Disease relevance of TRK1

• A TRK1 overexpression strain of C. albicans, constructed by integrating an additional TRK1 gene into wild-type cells, demonstrated cytoplasmic sequestration of Trk1 protein, along with somewhat diminished toxicity of Hst 5 [1].
• Cells deficient for both high and low affinity K+ transport (trk1 delta trk2) exhibit hypersensitivity to low extracellular pH that can be suppressed by high concentrations of K+ but not Na+ [2].

High impact information on TRK1

• The present study uses whole-cell patch-clamp experiments to show that yeast strains which grow poorly on submillimolar K+ due to the deletion of two K(+)-transporter genes (TRK1 and TRK2) are in fact missing a prominent K+ inward current present in wild-type cells [3].
• As this pump is electrogenic, the activity of the Trk1 and -2 K+ uptake system is crucial for sustained Pma1p operation [4].
• These results are consistent with a model in which the Ppz1-Hal3 interaction is a sensor of intracellular pH that modulates H+ and K+ homeostasis through the regulation of Trk1p activity [4].
• Genetic evidence indicates that Trk1p is activated by the Hal4 and -5 kinases and inhibited by the Ppz1 and -2 phosphatases, which, in turn, are inhibited by their regulatory subunit, Hal3p [4].
• We show that Trk1p, present in plasma membrane "rafts", physically interacts with Ppz1p, that Trk1p is phosphorylated in vivo, and that its level of phosphorylation increases in ppz1 and -2 mutants [4].

Chemical compound and disease context of TRK1

• Furthermore, Trk1p-mediated chloride conductance was blocked by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid in all of the tested strains, strongly suggesting that the TRK1 protein provides the essential pathway for ATP loss and is the critical effector for Hst 5 toxicity in C. albicans [1].

Biological context of TRK1

• In cells containing a deletion of TRK1, transcription levels of TRK2 are extremely low and are limiting for growth in media containing low levels of K+ (Trk- phenotype) [5].
• TRK2 shares 55% amino acid sequence identity with TRK1 [6].
• Haploid cells that contain a null allele of TRK1 (trk1 delta) rely on a low-affinity transporter for potassium uptake and, under certain conditions, exhibit energy-dependent loss of potassium, directly exposing the activity of a transporter responsible for the efflux of this ion [7].
• All measured parameters (Hst 5 killing, Hst 5-stimulated ATP efflux, normal Trk1p-mediated K(+) ((86)Rb(+)) influx, and Trk1p-mediated chloride conductance) were similarly reduced (5-7-fold) by removal of a single copy of the TRK1 gene from this diploid organism and were fully restored by complementation of the missing allele [1].
• Although hybridization with a c-TRK1 probe revealed highly homologous sequences in the genomes of most Saccharomyces species, the TRK1 sequence in S. uvarum (u-TRK1) was detected only under low-stringency conditions [8].

Anatomical context of TRK1

• Among suppressors of the K+ transport defect in trk1 delta trk2 delta cells, we have identified members of the sugar transporter gene superfamily [9].
• It appears that, in addition to the H+ export by the PMA1-coded plasma membrane H(+)-ATPase, at least three different univalent-cation involving activities are present: the high-affinity transport system for K+ (TRK1), another system (possibly TRK2) with different responses to K+ and Rb+, vs. Tl+, and an active system for K+ export [10].
• The transcriptional and posttranslational modifications of Mpk1 were not observed when the internal K+ concentration (and thus turgor pressure) was lowered by disrupting the TRK1 and -2 K+ transporter genes or when the cell wall was stabilized by the addition of sorbitol [11].
• A cDNA encoding a novel mammalian member of the Clp/HSP104 family was isolated from a mouse macrophage-like cell line (J774.1) cDNA library by suppression of the growth defect of a Saccharomyces cerevisiae trk1 trk2 double mutant [12].

Associations of TRK1 with chemical compounds

• TKHp, the product of SpTRK exhibits high homology to TRK1 and TRK2 of Saccharomyces cerevisiae as well as to HKT1 of Triticum aestivum, but is not related to HAK1 of another ascomycete, Schwanniomyces occidentalis, suggesting that different routes for potassium uptake evolved independently [13].
• This can be explained by the observation that trk1 trk2 ppz1 or trk1 trk2 ppz1 ppz2 strains display a very poor rubidium uptake, with markedly increased Km values [14].
• Growth of trk1 Delta trk2 Delta cells is also inhibited by lithium and ammonium; however, these ions do not inhibit NSC1, but instead enter yeast cells via NSC1 [15].
• The highest rate of ATP hydrolysis in vitro was found with the trk1 delta trk2 delta mutant where glucose-, as well as KCl-induced acidification were lowest [16].
• Factors which suppress NSC1-mediated inward currents and inhibit growth of trk1 Delta trk2 Delta cells include (i) elevating extracellular calcium over the range of 10 microM-10 mM, (ii) lowering extracellular pH over the range 7.5-4, (iii) blockade of NSC1 by hygromycin B, and (iv) to a lesser extent by TEA(+) [15].

Regulatory relationships of TRK1

• The N. crassa TRK1 and HAK1 transporters expressed by the corresponding cDNAs in a trk1 delta trk2 delta mutant of S. cerevisiae exhibited a high affinity for Rb+ and K+ [17].

Other interactions of TRK1

• TRK1 is nonessential in S. cerevisiae and maps to a locus unlinked to PMA1, the gene that encodes the plasma membrane ATPase [7].
• Characterization of potassium transport in wild-type and isogenic yeast strains carrying all combinations of trk1, trk2 and tok1 null mutations [18].
• The effect of HAL1 on intracellular K+ was independent of the TRK1 and TOK1 genes, corresponding to a major K+ uptake system and to a K+ efflux system activated by depolarization, respectively [19].
• This phenotype is suppressed by the TRK1 and HAL5 genes in high-copy number consistent with a defect in K(+) uptake mediated by the Trk system [20].
• The hal4-null phenotype is complemented by overexpression of the Trk1 potassium transporter or increased K(+) in the growth medium, suggesting that Hal4 promotes K(+) uptake, which consequently increases cellular resistance to other cations [21].

Analytical, diagnostic and therapeutic context of TRK1

• Northern blot analysis and comparison of the kinetic characteristics of the two transporters in the trk1 delta trk2 delta mutant with the kinetic characteristics of K+ uptake in N. crassa cells allowed TRK1 to be identified as the dominant K+ transporter and HAK1 as a transporter that is only expressed when the cells are K+ starved [17].

References