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.

Potassium transporter TrkH/TrkA
Identifiers
SymbolTrk
PfamPF02386
InterProIPR003445
TCDB2.A.38
OPM superfamily8
OPM protein4j7c

Homology

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

  1. the Gram-negative bacterial Trk proteins,
  2. the Gram-negative and Gram-positive bacterial Ktr proteins,
  3. the yeast proteins and
  4. the plant proteins comprising four distinct clusters.[1]

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.[2] Homology has been established between Trk carriers and VIC family channels.[3]

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.[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]

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.[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).

References
  1. Saier MH, Eng BH, Fard S, Garg J, Haggerty DA, Hutchinson WJ, Jack DL, Lai EC, Liu HJ, Nusinew DP, Omar AM, Pao SS, Paulsen IT, Quan JA, Sliwinski M, Tseng TT, Wachi S, Young GB (February 1999). "Phylogenetic characterization of novel transport protein families revealed by genome analyses". Biochimica et Biophysica Acta (BBA) - Reviews on Biomembranes. 1422 (1): 1–56. doi:10.1016/s0304-4157(98)00023-9. PMID 10082980.
  2. Matsuda N, Kobayashi H, Katoh H, Ogawa T, Futatsugi L, Nakamura T, Bakker EP, Uozumi N (December 2004). "Na+-dependent K+ uptake Ktr system from the cyanobacterium Synechocystis sp. PCC 6803 and its role in the early phases of cell adaptation to hyperosmotic shock". The Journal of Biological Chemistry. 279 (52): 54952–62. doi:10.1074/jbc.M407268200. PMID 15459199.
  3. Yu FH, Yarov-Yarovoy V, Gutman GA, Catterall WA (December 2005). "Overview of molecular relationships in the voltage-gated ion channel superfamily". Pharmacological Reviews. 57 (4): 387–95. doi:10.1124/pr.57.4.13. PMID 16382097. S2CID 2643413.
  4. Kato Y, Sakaguchi M, Mori Y, Saito K, Nakamura T, Bakker EP, Sato Y, Goshima S, Uozumi N (May 2001). "Evidence in support of a four transmembrane-pore-transmembrane topology model for the Arabidopsis thaliana Na+/K+ translocating AtHKT1 protein, a member of the superfamily of K+ transporters". Proceedings of the National Academy of Sciences of the United States of America. 98 (11): 6488–93. Bibcode:2001PNAS...98.6488K. doi:10.1073/pnas.101556598. PMC 33495. PMID 11344270.
  5. Zeng GF, Pypaert M, Slayman CL (January 2004). "Epitope tagging of the yeast K(+) carrier Trk2p demonstrates folding that is consistent with a channel-like structure". The Journal of Biological Chemistry. 279 (4): 3003–13. doi:10.1074/jbc.M309760200. PMID 14570869.
  6. "2.A.38 The K+ Transporter (Trk) Family". TCDB. Retrieved 2016-04-16.
  7. Bateman A, Birney E, Durbin R, Eddy SR, Howe KL, Sonnhammer EL (January 2000). "The Pfam protein families database". Nucleic Acids Research. 28 (1): 263–6. doi:10.1093/nar/28.1.263. PMC 102420. PMID 10592242.
  8. Roosild TP, Miller S, Booth IR, Choe S (June 2002). "A mechanism of regulating transmembrane potassium flux through a ligand-mediated conformational switch". Cell. 109 (6): 781–91. doi:10.1016/s0092-8674(02)00768-7. PMID 12086676. S2CID 9265433.

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