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TCID	Name	Domain	Kingdom/Phylum	Protein(s)
2.A.38.1.1	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 IIA^Ntr of the PTS (TC# 4.A) which inhibits transport (Lee et al. 2007).	Bacteria	Pseudomonadota	TrkHA of E. coli TrkH (P0AFZ7) TrkA (P23868)
2.A.38.1.2	K⁺ uptake transporter, TrkIA (Km=1 mM) (Kraegeloh et al., 2005)	Bacteria	Pseudomonadota	TrkIA of Halomonas elongata TrkI (AAR91793) TrkA (AAR91791)
2.A.38.1.3	K⁺ uptake transporter, TrkHA (Km=3 mM) (Kraegeloh et al., 2005)	Bacteria	Pseudomonadota	TrkHA of Halomonas elongata TrkH (Q6T3V7) TrkA (Q6T3V8)
2.A.38.1.4	Two 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).	Bacteria	Thermotogota	TrkA of Thermotoga maritima TM1088A (gi #93279835) TM1089 (Q9X0H4)
2.A.38.1.5	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).	Bacteria	Pseudomonadota	TrkH of Vibrio parahaemolyticus (Q87TN7)
2.A.38.1.6	Trk system potassium uptake protein TrkG	Bacteria	Pseudomonadota	TrkG of Escherichia coli
2.A.38.2.1	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.	Eukaryota	Fungi, Ascomycota	Trk1 of Saccharomyces cerevisiae
2.A.38.2.2	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).	Eukaryota	Fungi, Ascomycota	Trk1 of Neurospora crassa
2.A.38.2.3	Low affinity K⁺ transporter, Trk2 (Bertl et al., 2003)	Eukaryota	Fungi, Ascomycota	Trk2 of Saccharomyces cerevisiae
2.A.38.2.4	The K⁺ uptake transporter, Trk1 (Prista et al., 2007)	Eukaryota	Fungi, Ascomycota	DhTrk1 of Debaryomyces hansenii (Q6BYD8)
2.A.38.2.5	K⁺ and Na⁺ channel protein Trk1 (inward rectifying current; a single file pore mechanism is suggested (Corratgé et al., 2007).	Eukaryota	Fungi, Basidiomycota	Trk1 of Hebeloma cylindrosporum (A4H1L2)
2.A.38.3.1	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).	Eukaryota	Viridiplantae, Streptophyta	Hkt1 of Triticum aestivum
2.A.38.3.2	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.	Eukaryota	Viridiplantae, Streptophyta	AtHKT1 of Arabidopsis thaliana (Q84TI7)
2.A.38.3.3	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).	Eukaryota	Viridiplantae, Streptophyta	HKT1 of Oryza sativa (Q0D9S3)
2.A.38.3.4	The Na⁺:K⁺cotransporter, HKT2;4. (also transports Mg²⁺ and Ca²⁺ in the absence of K⁺) (Horie et al., 2011).	Eukaryota	Viridiplantae, Streptophyta	The HKT2;4 protein of Oryza sativa (A3FFK5)
2.A.38.3.5	Cation transporter HKT4 (OsHKT4)	Eukaryota	Viridiplantae, Streptophyta	HKT4 of Oryza sativa
2.A.38.3.6	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).	Eukaryota	Viridiplantae, Streptophyta	HKT1;1 of Vitis vinifera
2.A.38.3.7	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).	Eukaryota	Viridiplantae, Streptophyta	SeHKT1;2 of the halophyte, Salicornia europaea
2.A.38.4.1	Low affinity K⁺ transporter, KtrII (K⁺:Na⁺ symporter)	Bacteria	Bacillota	NtpJ of Enterococcus hirae
2.A.38.4.2	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.	Bacteria	Pseudomonadota	KtrAB of Vibrio alginolyticus
2.A.38.4.3	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).	Bacteria	Bacillota	KtrAB (YuaA/YubG) of Bacillus subtilis
2.A.38.4.4	Very low affinity (10 mM) K⁺ uptake transporter, KtrCD	Bacteria	Bacillota	KtrCD (YkqB/YkrM) of Bacillus subtilis
2.A.38.4.5	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.	Bacteria	Cyanobacteriota	KtrABE of Synechocystis sp. strain PPC6803 KtrA (Q55496) KtrB (P73949) KtrE (P73948)
2.A.38.4.6	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).	Bacteria	Bacillota	KtrABD of Staphylococcus aureus
2.A.38.5.1	The Na⁺-independent K⁺ uptake transporter, HKT1 (Mosimann et al., 2010).	Eukaryota	Euglenozoa	TbHKT1 of Trypanosoma brucei (Q38BH0)