coli TrkH and TrkG proteins are complexed to two peripheral membrane proteins, TrkA, the ATP/NAD+-binding protein, and TrkE, an ATP-binding protein. The peripheral membrane proteins are thought to function in regulation rather than energy coupling. TrkE maps to the sapABCDF operon which encodes an ABC transporter (TC #3.A.1.5.5), and the SapD ATP binding cassette (ABC) protein of E. coli can stimulate K+ uptake via either TrkH or TrkG (Harms et al., 2001). Therefore, SapD is probably TrkE. ATP binding to SapD, rather than ATP hydrolysis, appears to activate. Thus, the pmf drives transport while ATP binding activates transport. At least one other ATP-activating protein is probably present in the E. coli cell (Harms et al., 2001).

TrkA family members regulate various K+ transporters in all three domains of life. These regulatory subunits are generally called K+ transport/nucleotide binding subunits (Bateman et al., 2000). Systems regulated include TrkG/H (TC #2.A.38.1.1) regulated by TrkA (with two KTN domains) and KtrB (TC #2.A.38.4.2) regulated by KtrA (Roosild et al., 2002). In addition, homologous regulatory domains are sometimes incorporated into the transporter polypeptide chains as in the case of the KefC and KefB proteins of E. coli (TC #2.A.37.1.1 and 2.A.37.1.2, respectively).

TrkA domains can bind NAD+ and NADH, possibly allowing K+ transporters to be responsive to the redox state of the cell. The ratio of NADH/NAD+ may control gating. Multiple crystal structures of two KTN domains complexed with NAD+ or NADH reveal that these ligands control the oligomeric (tetrameric) state of KTN. The results suggest that KTN is inherently flexible, undergoing a large conformational change through a hinge motion (Roosild et al., 2002). The KTN domains of Kef channels interact dynamically with the transporter. The KTN conformation then controls permease activity (Roosild et al., 2002). 

Both yeast transport systems are believed to function by K+:H+ symport, but the wheat protein functions by K+:Na+ symport. It is possible that some of these proteins can function by a channel-type mechanism. Positively charged residues in TMS8 of several ktr/Trk/HKT transporters probably face the channel and block a conformational change that is essential for channel activity while allowing secondary active transport (Kato et al., 2007).

KtrB is the K+-translocating subunit of the K+-uptake system KtrAB. It is a member of the superfamily of K+transporters (SKT proteins) with other sub-families occurring in archaea, bacteria, fungi, plants and trypanosomes (Hänelt et al., 2011). SKT proteins may have originated from small K+ channels by at least two gene duplication and two gene fusion events. They contain four covalently linked M(1)PM(2) domains, in which M(1) and M(2) stand for transmembrane stretches, and P for a P-loop, which folds back from the external medium into the membrane. SKT proteins distinguish themselves in two important aspects from K+ channels: first, with just one conserved glycine residue in their P-loops they contain a much simpler K+-selectivity filter sequence than K+ channels with their conserved Thr-Val-Gly-Tyr-Gly sequence. Secondly, the middle part M(2C2) from the long transmembrane stretch M(2C) of KtrB from the bacterium Vibrio alginolyticus forms a gate inside the membrane, which prevents K+ permeation to the cytoplasm. Hänelt et al. (2011) discuss the mechanism of K+ transport via KtrB and other SKT proteins and how the KtrA protein regulates the transport activity.

The generalized transport reaction catalyzed by the Trk family members is therefore probably:

K+ (out) + H+ (out) ⇌ K+ (in) + H+ (in)

This family belongs to the VIC Superfamily.



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TC#NameOrganismal TypeExample

K+ uptake transporter (K+:H+ symporter), TrkHA. The 3-d structure of TrkH is known (Cao et al. 2011).  Molecular dynamics simulations of TrkH have been reported (Domene and Furini, 2012).  TrkA, which forms a tertameric ring that needs to bind ATP to activate transport, interacts with the Enzyme IIANtr of the PTS (TC# 4.A) which inhibits transport (Lee et al. 2007).


TrkHA of E. coli
TrkH (P0AFZ7)
TrkA (P23868)

2.A.38.1.2K+ uptake transporter, TrkIA (Km=1 mM) (Kraegeloh et al., 2005)BacteriaTrkIA of Halomonas elongata
TrkI (AAR91793)
TrkA (AAR91791)
2.A.38.1.3K+ uptake transporter, TrkHA (Km=3 mM) (Kraegeloh et al., 2005)


TrkHA of Halomonas elongata
TrkH (Q6T3V7)
TrkA (Q6T3V8)

2.A.38.1.4Two component K+ transporter, TrkA (TM1088A/B/TM1089) (The x-ray structure (2G1U_A) of TM1088A, the nucleotide-binding subunit, shows a Rossmann fold, and the crystal structure has AMP tightly bound to the GXGXXG motif. TM1089 is the membrane subunit) (Johnson et al., 2009).


TrkA of Thermotoga maritima
TM1088A (gi #93279835)
TM1089 (Q9X0H4)


Cation (K+,Rb+>Na+,Li+) transporter, TrkH. (A homodimer with each monomer containing an ion permeation pathway. 3d structure known (PDB acc#3PJZ; Cao et al., 2011).


TrkH of Vibrio parahaemolyticus (Q87TN7)

2.A.38.1.6Trk system potassium uptake protein TrkGBacteriaTrkG of Escherichia coli

TC#NameOrganismal TypeExample

High affinity K+ transporter (K+:H+ symporter). Trk1 is functional without its "long hydrophilic loop", but this LHL regulates cation translocation activity and selectivity (Kale et al. 2019). Possibly, LHL influences a transmembrane helix (M2A) which can switch between bent and straight conformations, thereby directly modifying the radius of the pore and selectivity filter.


Trk1 of Saccharomyces cerevisiae


Probable K+ uniporter, Trk1. Synthesized constitutively at low levels; synthesis is insensitive to extracellular K+ or the expression of Trk1 (2.A.38.2.2) (Rivetta et al. 2013).


Trk1 of Neurospora crassa

2.A.38.2.3Low affinity K+ transporter, Trk2 (Bertl et al., 2003) YeastTrk2 of Saccharomyces cerevisiae
2.A.38.2.4The K+ uptake transporter, Trk1 (Prista et al., 2007)YeastDhTrk1 of Debaryomyces hansenii (Q6BYD8)

K+ and Na+ channel protein Trk1 (inward rectifying current; a single file pore mechanism is suggested (Corratgé et al., 2007).


Trk1 of Hebeloma cylindrosporum (A4H1L2)


TC#NameOrganismal TypeExample

High affinity K+ transporter, Hkt1 (K+:Na+ symporter) (Kato et al., 2007). The ortholog in Populus trichocarpa enhances salt tolerance and improves the efficiency of antioxidant systems, suggesting that PeHKT1;1 plays an important role in response to salt stress (Xu et al. 2018). Similarly, the barley orthologs in Hordeum vulgaris and the halophilic H. maritimum are K+:Na+ sympoorters, but the ortholog of the halophyte, H. maritium (93% identical to the wheat homologue), has higher affinity for both cations, and plays a role in adaptation to high salenity (Hmidi et al. 2019).


Hkt1 of Triticum aestivum


K+ (high affinity) Na+ -selective (low affinity) uptake symporter, AtHKT1 (does not catalyze recirculation from shoots to roots as proposed by Berthomieu et al., 2003, but mediates Na+ unloading from the xylem sap and thereby protects plant leaves from toxic Na+ over accumulation (Sunarpi et al. 2005). Kato et al. (2007) have reported that AtHKT1 is a Na+ uniporter.


AtHKT1 of Arabidopsis thaliana (Q84TI7)


K+ or Na+ uptake uniporter or Na+-K+ uptake symporter, HKT1. (Banuelos et al, 2008). The high affinity HKT2;4 paralogue can function as a Ca2+-permeable cation channel (Lan et al., 2010).


HKT1 of Oryza sativa (Q0D9S3)


The Na+:K+ cotransporter, HKT2;4. (also transports Mg2+ and Ca2+ in the absence of K+) (Horie et al., 2011).


The HKT2;4 protein of Oryza sativa (A3FFK5)

2.A.38.3.5Cation transporter HKT4 (OsHKT4)


HKT4 of Oryza sativa


High affinity K+ transporter of 544 aas and 11 TMSs, HKT1;1. Natural variants with differing capacities to exclude K+ have been identified (Henderson et al. 2018).

HKT1;1 of Vitis vinifera


High affinity Na+ (not K+) transporter, SeHKT1;2 of 536 aas and 11 putative TMSs. The addition of K+ along with NaCl relieved Na+ sensitivity, and heterologous expression of SeHKT1;2 in a sos1 mutant of Arabidopsis thaliana increased salt sensitivity and could not rescued the transgenic plants (Haxim et al. 2023).

SeHKT1;2 of the halophyte, Salicornia europaea


TC#NameOrganismal TypeExample
2.A.38.4.1Low affinity K+ transporter, KtrII (K+:Na+ symporter) Bacteria NtpJ of Enterococcus hirae

High affinity (Km <50 μM) KtrA binds ATP to a β-α-β-Rossman fold. Other nucleotides bound, but only ATP bound with high affinity and changed the conformation of KtrA so that KtrA and KtrB associate (Kroning et al., 2007) in vivo. Both ATP and ΔΨ were required for transport activity. K+ uptake may occur by a Na+:K+ symport mechanism.  However, one region of KtrB, the transmembrane helix (M(2C); 40aas long) seems to control K+ transport non-coordinately with Na+ symport (Haenelt et al., 2010). M2C2 may form a flexible "gate" controlling K+ translocation at the cytoplasmic side of KtrB (Hänelt et al. 2010).  M(2C2) seems to be required for the interaction between KtrA and KtrB.


KtrAB of Vibrio alginolyticus


Low affinity (1 mM) K+ uptake transporter, KtrAB. Evidence for structural similarities between potassium channels and KtrB proteins in the extracellular half of the molecule and differences in the cytoplasmic regions has been obtained (Albright et al., 2007). This system is responsible for K+ accumulation in the cell cytoplasm, allowing oscillatory release of K+ from a biofilm through the BikC (YugO) K+ channel for the attraction of other bacteria (both of the same and different species) to the biofilm (Humphries et al. 2017).


KtrAB (YuaA/YubG) of Bacillus subtilis

2.A.38.4.4Very low affinity (10 mM) K+ uptake transporter, KtrCDBacteriaKtrCD (YkqB/YkrM) of Bacillus subtilis

The Na+-dependent K+ uptake transporter, KtrABE, required for adaption to salinity stress and high osmolarity (Zulkifli et al., 2010). KtrB alone confers low K+ uptake activity; KtrA+KtrE are required for Na+ dependency and higher K+ uptake rates.


KtrABE of Synechocystis sp. strain PPC6803
KtrA (Q55496)
KtrB (P73949)
KtrE (P73948)


K+ uptake system, KtrABD.  S. aureus devoid of the Ktr system became sensitive to hyperosmotic conditions, exhibited a hyperpolarized plasma membrane, and increased susceptibility to aminoglycoside antibiotics and cationic antimicrobials. In contrast to other organisms, the S. aureus Ktr system was shown to be important for low-K+ growth under alkaline conditions but played only a minor role in neutral and acidic conditions. In a mouse competitive index model of bacteraemia, the ktrA mutant was significantly outcompeted by the parental strain (Gries et al. 2013).


KtrABD of Staphylococcus aureus


TC#NameOrganismal TypeExample

The Na+-independent K+ uptake transporter, HKT1 (Mosimann et al., 2010).


TbHKT1 of Trypanosoma brucei (Q38BH0)