Potassium transporter family

Potassium transporter TrkH/TrkA
Identifiers
Symbol Trk
Pfam PF02386
InterPro IPR003445
TCDB 2.A.38
OPM superfamily 8
OPM protein 4j7c

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

  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.[6][3]

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, M. H.; Eng, B. H.; Fard, S.; Garg, J.; Haggerty, D. A.; Hutchinson, W. J.; Jack, D. L.; Lai, E. C.; Liu, H. J. (1999-02-25). "Phylogenetic characterization of novel transport protein families revealed by genome analyses". Biochimica Et Biophysica Acta 1422 (1): 1–56. ISSN 0006-3002. PMID 10082980.
  2. 1 2 Matsuda, Nobuyuki; Kobayashi, Hiroshi; Katoh, Hirokazu; Ogawa, Teruo; Futatsugi, Lui; Nakamura, Tatsunosuke; Bakker, Evert P.; Uozumi, Nobuyuki (2004-12-24). "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): 54952–54962. doi:10.1074/jbc.M407268200. ISSN 0021-9258. PMID 15459199.
  3. 1 2 Yu, Frank H.; Yarov-Yarovoy, Vladimir; Gutman, George A.; Catterall, William A. (2005-12-01). "Overview of Molecular Relationships in the Voltage-Gated Ion Channel Superfamily". Pharmacological Reviews 57 (4): 387–395. doi:10.1124/pr.57.4.13. ISSN 1521-0081. PMID 16382097.
  4. 1 2 Kato, Y.; Sakaguchi, M.; Mori, Y.; Saito, K.; Nakamura, T.; Bakker, E. P.; Sato, Y.; Goshima, S.; Uozumi, N. (2001-05-22). "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): 6488–6493. doi:10.1073/pnas.101556598. ISSN 0027-8424. PMC 33495. PMID 11344270.
  5. ↑ Zeng, Ge-Fei; Pypaert, Marc; Slayman, Clifford L. (2004-01-23). "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): 3003–3013. doi:10.1074/jbc.M309760200. ISSN 0021-9258. PMID 14570869.
  6. ↑ "2.A.38 The K+ Transporter (Trk) Family". TCDB. Retrieved 2016-04-16.
  7. ↑ Bateman, A.; Birney, E.; Durbin, R.; Eddy, S. R.; Howe, K. L.; Sonnhammer, E. L. (2000-01-01). "The Pfam protein families database". Nucleic Acids Research 28 (1): 263–266. ISSN 0305-1048. PMC 102420. PMID 10592242.
  8. 1 2 Roosild, Tarmo P.; Miller, Samantha; Booth, Ian R.; Choe, Senyon (2002-06-14). "A mechanism of regulating transmembrane potassium flux through a ligand-mediated conformational switch". Cell 109 (6): 781–791. ISSN 0092-8674. PMID 12086676.

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