Family 2.A.72 - The K+ Uptake Permease Family       

Family ID: 52646

Proteins of the KUP family include the KUP (TrkD) protein of E. coli, a partially sequenced ORF from Lactococcus lactis, high affinity K+ uptake systems (Hak1) of the yeast Debaryomyces occidentalis as well as the fungus, Neurospora crassa, and several homologues in plants. Arabidopsis thaliana possesses multiple KUP family paralogues. While the 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. While the E. coli KUP protein is assumed to be a secondary transporter, and uptake is blocked by protonophores such as CCCP (but not arsenate), the energy coupling mechanism has not been defined. However, the N. crassa protein has been shown to be a K+:H+ symporter, establishing that the KUP family consists of secondary carriers.

The yeast high affinity (KM = 1µM) 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 affinity (20mM) K+ transporter can complement K+ uptake defects in E. coli.