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 superfamilyStructure
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 ''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).
References
{{CCBYSASource, sourcepath=http://www.tcdb.org/search/result.php?tc=2.A.38, sourcearticle=2.A.38 The K+ Transporter (Trk) Family, revision=699838558 Protein families Membrane proteins Transmembrane proteins Transmembrane transporters Transport proteins Integral membrane proteins