Biology: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
- 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.[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
- ↑ "Phylogenetic characterization of novel transport protein families revealed by genome analyses". Biochimica et Biophysica Acta (BBA) - Reviews on Biomembranes 1422 (1): 1–56. February 1999. doi:10.1016/s0304-4157(98)00023-9. PMID 10082980.
- ↑ 2.0 2.1 "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. December 2004. doi:10.1074/jbc.M407268200. PMID 15459199.
- ↑ 3.0 3.1 "Overview of molecular relationships in the voltage-gated ion channel superfamily". Pharmacological Reviews 57 (4): 387–95. December 2005. doi:10.1124/pr.57.4.13. PMID 16382097.
- ↑ 4.0 4.1 "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. May 2001. doi:10.1073/pnas.101556598. PMID 11344270. Bibcode: 2001PNAS...98.6488K.
- ↑ "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. January 2004. doi:10.1074/jbc.M309760200. PMID 14570869.
- ↑ "2.A.38 The K+ Transporter (Trk) Family". http://www.tcdb.org/search/result.php?tc=2.A.38.5.
- ↑ "The Pfam protein families database". Nucleic Acids Research 28 (1): 263–6. January 2000. doi:10.1093/nar/28.1.263. PMID 10592242.
- ↑ 8.0 8.1 "A mechanism of regulating transmembrane potassium flux through a ligand-mediated conformational switch". Cell 109 (6): 781–91. June 2002. doi:10.1016/s0092-8674(02)00768-7. PMID 12086676.
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