Engineering of the yeast ubiquitin ligase Rsp5: isolation of a new variant that induces constitutive inactivation of the general amino acid permease Gap1

A novel yeast-based screening system for potential compounds that can alleviate human α-synuclein toxicity

Aims

This study aimed to establish a yeast‐based screening system for potential compounds that can alleviate the toxicity of α‐synuclein (α‐syn), a neuropathological hallmark of Parkinson’s disease, either inhibition of α‐syn aggregation or promotion of ubiquitin‐mediated degradation of α‐syn.

Methods and Results

A powerful yeast‐based screening assay using the rsp5^A401E‐mutant strain, which is hypersensitive to α‐syn aggregation, was established by two‐step gene replacement and further overexpressed the GFP‐fused α‐syn in the drug‐sensitive yeast strain with a galactose‐inducible multicopy plasmid. The rsp5^A401E‐mutant strain treated with baicalein, a known α‐syn aggregation inhibitor, showed better α‐syn toxicity alleviation than the same background wild type strain as accessed by comparison on the reduction kinetics of viable dye resazurin fluorometrically (λ_ex540/λ_em590 nm). The rsp5^A401E‐mutant yeast‐based assay system showed high sensitivity as it could detect as low as 3.13 µmol l⁻¹ baicalein, the concentration that lower than previously report detected by the in vitro assay.

Conclusions

Our yeast‐based system has been effective for screening potential compounds that can alleviate α‐syn toxicity with high sensitivity and specificity.

Significance and Impact of the Study

Yeast‐based assay system can be used to discover novel neuroprotective drug candidates which may be either efficiently suppress‐α‐syn aggregation or enhance ubiquitin‐dependent degradation.

Molecular mechanisms and highly functional development for stress tolerance of the yeast Saccharomyces cerevisiae

In response to environmental stress, microorganisms adapt to drastic changes while exerting cellular functions by controlling gene expression, metabolic pathways, enzyme activities, and protein-protein interactions. Microbial cells that undergo a fermentation process are subjected to stresses, such as high temperature, freezing, drying, changes in pH and osmotic pressure, and organic solvents. Combinations of these stresses that continue over long terms often inhibit cells' growth and lead to their death, markedly limiting the useful functions of microorganisms (eg their fermentation ability). Thus, high stress tolerance of cells is required to improve productivity and add value to fermented/brewed foods and biofuels. This review focuses on stress tolerance mechanisms, including l-proline/l-arginine metabolism, ubiquitin system, and transcription factors, and the functional development of the yeast Saccharomyces cerevisiae, which has been used not only in basic science as a model of higher eukaryotes but also in fermentation processes for making alcoholic beverages, food products, and bioethanol.

The C2 domain of the ubiquitin ligase Rsp5 is required for ubiquitination of the endocytic protein Rvs167 upon change of nitrogen source

Ubiquitination is a key signal for endocytosis of proteins on the plasma membrane. The ubiquitin ligase Rsp5 of Saccharomyces cerevisiae, which contains an amino-terminal membrane-binding C2 domain, three substrate-recognizing tryptophan-tryptophan (WW) domains and a carboxyl-terminal catalytic homologous to the E6-AP carboxyl terminus (HECT) domain, can ubiquitinate plasma membrane proteins directing them for endocytosis. Here, we examined the roles of the C2 domain in endocytosis for the downregulation of the general amino acid permease Gap1, which is one of nitrogen-regulated permeases in S. cerevisiae. First, we constructed several rsp5 mutants producing Rsp5 variants without the C2 domain or with amino acid changes of membrane-binding lysine residues. These mutants showed defects in endocytosis of Gap1 in response to a preferred nitrogen source. Intriguingly, we found that ubiquitination of Gap1 in these mutant cells was highly similar to that in wild-type cells during endocytosis. These results indicate that the C2 domain is essential for endocytosis but not for ubiquitination of substrates such as Gap1. Moreover, genetic and biochemical analyses showed that the endocytic protein Rvs167 was ubiquitinated via Rsp5 and the C2 domain was required for efficient ubiquitination in response to a preferred nitrogen source. Here, we propose a mechanism for the C2 domain-mediated endocytosis of plasma membrane permeases.

Sodium Acetate Responses in Saccharomyces cerevisiae and the Ubiquitin Ligase Rsp5

Recent studies have revealed the feasibility of sodium acetate as a potentially novel inhibitor/stressor relevant to the fermentation from neutralized lignocellulosic hydrolysates. This mini-review focuses on the toxicity of sodium acetate, which is composed of both sodium and acetate ions, and on the involved cellular responses that it elicits, particularly via the high-osmolarity glycerol (HOG) pathway, the Rim101 pathway, the P-type ATPase sodium pumps Ena1/2/5, and the ubiquitin ligase Rsp5 with its adaptors. Increased understanding of cellular responses to sodium acetate would improve our understanding of how cells respond not only to different stimuli but also to composite stresses induced by multiple components (e.g., sodium and acetate) simultaneously. Moreover, unraveling the characteristics of specific stresses under industrially related conditions and the cellular responses evoked by these stresses would be a key factor in the industrial yeast strain engineering toward the increased productivity of not only bioethanol but also advanced biofuels and valuable chemicals that will be in demand in the coming era of bio-based industry.

Isolation and functional analysis of yeast ubiquitin ligase Rsp5 variants that alleviate the toxicity of human α-synuclein

The essential ubiquitin ligase Rsp5 is a key enzyme involved in the degradation of abnormal or unfavourable proteins in the yeast Saccharomyces cerevisiae. Overexpression of human α-synuclein (α-syn), a small lipid-binding protein implicated in several neurodegenerative diseases, in S. cerevisiae leads to growth inhibition due to many intracellular defects, including accumulation of reactive oxygen species (ROS). Here, to understand the mechanism of Rsp5-mediated detoxification of α-syn, we isolated novel Rsp5 variants (T255A, D295G, P343S and N427D), which conferred α-syn tolerance to yeast cells. Interestingly, these mutants were phenotypically distinguished from our previously identified RSP5(T357A) mutation, which increases ubiquitination of the general amino acid permease Gap1. Among them, the RSP5(P343S) substitution accelerated the degradation of α-syn, suppressed the accumulation of intracellular ROS and enhanced the interaction with α-syn and its ubiquitination. In contrast, the RSP5(T255A) mutation did not contribute to degradation of α-syn, but improved cell growth under acetate stress conditions, possibly leading to alleviation of the α-syn toxicity. Thus, these novel mutations might be useful not only in elucidating the molecular basis by which disused proteins are specifically recognized and effectively removed but also in screening drug candidates for neurodegenerative diseases or in improving ethanol production under acidic fermentation conditions.