Potassium transporter family

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.^[4] and Trk2 of S. cerevisiae.^[5] This folding pattern resembles quadruplicated primitive K⁺ channels of the VIC superfamily (TC #1.A.1) instead of typical 12 TMS carriers.^[2] As homology has been established between Trk carriers and VIC family channels.^[3][6]

Trk family members regulate various K⁺ transporters in all three domains of life. These regulatory subunits are generally called K⁺ transport/nucleotide binding subunits.^[7] 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.^[8] The KTN domains of Kef channels interact dynamically with the transporter. The KTN conformation then controls permease activity.^[8]

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.^[4]

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

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

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