Potassium uptake permease

The E. coli protein is 622 amino acyl residues long and has 12 established transmembrane spanners (440 residues) with a requisite hydrophilic, C-terminal domain of 182 residues, localized to the cytoplasmic side of the membrane.[3] 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.[4] The N. crassa protein was earlier shown to be a K⁺:H⁺ symporter, establishing that the KUP family consists of secondary carriers.

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.

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.[5] 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.[5]