2.A.72 The K⁺ Uptake Permease (KUP) Family Proteins of the KUP family include the KUP (TrkD) protein of E. coli and homologues in both Gram-positive and Gram-negative bacteria. High affinity (20 mM) 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 putative transmembrane spanners (440 residues) with a requisite hydrophilic, C-terminal domain of 182 residues, localized to the cytoplasmic side of the membrane. 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. The yeast high affinity (K_M = 1 µM) K⁺ transporter Hak1 is 762 amino acyl residues long with 12 putative transmembrane segments. 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. The generalized transport reaction for members of the KUP family is: K⁺ (out) + energy Æ K⁺ (in).
References:
Bañuelos, M.A., R.D. Klein, S.J. Alexander-Bowman, and A. Rodrîguez-Navarro. (1995). A potassium transporter of the yeast Schwanniomyces occidentalis homologous to the Kup system of Escherichia coli has a high concentrative capacity. EMBO J. 14: 3021-3027.
Fu, H.-H. and S. Luan. (1998). AtKUP1: a dual-affinity K⁺ transporter from Arabidopsis. Plant Cell 10: 63-73.
Haro, R., L. Sainz, F. Rubio, and A. Rodríguez-Navarro. (1999). Cloning of two genes encoding potassium transporters in Neurospora crassa and expression of the corresponding cDNAs in Saccharomyces cerevisiae. Mol. Microbiol. 31: 511-520.
Kim, E.J., J.M. Kwak, N. Uozumi, and J.I. Schroeder. (1998). AtKUP1, an Arabidopsis gene encoding high-affinity potassium transport activity. Plant Cell 10: 51-62.
Schleyer, M. and E.P. Bakker. (1993). Nucleotide sequence and 3'-end deletion studies indicate that the K⁺-uptake protein Kup from Escherichia coli is composed of a hydrophobic core linked to a large and partially essential hydrophilic C terminus. J. Bacteriol. 175: 6925-6931.
Senn, M.E., F. Rubio, M.A. Bañuelos, and A. Rodríguez-Navarro. (2001). Comparative functional features of plant potassium HvHAK1 and HvHAK2 transporters. J. Biol. Chem. 276: 44563-44569.
Trchounian, A. and H. Kobayashi. (1999). Kup is the major K⁺ uptake system in Escherichia coli upon hyper-osmotic stress at a low pH. FEBS Lett. 447: 144-148.
Zakharyan, E. and A. Trchounian. (2001). K⁺ influx by Kup in Escherichia coli is accompanied by a decrease in H⁺ efflux. FEMS Microbiol. Lett. 204: 61-64.
Examples:
TC#	Name	Organismal Type	Example
2.A.72.1.1	K⁺ uptake permease	Bacteria	KUP of E. coli

2.A.72.2.1	K⁺ uptake permease	Yeast	Hak1 of Debaryomyces (Schwanniomyces) occidentalis

2.A.72.3.1	High affinity K⁺ transporter	Plants	AtKUP1 of Arabidopsis thaliana

2.A.72.3.2	K⁺:H⁺ symporter	Fungi	Hak1 of Neurospora crassa

2.A.72.3.3	High affinity K⁺ uptake transporter	Plants	Hak1 of Hordeum vulgare

2.A.72.3.4	Low affinity, Na⁺-sensitive K⁺ uptake transporter (vacuolar ?)	Plants	Hak2 of Hordeum vulgare