Potassium Starvation in Yeast: Mechanisms of Homeostasis Revealed by Mathematical Modeling

Abstract

The intrinsic ability of cells to adapt to a wide range of environmental conditions is a fundamental process required for survival. Potassium is the most abundant cation in living cells and is required for essential cellular processes, including the regulation of cell volume, pH and protein synthesis. Yeast cells can grow from low micromolar to molar potassium concentrations and utilize sophisticated control mechanisms to keep the internal potassium concentration in a viable range. We developed a mathematical model for Saccharomyces cerevisiae to explore the complex interplay between biophysical forces and molecular regulation facilitating potassium homeostasis. By using a novel inference method (“the reverse tracking algorithm”) we predicted and then verified experimentally that the main regulators under conditions of potassium starvation are proton fluxes responding to changes of potassium concentrations. In contrast to the prevailing view, we show that regulation of the main potassium transport systems (Trk1,2 and Nha1) in the plasma membrane is not sufficient to achieve homeostasis.

Coordination of K + Transporters in Neurospora: TRK1 Is Scarce and Constitutive, while HAK1 Is Abundant and Highly Regulated

“…An important implication of this finding is that potassium's primary effect in metabolic regulation, at least in the presence of abundant ammonium ions, should be on signaling rather than upon simple enhancement of cytoplasmic enzyme activities. The same conclusion has also been reached via theoretical analysis of K ϩ transport in Saccharomyces (77,78). That seems like a sensible arrangement; given that K ϩ is a bulk constituent of cytoplasm (also partially sequestered in vacuoles), its actual concentration should respond only slowly to threats.…”

Regulation of Cation Balance inSaccharomyces cerevisiae

“…Protons are pumped out of the cell by Pma1; however, HCO 3 2 accumulates inside the cell and becomes a negative-charge sink that increases K + retention inside the cell. Experimental studies confirm that NCE103 expression increases under conditions of K + starvation (Kahm et al 2012) and also show that while cells maintain a minimal intracellular K + concentration, the amount of K + in a yeast cell varies as a function of extracellular K + concentration and is an example of nonperfect adaptation (Kahm et al 2012).…”

“…K + is continually taken up and extruded by cells, and both processes are important; membrane potential increases when potassium influx is crippled and decreases in cells defective for K + efflux (Madrid et al 1998;Kinclova-Zimmermannova et al 2006); furthermore, K + efflux systems are regulated in response to changes in the membrane potential . K + transport is also closely coordinated with H + -ATPase activity, and Pma1 is activated when K + uptake increases and also under K + starvation conditions (Seto- Young and Perlin 1991;Kahm et al 2012).…”

“…A computational model was recently developed to explain how cells maintain a minimal concentration of intracellular K + under K + starvation conditions. Surprisingly, these investigations suggest that increased levels of Pma1 activity and bicarbonate ion, rather than regulation of Trk1/2 or Nha1, are critical under these conditions (Kahm et al 2012). Intracellular CO 2 , produced in many enzymatic reactions, is converted by carbonic anhydrase (encoded by NCE103) to carbonic acid, which dissociates into bicarbonate (HCO 3 2 ) and protons.…”

A physiological, biochemical and proteomic characterization of Saccharomyces cerevisiae trk1,2 transport mutants grown under limiting potassium conditions

“…In the context of systems biology, mathematical modelling predicting the complexity of ion regulation allowed the proposal of mechanisms of cation homeostasis (Kahm et al, 2012;Ke et al, 2013). Detailed comparisons between wild-type and trk1,2 double mutant strains showed that, in addition to the lack of high affinity K + -transport: (i) the mutants grown under non-limiting potassium were hyperpolarized and had a lower intracellular pH than the wildtype (Navarrete et al, 2010); (ii) regulation of the Trk1,2 system is important but not sufficient to achieve homeostasis, and adaptation to low potassium requires modulation of proton fluxes, production of bicarbonate and activation of the H + -ATPase (Kahm et al, 2012); and (iii) Trk1 is not essential for adaptation to external potassium changes since trk1,2 mutants are still able to adjust intracellular cation content in response to rapid changes in external potassium, although the dynamics of potassium exchange in the mutant are very different from the wild-type (Herrera et al, 2014).…”