Evaluating the fitness cost of protein expression in Saccharomyces cerevisiae

Protein metabolism is one of the most costly processes in the cell and is therefore expected to be under the effective control of natural selection. We stimulated yeast strains to overexpress each single gene product to approximately 1% of the total protein content. Consistent with previous reports, we found that excessive expression of proteins containing disordered or membrane-protruding regions resulted in an especially high fitness cost. We estimated these costs to be nearly twice as high as for other proteins. There was a ten-fold difference in cost if, instead of entire proteins, only the disordered or membrane-embedded regions were compared with other segments. Although the cost of processing bulk protein was measurable, it could not be explained by several tested protein features, including those linked to translational efficiency or intensity of physical interactions after maturation. It most likely included a number of individually indiscernible effects arising during protein synthesis, maturation, maintenance, (mal)functioning, and disposal. When scaled to the levels normally achieved by proteins in the cell, the fitness cost of dealing with one amino acid in a standard protein appears to be generally very low. Many single amino acid additions or deletions are likely to be neutral even if the effective population size is as large as that of the budding yeast. This should also apply to substitutions. Selection is much more likely to operate if point mutations affect protein structure by, for example, extending or creating stretches that tend to unfold or interact improperly with membranes

Molecular chaperones and selection against mutations

<p>Abstract</p> <p>Background</p> <p>Molecular chaperones help to restore the native states of proteins after their destabilization by external stress. It has been proposed that another function of chaperones is to maintain the activity of proteins destabilized by mutation, weakening the selection against suboptimal protein variants. This would allow for the accumulation of genetic variation which could then be exposed during environmental perturbation and facilitate rapid adaptation.</p> <p>Results</p> <p>We focus on studies describing interactions of chaperones with mutated polypeptides. There are some examples that chaperones can alleviate the deleterious effects of mutations through increased assistance of destabilized proteins. These experiments are restricted to bacteria and typically involve overexpression of chaperones. In eukaryotes, it was found that the malfunctioning of chaperones aggravated phenotypic aberrations associated with mutations. This effect could not be linked to chaperone-mediated stabilization of mutated proteins. More likely, the insufficient activity of chaperones inflicted a deregulation of multiple cellular systems, including those responsible for signaling and therefore important in development. As to why the assistance of mutated proteins by chaperones seems difficult to demonstrate, we note that chaperone-assisted folding can often co-exist with chaperone-assisted degradation. There is growing evidence that some chaperones, including those dependent on Hsp90, can detect potentially functional but excessively unstable proteins and direct them towards degradation instead of folding. This implies that at least some mutations are exposed rather than masked by the activity of molecular chaperones.</p> <p>Conclusion</p> <p>It is at present impossible to determine whether molecular chaperones are mostly helpers or examiners of mutated proteins because experiments showing either of these roles are very few. Depending on whether assistance or disposal prevails, molecular chaperones could speed up or slow down evolution of protein sequences. Similar uncertainties arise when the concept of chaperones (mostly Hsp90) as general regulators of evolvability is considered. If the two roles of chaperones are antagonistic, then any (even small) modification of the chaperone activities to save mutated polypeptides could lead to increased misfolding and aggregation of other proteins. This would be a permanent burden, different from the stochastic cost arising from indiscriminate buffering of random mutations of which many are maladaptive.</p> <p>Reviewers</p> <p>This article was reviewed by A. S. Kondrashov, J. Höhfeld (nominated by A. Eyre-Walker) and D. A. Drummond (nominated by C. Adami). For the full reviews, please go to the Reviewers' comments section.</p

Convergent lifespan reaction norms in the yeast cultures exposed to different environmental stresses

Lifespan extension under mild stress is frequently observed although difficult to quantify and generalize as previous studies differed substantially in specific experimental arrangements. We cultured the budding yeast in several environments defined by different temperature, source of energy, saline concentration or combinations of these factors. Cells obtained under different growth regimes were transferred to identical and generally nonstressful conditions except for an absence of organic carbon. Chronological lifespan (CL) of the starving cells showed an approximately common norm of reaction when plotted against the growth rate which served as a measure of stress intensity. CL increased roughly 50% in cultures raised at moderately slower pace, regardless of what particular single or multiple stress signals were present, and then decreased gradually with a deepening growth deceleration. We suggest that the strongly nonlinear relation between the metabolic rate and longevity can be a potent constraint controlling norms of phenotypic reaction in a variety of environmental gradients

Adaptive roles of SSY1 and SIR3 during cycles of growth and starvation in Saccharomyces cerevisiae populations enriched for quiescent or nonquiescent cells

Over its evolutionary history, Saccharomyces cerevisiae has evolved to be well-adapted to fluctuating nutrient availability. In the presence of sufficient nutrients, yeast cells continue to proliferate, but upon starvation haploid yeast cells enter stationary phase and differentiate into nonquiescent (NQ) and quiescent (Q) cells. Q cells survive stress better than NQ cells and show greater viability when nutrient-rich conditions are restored. To investigate the genes that may be involved in the differentiation of Q and NQ cells, we serially propagated yeast populations that were enriched for either only Q or only NQ cell types over many repeated growth–starvation cycles. After 30 cycles (equivalent to 300 generations), each enriched population produced a higher proportion of the enriched cell type compared to the starting population, suggestive of adaptive change. We also observed differences in each population’s fitness suggesting possible tradeoffs: clones from NQ lines were better adapted to logarithmic growth, while clones from Q lines were better adapted to starvation. Whole-genome sequencing of clones from Q- and NQ-enriched lines revealed mutations in genes involved in the stress response and survival in limiting nutrients (ECM21, RSP5, MSN1, SIR4, and IRA2) in both Q and NQ lines, but also differences between the two lines: NQ line clones had recurrent independent mutations affecting the Ssy1p-Ptr3p-Ssy5p (SPS) amino acid sensing pathway, while Q line clones had recurrent, independent mutations in SIR3 and FAS1. Our results suggest that both sets of enriched-cell type lines responded to common, as well as distinct, selective pressures

Crystallization of Lanthanide—Ho3+ and Tm3+ Ions Doped Tellurite Glasses

In the presented work, the tellurite glasses TeO2-WO3-ZnO doped with Tm3+ and Ho3+ ions were prepared by the same glass forming method. X-ray diffraction (XRD) and differential thermal analysis (DTA) techniques were used to study the effects of the forming technology on the thermal and structural properties of the fabricated glasses. After controlled crystallization of investigated glasses, the emission in the VIS- and NIR range was determined. The effect of silver doping on emission intensity was investigated. The value of the activation energy of the glass crystallization process was determined, while the Ea value for pure TeO2 glass was much lower than for tellurite glasses TeO2-WO3-ZnO

Andropauza i proces starzenia się a jakość życia mężczyzn po 50. roku życia

INTRODUCTION. Andropause and physiological aging of men are the natural stages of their lives. Nevertheless, these issues are often overlooked by men themselves as well as by medical personnel. The primary purpose of self-study is to determine the level of knowledge about the impact of andropause in the aging process of people over 50 years of age. The research is aimed at demonstrating whether the symptoms associated with andropause reduce the comfort of men’s life after the age of 50. MATERIAL AND METHODS. The study was conducted on a group of 50 men aged 50 years. The author used a questionnaire that used the knowledge of the respondents on andropause, their symptoms and their effects on the well-being of men. RESULTS. Men in the study showed varying degrees of knowledge about aging and andro­pause. At different frequencies, they also experienced symptoms characteristic for an ­dropause. CONCLUSIONS. The analysis of the results obtained from the study of andropause and the physiological aging of the organism revealed that the level of knowledge of men in this area was low. Analyzes have also shown that men after 50 years of age experience different ailments characteristic for andropause, and the ailments themselves lessen the comfort of their lives.WSTĘP: Andropauza oraz fizjologiczne starzenie się mężczyzn są naturalnymi etapami ich życia. Niemniej jednak wspomniane zagadnienia są bardzo często pomijane, tak przez samych mężczyzn jak również przez personel medyczny. CEL: Zasadniczym celem badań własnych jest określenie poziomu wiedzy na temat wpływu andropauzy w procesie starzenia się osób po 50 roku życia. Badania mają na celu wykazanie czy objawy towarzyszące andropauzie zmniejszają komfort życia mężczyzn po 50 roku życia. MATERIAŁ I METODA: Badania przeprowadzone zostały na grupie 50 mężczyzn po 50 roku życia. Do ich przeprowadzenia wykorzystano autorski kwestionariusz ankiety, który odnosił się do wiedzy respondentów na temat andropauzy oraz występowania u nich objawów charakterystycznych dla tego stanu i ich wpływu na samopoczucie mężczyzn. WYNIKI: Mężczyźni objęci badaniem wykazywali się różnym stopniem wiedzy na temat procesu starzenia się oraz andropauzy. Z różną częstotliwością odczuwali oni również objawy charakterystyczne dla andropauzy. WNIOSKI: Analiza wyników uzyskanych po przeprowadzeniu badań odnoszących się do zagadnienia andropauzy i fizjologicznego starzenia się organizmu pozwoliły na stwierdzenie, że poziom wiedzy mężczyzn w tym zakresie jest niski. Analizy wykazały również, że mężczyźni po 50 roku życia z różną częstotliwością odczuwają dolegliwości charakterystyczne dla andropauzy, a same dolegliwości zmniejszają komfort ich życia