Potassium transporter family
# Potassium transporter family

The $K^+$ Transporter (Trk) Family is a member of the voltage-gated ion channel (VIC) superfamily. The proteins of the Trk family are derived from Gram-negative and Gram-positive bacteria, yeast and plants.

## Homology

The phylogenetic tree reveals that the proteins cluster according to phylogeny of the source organism with

* the Gram-negative bacterial Trk proteins,
* the Gram-negative and Gram-positive bacterial Ktr proteins,
* the yeast proteins and
* the plant proteins comprising four distinct clusters.

S. cerevisiae possesses at least two paralogues, high- and low-affinity $K^+$ transporters. Folding pattern seen in Trk proteins resembles quadruplicated primitive $K^+$ channels of the VIC superfamily (TC #1.A.1) instead of typical 12 TMS carriers. Homology has been established between Trk carriers and VIC family channels.

## Structure

The sizes of the Trk family members vary from 423 residues to 1235 residues. The bacterial proteins are of 423-558 residues, the Triticum aestivum protein is 533 residues, and the yeast proteins vary between 841 and 1241 residues. These proteins possess 8 putative transmembrane α-helical spanners (TMSs). An 8 TMS topology with N- and C-termini on the inside, has been established for AtHKT1 of A. thaliana. and Trk2 of S. cerevisiae. This folding pattern resembles quadruplicated primitive $K^+$ channels of the VIC superfamily (TC #1.A.1) instead of typical 12 TMS carriers. As homology has been established between Trk carriers and VIC family channels.

## Function

Trk family members regulate various $K^+$ transporters in all three domains of life. These regulatory subunits are generally called $K^+$ transport/nucleotide binding subunits. TrkA domains can bind $NAD^+$ and NADH, possibly allowing $K^+$ transporters to be responsive to the redox state of the cell. The ratio of $NADH/NAD^+$ may control gating. Multiple crystal structures of two KTN domains complexed with $NAD^+$ or NADH reveal that these ligands control the oligomeric (tetrameric) state of KTN. The results suggest that KTN is inherently flexible, undergoing a large conformational change through a hinge motion. The KTN domains of Kef channels interact dynamically with the transporter. The KTN conformation then controls permease activity.

Both yeast transport systems are believed to function by $K^+:H^+$ symport, but the wheat protein functions by $K^+:Na^+$ symport. It is possible that some of these proteins can function by a channel-type mechanism. Positively charged residues in TMS8 of several ktr/Trk/HKT transporters probably face the channel and block a conformational change that is essential for channel activity while allowing secondary active transport.

The putative generalized transport reaction catalyzed by the Trk family members is:

   $K^+$ (out) + $H^+$ (out) ⇌ $K^+$ (in) + $H^+$ (in).
