2.A.72 The K⁺ Uptake Permease (KUP) Family

Proteins of the KUP/HAK/KT family include the KUP (TrkD) protein of E. coli and homologues in both Gram-positive and Gram-negative bacteria. High affinity (20 μM) K⁺ uptake systems (Hak1) of the yeast Debaryomyces occidentalis as well as the fungus, Neurospora crassa, and several homologues in plants have been characterized. Arabidopsis thaliana and other plants possess multiple KUP family paralogues. While many plant proteins cluster tightly together, the Hak1 proteins from yeast as well as the two Gram-positive and Gram-negative bacterial proteins are distantly related on the phylogenetic tree for the KUP family. The E. coli protein is 622 amino acyl residues long and has 12 estabilshed transmembrane spanners (440 residues) with a requisite hydrophilic, C-terminal domain of 182 residues, localized to the cytoplasmic side of the membrane (Sato et al. 2014). Deletion of most of the hydrophilic domain reduces but does not abolish KUP transport activity. The function of the C-terminal domain is not known. The E. coli KUP protein is believed to be a secondary transporter. Uptake is blocked by protonophores such as CCCP (but not arsenate), and evidence for a proton symport mechanism has been presented (Zakharyan and Trchounian, 2001). The N. crassa protein was earlier shown to be a K⁺:H⁺ symporter, establishing that the KUP family consists of secondary carriers. There are 10 members of the KUP (HAK) family in sugar beets (Beta vulgaris) and the results suggest that the BvHAK gene family plays important roles in the response of sugar beet to salt stress (Yang et al. 2022).

The yeast high affinity (K_M = 1 µM) K⁺ transporter Hak1 is 762 amino acyl residues long with 12 putative TMSs. Like the E. coli KUP protein, it possesses a C-terminal hydrophilic domain, probably localized to the cytoplasmic side of the membrane. Hak1 may be able to accumulate K⁺ 10⁶-fold against a concentration gradient. The plant high and low affinity K⁺ transporters can complement K⁺ uptake defects in E. coli.  Plant genomes contain large numbers of HAK/KUP/KT transporters, and they show diverse roles in K⁺ uptake and translocation, salt tolerance and osmotic potential regulation, as well as in controlling root morphology and shoot phenotyping (Li et al. 2018). HAK/KUP/KT transporters can be phosphorylated by CIPK-CBL complexes for activating K⁺ uptake and probably signaling (Li et al. 2018).

TRK transporters, responsible for the  bulk of K⁺ accumulation in plants, fungi, and bacteria, mediate ion currents driven by the large membrane voltages (-150 to -250 mV) common to non-animal cells. Bacterial TRK proteins resemble K⁺ channels in their primary sequence, crystallize as membrane dimers having intramolecular K⁺-channel-like folding, and complex with a cytoplasmic collar formed of four RCK domains (Pardo et al. 2015). Fungal TRK proteins possess a large built-in regulatory domain and a highly conserved pair of transmembrane helices (TMSs 7 and 8, ahead of the C-terminus), postulated to facilitate intramembranal oligomerization.These fungal HAK proteins are chloride channels mediating efflux, a process suppressed by osmoprotective agents. It involve hydrophobic gating and resembles conduction by Cys-loop ligand-gated anion channels. Possibly, the tendency of hydrophobic or amphipathic transmembrane helices to self-organize into oligomers creates novel ionic pathways through membranes: hydrophobic nanopores, pathways of low selectivity governed by the chaotropic behavior of individual ionic species under the influence of membrane voltage (Pardo et al. 2015).

Plant HAK/KUP/KT family members function as plasma membrane (PM) H⁺/K⁺ symporters and modulate chemiosmotically-driven polar auxin transport (PAT). Yang et al. 2020 showed that inactivation of OsHAK5 (TC# 2.A.72.3.9) in rice decreased rootward and shootward PAT, tiller number, and the length of both lateral roots and root hairs, while OsHAK5 overexpression had the opposite effect, irrespective of the K⁺ supply. Inhibitors of ATP-binding-cassette type-B transporters, NPA and BUM, abolished the OsHAK5-overexpression effect on PAT. The mechanistic basis of these changes included the OsHAK5-mediated decrease of transmembrane potential (depolarization), increase of extracellular pH, and increase of PM-ATPase activity. Thus, dual roles of OsHAK5 in altering cellular chemiosmotic gradients (generated continuously by PM H⁺-ATPase) and regulating ATP-dependent auxin transport. Both functions may underlie the prominent effect of OsHAK5 on rice architecture (Yang et al. 2020).

The generalized transport reaction for members of the KUP family is:

K⁺ (out) + Na⁺ or H⁺ (out) → K⁺ (in) + Na⁺ or H⁺ (in).