The NPR/Hal family of protein kinases in yeasts: biological role, phylogeny and regulation under environmental challenges

The role of ion homeostasis in adaptation and tolerance to acetic acid stress in yeasts

Maintenance of asymmetric ion concentrations across cellular membranes is crucial for proper yeast cellular function. Disruptions of these ionic gradients can significantly impact membrane electrochemical potential and the balance of other ions, particularly under stressful conditions such as exposure to acetic acid. This weak acid, ubiquitous to both yeast metabolism and industrial processes, is a major inhibitor of yeast cell growth in industrial settings and a key determinant of host colonization by pathogenic yeast. Acetic acid toxicity depends on medium composition, especially on the pH (H+ concentration), but also on other ions' concentrations. Regulation of ion fluxes is essential for effective yeast response and adaptation to acetic acid stress. However, the intricate interplay among ion balancing systems and stress response mechanisms still presents significant knowledge gaps. This review offers a comprehensive overview of the mechanisms governing ion homeostasis, including H+, K+, Zn2+, Fe2+/3+, and acetate, in the context of acetic acid toxicity, adaptation, and tolerance. While focus is given on Saccharomyces cerevisiae due to its extensive physiological characterization, insights are also provided for biotechnologically and clinically relevant yeast species whenever available.

The Hrk1 kinase is a determinant of acetic acid tolerance in yeast by modulating H+ and K+ homeostasis

Acetic acid-induced stress is a common challenge in natural environments and industrial bioprocesses, significantly affecting the growth and metabolic performance of Saccharomyces cerevisiae. The adaptive response and tolerance to this stress involves the activation of a complex network of molecular pathways. This study aims to delve deeper into these mechanisms in S. cerevisiae, particularly focusing on the role of the Hrk1 kinase. Hrk1 is a key determinant of acetic acid tolerance, belonging to the NPR/Hal family, whose members are implicated in the modulation of the activity of plasma membrane transporters that orchestrate nutrient uptake and ion homeostasis. The influence of Hrk1 on S. cerevisiae adaptation to acetic acid-induced stress was explored by employing a physiological approach based on previous phosphoproteomics analyses. The results from this study reflect the multifunctional roles of Hrk1 in maintaining proton and potassium homeostasis during different phases of acetic acid-stressed cultivation. Hrk1 is shown to play a role in the activation of plasma membrane H+-ATPase, maintaining pH homeostasis, and in the modulation of plasma membrane potential under acetic acid stressed cultivation. Potassium (K+) supplementation of the growth medium, particularly when provided at limiting concentrations, led to a notable improvement in acetic acid stress tolerance of the hrk1Δ strain. Moreover, abrogation of this kinase expression is shown to confer a physiological advantage to growth under K+ limitation also in the absence of acetic acid stress. The involvement of the alkali metal cation/H+ exchanger Nha1, another proposed molecular target of Hrk1, in improving yeast growth under K+ limitation or acetic acid stress, is proposed.