The Molecular Chaperone DnaK Is a Source of Mutational Robustness

Selection for robust metabolism in domesticated yeasts is driven by adaptation to Hsp90 stress

Protein folding both promotes and constrains adaptive evolution. We uncover this surprising duality in the role of the protein-folding chaperone heat shock protein 90 (Hsp90) in maintaining the integrity of yeast metabolism amid proteotoxic stressors within industrial domestication niches. Ethanol disrupts critical Hsp90-dependent metabolic pathways and exerts strong selective pressure for redundant duplications of key genes within these pathways, yielding the classical genomic signatures of beer and bread domestication. This work demonstrates a mechanism of adaptive canalization in an ecology of major economic importance and highlights Hsp90-dependent variation as an important source of phantom heritability in complex traits.

Mutational robustness and the role of buffer genes in evolvability

Organisms rely on mutations to fuel adaptive evolution. However, many mutations impose a negative effect on fitness. Cells may have therefore evolved mechanisms that affect the phenotypic effects of mutations, thus conferring mutational robustness. Specifically, so-called buffer genes are hypothesized to interact directly or indirectly with genetic variation and reduce its effect on fitness. Environmental or genetic perturbations can change the interaction between buffer genes and genetic variation, thereby unmasking the genetic variation's phenotypic effects and thus providing a source of variation for natural selection to act on. This review provides an overview of our understanding of mutational robustness and buffer genes, with the chaperone gene HSP90 as a key example. It discusses whether buffer genes merely affect standing variation or also interact with de novo mutations, how mutational robustness could influence evolution, and whether mutational robustness might be an evolved trait or rather a mere side-effect of complex genetic interactions.

An essential protease, FtsH, influences daptomycin resistance acquisition in Enterococcus faecalis

Daptomycin is a last-line antibiotic commonly used to treat vancomycin-resistant Enterococci, but resistance evolves rapidly and further restricts already limited treatment options. While genetic determinants associated with clinical daptomycin resistance (DAPR) have been described, information on factors affecting the speed of DAPR acquisition is limited. The multiple peptide resistance factor (MprF), a phosphatidylglycerol-modifying enzyme involved in cationic antimicrobial resistance, is linked to DAPR in pathogens such as methicillin-resistant Staphylococcus aureus. Since Enterococcus faecalis encodes two paralogs of mprF and clinical DAPR mutations do not map to mprF, we hypothesized that functional redundancy between the paralogs prevents mprF-mediated resistance and masks other evolutionary pathways to DAPR. Here, we performed in vitro evolution to DAPR in mprF mutant background. We discovered that the absence of mprF results in slowed DAPR evolution and is associated with inactivating mutations in ftsH, resulting in the depletion of the chaperone repressor HrcA. We also report that ftsH is essential in the parental, but not in the ΔmprF, strain where FtsH depletion results in growth impairment in the parental strain, a phenotype associated with reduced extracellular acidification and reduced ability for metabolic reduction. This presents FtsH and HrcA as enticing targets for developing anti-resistance strategies.

A genome-centric view of the role of the Acropora kenti microbiome in coral health and resilience

Microbial diversity has been extensively explored in reef-building corals. However, the functional roles of coral-associated microorganisms remain poorly elucidated. Here, we recover 191 bacterial and 10 archaeal metagenome-assembled genomes (MAGs) from the coral Acropora kenti (formerly A. tenuis) and adjacent seawater, to identify microbial functions and metabolic interactions within the holobiont. We show that 82 MAGs were specific to the A. kenti holobiont, including members of the Pseudomonadota, Bacteroidota, and Desulfobacterota. A. kenti-specific MAGs displayed significant differences in their genomic features and functional potential relative to seawater-specific MAGs, with a higher prevalence of genes involved in host immune system evasion, nitrogen and carbon fixation, and synthesis of five essential B-vitamins. We find a diversity of A. kenti-specific MAGs encode the biosynthesis of essential amino acids, such as tryptophan, histidine, and lysine, which cannot be de novo synthesised by the host or Symbiodiniaceae. Across a water quality gradient spanning 2° of latitude, A. kenti microbial community composition is correlated to increased temperature and dissolved inorganic nitrogen, with corresponding enrichment in molecular chaperones, nitrate reductases, and a heat-shock protein. We reveal mechanisms of A. kenti-microbiome-symbiosis on the Great Barrier Reef, highlighting the interactions underpinning the health of this keystone holobiont.

Human Hsp70 Substrate-Binding Domains Recognize Distinct Client Proteins

The 13 Hsp70 proteins in humans act on unique sets of substrates with diversity often being attributed to J-domain-containing protein (Hsp40 or JDP) cofactors. We were therefore surprised to find drastically different binding affinities for Hsp70-peptide substrates, leading us to probe substrate specificity among the 8 canonical Hsp70s from humans. We used peptide arrays to characterize Hsp70 binding and then mined these data using machine learning to develop an algorithm for isoform-specific prediction of Hsp70 binding sequences. The results of this algorithm revealed recognition patterns not predicted based on local sequence alignments. We then showed that none of the human isoforms can complement heat-shocked DnaK knockout Escherichia coli cells. However, chimeric Hsp70s consisting of the human nucleotide-binding domain and the substrate-binding domain of DnaK complement during heat shock, providing further evidence in vivo of the divergent function of the Hsp70 substrate-binding domains. We also demonstrated that the differences in heat shock complementation among the chimeras are not due to loss of DnaJ binding. Although we do not exclude JDPs as additional specificity factors, our data demonstrate substrate specificity among the Hsp70s, which has important implications for inhibitor development in cancer and neurodegeneration.

Maltodextrin-binding protein as a key factor in Cronobacter sakazakii survival under desiccation stress

Cronobacter sakazakii (C. sakazakii) is a notorious pathogen responsible for infections in infants and newborns, often transmitted through contaminated infant formula. Despite the use of traditional pasteurization methods, which can reduce microbial contamination, there remains a significant risk of pathogenic C. sakazakii surviving due to its exceptional stress tolerance. In our study, we employed a comparative proteomic approach by comparing wild-type strains with gene knockout strains to identify the essential genes crucial for the successful survival of C. sakazakii during desiccation. Our investigation revealed the significance of envZ-ompR, recA, and flhD gene cassettes in contributing to desiccation tolerance in C. sakazakii. Furthermore, through our comparative proteomic profiling, we identified the maltodextrin-binding protein encoded by ESA_03421 as a potential factor influencing dry tolerance. This protein is regulated by EnvZ-OmpR, RecA, and FlhD. Notably, the knockout of ESA_03421 resulted in a 150% greater reduction in Log CFU compared to the wild-type C. sakazakii. Overall, our findings offer valuable insights into the mechanisms underlying C. sakazakii desiccation tolerance and provide potential targets for the development of new antimicrobial strategies aimed at reducing the risk of infections in infants and newborns.

Viral Evolution Shaped by Host Proteostasis Networks

Understanding the factors that shape viral evolution is critical for developing effective antiviral strategies, accurately predicting viral evolution, and preventing pandemics. One fundamental determinant of viral evolution is the interplay between viral protein biophysics and the host machineries that regulate protein folding and quality control. Most adaptive mutations in viruses are biophysically deleterious, resulting in a viral protein product with folding defects. In cells, protein folding is assisted by a dynamic system of chaperones and quality control processes known as the proteostasis network. Host proteostasis networks can determine the fates of viral proteins with biophysical defects, either by assisting with folding or by targeting them for degradation. In this review, we discuss and analyze new discoveries revealing that host proteostasis factors can profoundly shape the sequence space accessible to evolving viral proteins. We also discuss the many opportunities for research progress proffered by the proteostasis perspective on viral evolution and adaptation.

Selection of Multi-Drug Targets against Drug-Resistant Mycobacterium tuberculosis XDR1219 Using the Hyperbolic Mapping of the Protein Interaction Network

Tuberculosis remains the leading cause of death from a single pathogen. On the other hand, antimicrobial resistance (AMR) makes it increasingly difficult to deal with this disease. We present the hyperbolic embedding of the Mycobacterium tuberculosis protein interaction network (mtbPIN) of resistant strain (MTB XDR1219) to determine the biological relevance of its latent geometry. In this hypermap, proteins with similar interacting partners occupy close positions. An analysis of the hypermap of available drug targets (DTs) and their direct and intermediate interactors was used to identify potentially useful drug combinations and drug targets. We identify rpsA and rpsL as close DTs targeted by different drugs (pyrazinamide and aminoglycosides, respectively) and propose that the combination of these drugs could have a synergistic effect. We also used the hypermap to explain the effects of drugs that affect multiple DTs, for example, forcing the bacteria to deal with multiple stresses like ethambutol, which affects the synthesis of both arabinogalactan and lipoarabinomannan. Our strategy uncovers novel potential DTs, such as dprE1 and dnaK proteins, which interact with two close DT pairs: arabinosyltransferases (embC and embB), Ser/Thr protein kinase (pknB) and RNA polymerase (rpoB), respectively. Our approach provides mechanistic explanations for existing drugs and suggests new DTs. This strategy can also be applied to the study of other resistant strains.

Ethanol Drives Evolution of Hsp90-Dependent Robustness by Redundancy in Yeast Domestication

Protein folding promotes and constrains adaptive evolution. We uncover this surprising duality in the role the protein-folding chaperone Hsp90 plays in mediating the interplay between proteome and the genome which acts to maintain the integrity of yeast metabolism in the face of proteotoxic stressors in anthropic niches. Of great industrial relevance, ethanol concentrations generated by fermentation in the making of beer and bread disrupt critical Hsp90-dependent nodes of metabolism and exert strong selective pressure for increased copy number of key genes encoding components of these nodes, yielding the classical genetic signatures of beer and bread domestication. This work establishes a mechanism of adaptive canalization in an ecology of major economic significance and highlights Hsp90-contingent variation as an important source of phantom heritability in complex traits.

Critical Beginnings: Selective Tuning of Solubility and Structural Accuracy of Newly Synthesized Proteins by the Hsp70 Chaperone System

Proteins are particularly prone to aggregation immediately after release from the ribosome, and it is therefore important to elucidate the role of chaperones during these key steps of protein life. The Hsp70 and trigger factor (TF) chaperone systems interact with nascent proteins during biogenesis and immediately post-translationally. It is unclear, however, whether these chaperones can prevent formation of soluble and insoluble aggregates. Here, we address this question by monitoring the solubility and structural accuracy of globin proteins biosynthesized in an Escherichia coli cell-free system containing different concentrations of the bacterial Hsp70 and TF chaperones. We find that Hsp70 concentrations required to grant solubility to newly synthesized proteins are extremely sensitive to client-protein sequence. Importantly, Hsp70 concentrations yielding soluble client proteins are insufficient to prevent formation of soluble aggregates. In fact, for some aggregation-prone protein variants, avoidance of soluble-aggregate formation demands Hsp70 concentrations that exceed cellular levels in E. coli. In all, our data highlight the prominent role of soluble aggregates upon nascent-protein release from the ribosome and show the limitations of the Hsp70 chaperone system in the case of highly aggregation-prone proteins. These results demonstrate the need to devise better strategies to prevent soluble-aggregate formation upon release from the ribosome.

Discovery and Development of a Selective Inhibitor of the ER Resident Chaperone Grp78

A recent study illustrated that a fluorescence polarization assay can be used to identify substrate-competitive Hsp70 inhibitors that can be isoform-selective. Herein, we use that assay in a moderate-throughput screen and report the discovery of a druglike amino-acid-based inhibitor with reasonable specificity for the endoplasmic reticular Hsp70, Grp78. Using traditional medicinal chemistry approaches, the potency and selectivity were further optimized through structure-activity relationship (SAR) studies in parallel assays for six of the human Hsp70 isoforms. The top compounds were all tested against a panel of cancer cell lines and disappointingly showed little effect. The top-performing compound, 8, was retested using a series of endoplasmic reticulum (ER) stress-inducing agents and found to synergize with these agents. Finally, 8 was tested in a spheroid tumor model and found to be more potent than in two-dimensional models. The optimized Grp78 inhibitors are the first reported isoform-selective small-molecule-competitive inhibitors of an Hsp70-substrate interaction.

Bacterial Hsp90 predominantly buffers but does not potentiate the phenotypic effects of deleterious mutations during fluorescent protein evolution

Chaperones facilitate the folding of other ("client") proteins and can thus affect the adaptive evolution of these clients. Specifically, chaperones affect the phenotype of proteins via two opposing mechanisms. On the one hand, they can buffer the effects of mutations in proteins and thus help preserve an ancestral, premutation phenotype. On the other hand, they can potentiate the effects of mutations and thus enhance the phenotypic changes caused by a mutation. We study that how the bacterial Hsp90 chaperone (HtpG) affects the evolution of green fluorescent protein. To this end, we performed directed evolution of green fluorescent protein under low and high cellular concentrations of Hsp90. Specifically, we evolved green fluorescent protein under both stabilizing selection for its ancestral (green) phenotype and directional selection toward a new (cyan) phenotype. While Hsp90 did only affect the rate of adaptive evolution transiently, it did affect the phenotypic effects of mutations that occurred during adaptive evolution. Specifically, Hsp90 allowed strongly deleterious mutations to accumulate in evolving populations by buffering their effects. Our observations show that the role of a chaperone for adaptive evolution depends on the organism and the trait being studied.

Non-Equilibrium Protein Folding and Activation by ATP-Driven Chaperones

Recent experimental studies suggest that ATP-driven molecular chaperones can stabilize protein substrates in their native structures out of thermal equilibrium. The mechanism of such non-equilibrium protein folding is an open question. Based on available structural and biochemical evidence, I propose here a unifying principle that underlies the conversion of chemical energy from ATP hydrolysis to the conformational free energy associated with protein folding and activation. I demonstrate that non-equilibrium folding requires the chaperones to break at least one of four symmetry conditions. The Hsp70 and Hsp90 chaperones each break a different subset of these symmetries and thus they use different mechanisms for non-equilibrium protein folding. I derive an upper bound on the non-equilibrium elevation of the native concentration, which implies that non-equilibrium folding only occurs in slow-folding proteins that adopt an unstable intermediate conformation in binding to ATP-driven chaperones. Contrary to the long-held view of Anfinsen’s hypothesis that proteins fold to their conformational free energy minima, my results predict that some proteins may fold into thermodynamically unstable native structures with the assistance of ATP-driven chaperones, and that the native structures of some chaperone-dependent proteins may be shaped by their chaperone-mediated folding pathways.

Protein folding in vitro and in the cell: From a solitary journey to a team effort

Correct protein folding is essential for the health and function of living organisms. Yet, it is not well understood how unfolded proteins reach their native state and avoid aggregation, especially within the cellular milieu. Some proteins, especially small, single-domain and apparent two-state folders, successfully attain their native state upon dilution from denaturant. Yet, many more proteins undergo misfolding and aggregation during this process, in a concentration-dependent fashion. Once formed, native and aggregated states are often kinetically trapped relative to each other. Hence, the early stages of protein life are absolutely critical for proper kinetic channeling to the folded state and for long-term solubility and function. This review summarizes current knowledge on protein folding/aggregation mechanisms in buffered solution and within the bacterial cell, highlighting early stages. Remarkably, teamwork between nascent chain, ribosome, trigger factor and Hsp70 molecular chaperones enables all proteins to overcome aggregation propensities and reach a long-lived bioactive state.

The endoplasmic reticulum proteostasis network profoundly shapes the protein sequence space accessible to HIV envelope

The sequence space accessible to evolving proteins can be enhanced by cellular chaperones that assist biophysically defective clients in navigating complex folding landscapes. It is also possible, at least in theory, for proteostasis mechanisms that promote strict quality control to greatly constrain accessible protein sequence space. Unfortunately, most efforts to understand how proteostasis mechanisms influence evolution rely on artificial inhibition or genetic knockdown of specific chaperones. The few experiments that perturb quality control pathways also generally modulate the levels of only individual quality control factors. Here, we use chemical genetic strategies to tune proteostasis networks via natural stress response pathways that regulate the levels of entire suites of chaperones and quality control mechanisms. Specifically, we upregulate the unfolded protein response (UPR) to test the hypothesis that the host endoplasmic reticulum (ER) proteostasis network shapes the sequence space accessible to human immunodeficiency virus-1 (HIV-1) envelope (Env) protein. Elucidating factors that enhance or constrain Env sequence space is critical because Env evolves extremely rapidly, yielding HIV strains with antibody- and drug-escape mutations. We find that UPR-mediated upregulation of ER proteostasis factors, particularly those controlled by the IRE1-XBP1s UPR arm, globally reduces Env mutational tolerance. Conserved, functionally important Env regions exhibit the largest decreases in mutational tolerance upon XBP1s induction. Our data indicate that this phenomenon likely reflects strict quality control endowed by XBP1s-mediated remodeling of the ER proteostasis environment. Intriguingly, and in contrast, specific regions of Env, including regions targeted by broadly neutralizing antibodies, display enhanced mutational tolerance when XBP1s is induced, hinting at a role for host proteostasis network hijacking in potentiating antibody escape. These observations reveal a key function for proteostasis networks in decreasing instead of expanding the sequence space accessible to client proteins, while also demonstrating that the host ER proteostasis network profoundly shapes the mutational tolerance of Env in ways that could have important consequences for HIV adaptation.

The host cell’s endoplasmic reticulum proteostasis network has a profound, constraining impact on the protein sequence space accessible to HIV’s envelope protein, which is a major target of the host’s adaptive immune system; in particular, upregulation of stringent quality control pathways appears to restrict the viability of destabilizing envelope variants.

An allosteric inhibitor of bacterial Hsp70 chaperone potentiates antibiotics and mitigates resistance

DnaK is the bacterial homolog of Hsp70, an ATP-dependent chaperone that helps cofactor proteins to catalyze nascent protein folding and salvage misfolded proteins. In the pathogen Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), DnaK and its cofactors are proposed antimycobacterial targets, yet few small-molecule inhibitors or probes exist for these families of proteins. Here, we describe the repurposing of a drug called telaprevir that is able to allosterically inhibit the ATPase activity of DnaK and to prevent chaperone function by mimicking peptide substrates. In mycobacterial cells, telaprevir disrupts DnaK- and cofactor-mediated cellular proteostasis, resulting in enhanced efficacy of aminoglycoside antibiotics and reduced resistance to the frontline TB drug rifampin. Hence, this work contributes to a small but growing collection of protein chaperone inhibitors, and it demonstrates that these molecules disrupt bacterial mechanisms of survival in the presence of different antibiotic classes.

The interactions of molecular chaperones with client proteins: why are they so weak?

The major classes of molecular chaperones have highly variable sequences, sizes, and shapes, yet they all bind to unfolded proteins, limit their aggregation, and assist in their folding. Despite the central importance of this process to protein homeostasis, it has not been clear exactly how chaperones guide this process or whether the diverse families of chaperones use similar mechanisms. For the first time, recent advances in NMR spectroscopy have enabled detailed studies of how unfolded, "client" proteins interact with both ATP-dependent and ATP-independent classes of chaperones. Here, we review examples from four distinct chaperones, Spy, Trigger Factor, DnaK, and HscA-HscB, highlighting the similarities and differences between their mechanisms. One striking similarity is that the chaperones all bind weakly to their clients, such that the chaperone-client interactions are readily outcompeted by stronger, intra- and intermolecular contacts in the folded state. Thus, the relatively weak affinity of these interactions seems to provide directionality to the folding process. However, there are also key differences, especially in the details of how the chaperones release clients and how ATP cycling impacts that process. For example, Spy releases clients in a largely folded state, while clients seem to be unfolded upon release from Trigger Factor or DnaK. Together, these studies are beginning to uncover the similarities and differences in how chaperones use weak interactions to guide protein folding.

Transcriptional Response of Wolbachia to Dengue Virus Infection in Cells of the Mosquito Aedes aegypti

Aedes aegypti transmits one of the most significant mosquito-borne viruses, dengue virus (DENV). The absence of effective vaccines and clinical treatments and the emergence of insecticide resistance in A. aegypti necessitate novel vector control strategies. A new approach uses the endosymbiotic bacterium Wolbachia pipientis to reduce the spread of arboviruses. However, the Wolbachia-mediated antiviral mechanism is not well understood. To shed light on this mechanism, we investigated an unexplored aspect of Wolbachia-virus-mosquito interaction. We used RNA sequencing to examine the transcriptional response of Wolbachia to DENV infection in A. aegypti Aag2 cells transinfected with the wAlbB strain of Wolbachia. Our results suggest that genes encoding an endoribonuclease (RNase HI), a regulator of sigma 70-dependent gene transcription (6S RNA), essential cellular, transmembrane, and stress response functions and primary type I and IV secretion systems were upregulated, while a number of transport and binding proteins of Wolbachia, ribosome structure, and elongation factor-associated genes were downregulated due to DENV infection. Furthermore, bacterial retrotransposon, transposable, and phage-related elements were found among the up- and downregulated genes. We show that Wolbachia elicits a transcriptional response to virus infection and identify differentially expressed Wolbachia genes mostly at the early stages of virus infection. These findings highlight Wolbachia's ability to alter its gene expression in response to DENV infection of the host cell. IMPORTANCE Aedes aegypti is a vector of several pathogenic viruses, including dengue, Zika, chikungunya, and yellow fever viruses, which are of importance to human health. Wolbachia is an endosymbiotic bacterium currently used in transinfected mosquitoes to suppress replication and transmission of dengue viruses. However, the mechanism of Wolbachia-mediated virus inhibition is not fully understood. While several studies have shown mosquitoes' transcriptional responses to dengue virus infection, none have investigated these responses in Wolbachia, which may provide clues to the inhibition mechanism. Our results suggest changes in the expression of a number of functionally important Wolbachia genes upon dengue virus infection, including those involved in stress responses, providing insights into the endosymbiont's reaction to virus infection.

Genomic insights on DNase production in Streptococcus agalactiae ST17 and ST19 strains

Streptococcus agalactiae evasion from the human defense mechanisms has been linked to the production of DNases. These were proposed to contribute to the hypervirulence of S. agalactiae ST17/capsular-type III strains, mostly associated with neonatal meningitis. We performed a comparative genomic analysis between ST17 and ST19 human strains with different cell tropism and distinct DNase production phenotypes. All S. agalactiae ST17 strains, with the exception of 2211-04, were found to display DNase activity, while the opposite scenario was observed for ST19, where 1203-05 was the only DNase(+) strain. The analysis of the genetic variability of the seven genes putatively encoding secreted DNases in S. agalactiae revealed an exclusive amino acid change in the predicted signal peptide of GBS0661 (NucA) of the ST17 DNase(-), and an exclusive amino acid change alteration in GBS0609 of the ST19 DNase(+) strain. Further core-genome analysis identified some specificities (SNVs or indels) differentiating the DNase(-) ST17 2211-04 and the DNase(+) ST19 1203-05 from the remaining strains of each ST. The pan-genomic analysis evidenced an intact phage without homology in S. agalactiae and a transposon homologous to TnGBS2.3 in ST17 DNase(-) 2211-04; the transposon was also found in one ST17 DNase(+) strain, yet with a different site of insertion. A group of nine accessory genes were identified among all ST17 DNase(+) strains, including the Eco47II family restriction endonuclease and the C-5 cytosine-specific DNA methylase. None of these loci was found in any DNase(-) strain, which may suggest that these proteins might contribute to the lack of DNase activity. In summary, we provide novel insights on the genetic diversity between DNase(+) and DNase(-) strains, and identified genetic traits, namely specific mutations affecting predicted DNases (NucA and GBS0609) and differences in the accessory genome, that need further investigation as they may justify distinct DNase-related virulence phenotypes in S. agalactiae.

The Hsp70-Chaperone Machines in Bacteria

The ATP-dependent Hsp70s are evolutionary conserved molecular chaperones that constitute central hubs of the cellular protein quality surveillance network. None of the other main chaperone families (Tig, GroELS, HtpG, IbpA/B, ClpB) have been assigned with a comparable range of functions. Through a multitude of functions Hsp70s are involved in many cellular control circuits for maintaining protein homeostasis and have been recognized as key factors for cell survival. Three mechanistic properties of Hsp70s are the basis for their high versatility. First, Hsp70s bind to short degenerate sequence motifs within their client proteins. Second, Hsp70 chaperones switch in a nucleotide-controlled manner between a state of low affinity for client proteins and a state of high affinity for clients. Third, Hsp70s are targeted to their clients by a large number of cochaperones of the J-domain protein (JDP) family and the lifetime of the Hsp70-client complex is regulated by nucleotide exchange factors (NEF). In this review I will discuss advances in the understanding of the molecular mechanism of the Hsp70 chaperone machinery focusing mostly on the bacterial Hsp70 DnaK and will compare the two other prokaryotic Hsp70s HscA and HscC with DnaK.

Function, Therapeutic Potential, and Inhibition of Hsp70 Chaperones

Hsp70s are among the most highly conserved proteins in all of biology. Through an iterative binding and release of exposed hydrophobic residues on client proteins, Hsp70s can prevent aggregation and promote folding to the native state of their client proteins. The human proteome contains eight canonical Hsp70s. Because Hsp70s are relatively promiscuous they play a role in folding a large proportion of the proteome. Hsp70s are implicated in disease through their ability to regulate protein homeostasis. In recent years, researchers have attempted to develop selective inhibitors of Hsp70 isoforms to better understand the role of individual isoforms in biology and as potential therapeutics. Selective inhibitors have come from rational design, forced localization, and serendipity, but the development of completely selective inhibitors remains elusive. In the present review, we discuss the Hsp70 structure and function, the known Hsp70 client proteins, the role of Hsp70s in disease, and current efforts to discover Hsp70 modulators.

The DnaK Chaperone System Buffers the Fitness Cost of Antibiotic Resistance Mutations in Mycobacteria

Chaperones aid in protein folding and maintenance of protein integrity. In doing so, they have the unique ability to directly stabilize resistance-conferring amino acid substitutions in drug targets and to counter the stress imparted by these substitutions, thus supporting heritable antimicrobial resistance (AMR). We asked whether chaperones support AMR in Mycobacterium smegmatis, a saprophytic model of Mycobacterium tuberculosis, the causative agent of tuberculosis (TB). We show that DnaK associates with many drug targets and that DnaK associates more with AMR-conferring mutant RNA polymerase (RNAP) than with wild-type RNAP. In addition, frequency-of-resistance (FOR) and fitness studies reveal that the DnaK system of chaperones supports AMR in antimicrobial targets in mycobacteria, including RNAP and the ribosome. These findings highlight chaperones as potential targets for drugs to overcome AMR in mycobacteria, including M. tuberculosis, as well as in other pathogens.IMPORTANCE AMR is a global problem, especially for TB. Here, we show that mycobacterial chaperones support AMR in M. smegmatis, a nonpathogenic model of M. tuberculosis, the causative agent of TB. In particular, the mycobacterial DnaK system of chaperones supports AMR in the antimicrobial targets RNA polymerase and the ribosome. This is the first report showing a role for protein chaperones in mediating AMR in mycobacteria. Given the widespread role of protein chaperones in enabling genomic diversity, we anticipate that our findings can be extended to other microbes.

Contact Effect of a Methylobacterium sp. Extract on Biofilm of a Mycobacterium chimaera Strain Isolated from a 3T Heater-Cooler System

Mycobacterium chimaera is an opportunistic slowly growing non-tuberculous mycobacteriumof increasing importance due to the outbreak of cases associated with contaminated 3T heater-cooler device (HCD) extracorporeal membrane oxygenator (ECMO). The aim of this study was to evaluate the effect of pre-treating a surface with a Methylobacterium sp. CECT 7180 extract to inhibit the M. chimaera ECMO biofilm as well as of the treatment after different dehydration times. Surface adherence, biofilm formation and treatment effect were evaluated by estimating colony-forming units (CFU) per square centimeter and characterizing the amount of covered surface area, thickness, cell viability, and presence of intrinsic autofluorescence at different times using confocal laser scanning microscopy and image analysis. We found that exposing a surface to the Methylobacterium sp. CECT 7180 extract inhibited M. chimaera ECMO biofilm development. This effect could be result of the effect of Methylobacterium proteins, such as DNaK, trigger factor, and xanthine oxidase. In conclusion, exposing a surface to the Methylobacteriumsp. extract inhibits M. chimaera ECMO biofilm development. Furthermore, this extract could be used as a pre-treatment prior to disinfection protocols for equipment contaminated with mycobacteria after dehydration for at least 96 h.

The Ingenuity of Bacterial Genomes

The genomes of bacteria contain fewer genes and substantially less noncoding DNA than those of eukaryotes, and as a result, they have much less raw material to invent new traits. Yet, bacteria are vastly more taxonomically diverse, numerically abundant, and globally successful in colonizing new habitats compared to eukaryotes. Although bacterial genomes are generally considered to be optimized for efficient growth and rapid adaptation, nonadaptive processes have played a major role in shaping the size, contents, and compact organization of bacterial genomes and have allowed the establishment of deleterious traits that serve as the raw materials for genetic innovation.

Highly parallel lab evolution reveals that epistasis can curb the evolution of antibiotic resistance

Genetic perturbations that affect bacterial resistance to antibiotics have been characterized genome-wide, but how do such perturbations interact with subsequent evolutionary adaptation to the drug? Here, we show that strong epistasis between resistance mutations and systematically identified genes can be exploited to control spontaneous resistance evolution. We evolved hundreds of Escherichia coli K-12 mutant populations in parallel, using a robotic platform that tightly controls population size and selection pressure. We find a global diminishing-returns epistasis pattern: strains that are initially more sensitive generally undergo larger resistance gains. However, some gene deletion strains deviate from this general trend and curtail the evolvability of resistance, including deletions of genes for membrane transport, LPS biosynthesis, and chaperones. Deletions of efflux pump genes force evolution on inferior mutational paths, not explored in the wild type, and some of these essentially block resistance evolution. This effect is due to strong negative epistasis with resistance mutations. The identified genes and cellular functions provide potential targets for development of adjuvants that may block spontaneous resistance evolution when combined with antibiotics.

The antibiotic resistance crisis calls for new ways of restricting the ability of bacteria to evolve resistance. Here, Lukačišinová et al. perform highly controlled evolution experiments in E. coli strains to identify genetic perturbations that strongly limit the evolution of antibiotic resistance through epistasis.

Chaperone client proteins evolve slower than non-client proteins

Chaperones are important molecular machinery that assists proteins to attain their native three-dimensional structure crucial for function. Earlier studies using experimental evolution showed that chaperones impose a relaxation of sequence constraints on their "client" proteins, which may lead to the fixation of slightly deleterious mutations on the latter. However, we hypothesized that such a phenomenon might be harmful to the organism in a natural physiological condition. In this study, we investigated the evolutionary rates of chaperone client and non-client proteins in five model organisms from both prokaryotic and eukaryotic lineages. Our study reveals a slower evolutionary rate of chaperone client proteins in all five organisms. Additionally, the slower folding rate and lower aggregation propensity of chaperone client proteins reveal that the chaperone may play an essential role in rescuing the slightly disadvantageous effects due to random mutations and subsequent protein misfolding. However, the fixation of such mutations is less likely to be selected in the natural population.

Molecular Chaperones Accelerate the Evolution of Their Protein Clients in Yeast

Protein stability is a major constraint on protein evolution. Molecular chaperones, also known as heat-shock proteins, can relax this constraint and promote protein evolution by diminishing the deleterious effect of mutations on protein stability and folding. This effect, however, has only been stablished for a few chaperones. Here, we use a comprehensive chaperone–protein interaction network to study the effect of all yeast chaperones on the evolution of their protein substrates, that is, their clients. In particular, we analyze how yeast chaperones affect the evolutionary rates of their clients at two very different evolutionary time scales. We first study the effect of chaperone-mediated folding on protein evolution over the evolutionary divergence of Saccharomyces cerevisiae and S. paradoxus. We then test whether yeast chaperones have left a similar signature on the patterns of standing genetic variation found in modern wild and domesticated strains of S. cerevisiae. We find that genes encoding chaperone clients have diverged faster than genes encoding non-client proteins when controlling for their number of protein–protein interactions. We also find that genes encoding client proteins have accumulated more intraspecific genetic diversity than those encoding non-client proteins. In a number of multivariate analyses, controlling by other well-known factors that affect protein evolution, we find that chaperone dependence explains the largest fraction of the observed variance in the rate of evolution at both evolutionary time scales. Chaperones affecting rates of protein evolution mostly belong to two major chaperone families: Hsp70s and Hsp90s. Our analyses show that protein chaperones, by virtue of their ability to buffer destabilizing mutations and their role in modulating protein genotype–phenotype maps, have a considerable accelerating effect on protein evolution.

Improvement of solubility and yield of recombinant protein expression in E. coli using a two-step system

Overexpression of recombinant proteins in Escherichia coli results in inclusion body formation, and consequently decreased production yield and increased production cost. Co-expression of chaperon systems accompanied by recombinant protein is a general method to increase the production yield. However, it has not been successful enough due to imposed intense stress to the host cells. The aim of this study was to balance the rate of protein production and the imposed cellular stresses using a two-step expression system. For this purpose, in the first step, green fluorescent protein (GFP) was expressed as a recombinant protein model under control of the T7-TetO artificial promoter-operator, accompanied by Dnak/J/GrpE chaperon system. Then, in the next step, TetR repressor was activated automatically under the control of the stress promoter ibpAB and suppressed the GFP production after accumulation of inclusion bodies. Thus in this step incorrect folded proteins and inclusion bodies are refolded causing increased yield and solubility of the recombinant protein and restarting GFP expression again. Total GFP, soluble and insoluble GFP fractions, were measured by Synergy H1 multiple reader. Results showed that expression yield and soluble/insoluble ratio of GFP have been increased 5 and 2.5 times using this system in comparison with the single step process, respectively. The efficiency of this system in increasing solubility and production yield of recombinant proteins was confirmed. The two-step system must be evaluated for expression of various proteins to further confirm its applicability in the field of recombinant protein production.

The causes of evolvability and their evolution

Evolvability is the ability of a biological system to produce phenotypic variation that is both heritable and adaptive. It has long been the subject of anecdotal observations and theoretical work. In recent years, however, the molecular causes of evolvability have been an increasing focus of experimental work. Here, we review recent experimental progress in areas as different as the evolution of drug resistance in cancer cells and the rewiring of transcriptional regulation circuits in vertebrates. This research reveals the importance of three major themes: multiple genetic and non-genetic mechanisms to generate phenotypic diversity, robustness in genetic systems, and adaptive landscape topography. We also discuss the mounting evidence that evolvability can evolve and the question of whether it evolves adaptively.

Chaperonin overproduction and metabolic erosion caused by mutation accumulation in Escherichia coli

Bacterial cells adapting to a constant environment tend to accumulate mutations in portions of their genome that are not maintained by selection. This process has been observed in bacteria evolving under strong genetic drift, and especially in bacterial endosymbionts of insects. Here, we study this process in hypermutable Escherichia coli populations evolved through 250 single-cell bottlenecks on solid rich medium in a mutation accumulation experiment that emulates the evolution of bacterial endosymbionts. Using phenotype microarrays monitoring metabolic activity in 95 environments distinguished by their carbon sources, we observe how mutation accumulation has decreased the ability of cells to metabolize most carbon sources. We study if the chaperonin GroEL, which is naturally overproduced in bacterial endosymbionts, can ameliorate the process of metabolic erosion, because of its known ability to buffer destabilizing mutations in metabolic enzymes. Our results indicate that GroEL can slow down the negative phenotypic consequences of genome decay in some environments.

Immunogenic and protective antigens of Brucella as vaccine candidates

Brucella is an intracellular pathogen that causes abortion in domestic animals and undulant fever in humans. Due to the lack of a human vaccine against brucellosis, animal vaccines play an important role in the management of animal and human brucellosis for decades. Strain 19, RB51 and Rev1 are the approved Brucella spp. vaccine strains that are most commonly used to protect livestock against infection and abortion. However, due to some disadvantages of these vaccines, numerous studies have been conducted for the development of effective vaccines that could also be used in other susceptible animals. In this review, we compare different aspects of immunogenic antigens that have been a candidate for the brucellosis vaccine.

Metabolic Determinants of Enzyme Evolution in a Genome-Scale Bacterial Metabolic Network

Different genes and proteins evolve at very different rates. To identify the factors that explain these differences is an important aspect of research in molecular evolution. One such factor is the role a protein plays in a large molecular network. Here, we analyze the evolutionary rates of enzyme-coding genes in the genome-scale metabolic network of Escherichia coli to find the evolutionary constraints imposed by the structure and function of this complex metabolic system. Central and highly connected enzymes appear to evolve more slowly than less connected enzymes, but we find that they do so as a by-product of their high abundance, and not because of their position in the metabolic network. In contrast, enzymes catalyzing reactions with high metabolic flux-high substrate to product conversion rates-evolve slowly even after we account for their abundance. Moreover, enzymes catalyzing reactions that are difficult to by-pass through alternative pathways, such that they are essential in many different genetic backgrounds, also evolve more slowly. Our analyses show that an enzyme's role in the function of a metabolic network affects its evolution more than its place in the network's structure. They highlight the value of a system-level perspective for studies of molecular evolution.

Chaperones, Canalization, and Evolution of Animal Forms

Over half a century ago, British developmental biologist Conrad Hal Waddington proposed the idea of canalization, that is, homeostasis in development. Since the breakthrough that was made by Rutherford and Lindquist (1998), who proposed a role of Hsp90 in developmental buffering, chaperones have gained much attention in the study of canalization. However, recent studies have revealed that a number of other molecules are also potentially involved in canalization. Here, I introduce the emerging role of DnaJ chaperones in canalization. I also discuss how the expression levels of such buffering molecules can be altered, thereby altering organismal development. Since developmental robustness is maternally inherited in various organisms, I propose that dynamic bet hedging, an increase in within-clutch variation in offspring phenotypes that is caused by unpredictable environmental challenges to the mothers, plays a key role in altering the expression levels of buffering molecules. Investigating dynamic bet hedging at the molecular level and how it impacts upon morphological phenotypes will help our understanding of the molecular mechanisms of canalization and evolutionary processes.

Cochaperones enable Hsp70 to use ATP energy to stabilize native proteins out of the folding equilibrium

The heat shock protein 70 (Hsp70) chaperones, vital to the proper folding of proteins inside cells, consume ATP and require cochaperones in assisting protein folding. It is unclear whether Hsp70 can utilize the free energy from ATP hydrolysis to fold a protein into a native state that is thermodynamically unstable in the chaperone-free equilibrium. Here I present a model of Hsp70-mediated protein folding, which predicts that Hsp70, as a result of differential stimulation of ATP hydrolysis by its Hsp40 cochaperone, dissociates faster from a substrate in fold-competent conformations than from one in misfolding-prone conformations, thus elevating the native concentration above and suppressing the misfolded concentration below their respective equilibrium values. Previous models would not make or imply these predictions, which are experimentally testable. My model quantitatively reproduces experimental refolding kinetics, predicts how modulations of the Hsp70/Hsp40 chaperone system affect protein folding, and suggests new approaches to regulating cellular protein quality.

Protein evolution speed depends on its stability and abundance and on chaperone concentrations

Some biological evolution is slow (millions of years), and some is fast (months to years). The speed at which a protein evolves depends on how stable a protein’s folded structure is, how well it avoids aggregation, and how well-chaperoned it is. What are the mechanisms? We compute fitness landscapes by combining a model of protein-folding equilibria with sequence-change dynamics. We find that adapting to a new environment is fastest for proteins that are least stably folded, because those sit on steep downhill parts of fitness potentials. The modeling shows that cells should adapt to warmer environments faster than to colder ones, explains why increasing a protein’s abundance slows cell evolution, and explains how chaperones accelerate evolution by mitigating this effect.

Proteins evolve at different rates. What drives the speed of protein sequence changes? Two main factors are a protein’s folding stability and aggregation propensity. By combining the hydrophobic–polar (HP) model with the Zwanzig–Szabo–Bagchi rate theory, we find that: (i) Adaptation is strongly accelerated by selection pressure, explaining the broad variation from days to thousands of years over which organisms adapt to new environments. (ii) The proteins that adapt fastest are those that are not very stably folded, because their fitness landscapes are steepest. And because heating destabilizes folded proteins, we predict that cells should adapt faster when put into warmer rather than cooler environments. (iii) Increasing protein abundance slows down evolution (the substitution rate of the sequence) because a typical protein is not perfectly fit, so increasing its number of copies reduces the cell’s fitness. (iv) However, chaperones can mitigate this abundance effect and accelerate evolution (also called evolutionary capacitance) by effectively enhancing protein stability. This model explains key observations about protein evolution rates.

Mutations, protein homeostasis, and epigenetic control of genome integrity

From bacteria to humans, ancient stress responses enable organisms to contend with damage to both the genome and the proteome. These pathways have long been viewed as fundamentally separate responses. Yet recent discoveries from multiple fields have revealed surprising links between the two. Many DNA-damaging agents also target proteins, and mutagenesis induced by DNA damage produces variant proteins that are prone to misfolding, degradation, and aggregation. Likewise, recent studies have observed pervasive engagement of a p53-mediated response, and other factors linked to maintenance of genomic integrity, in response to misfolded protein stress. Perhaps most remarkably, protein aggregation and self-assembly has now been observed in multiple proteins that regulate the DNA damage response. The importance of these connections is highlighted by disease models of both cancer and neurodegeneration, in which compromised DNA repair machinery leads to profound defects in protein quality control, and vice versa.

Evolution of heat-shock protein expression underlying adaptive responses to environmental stress

Heat-shock proteins (Hsps) and their cognates are primary mitigators of cell stress. With increasingly severe impacts of climate change and other human modifications of the biosphere, the ability of the heat-shock system to affect evolutionary fitness in environments outside the laboratory and to evolve in response is topic of growing importance. Since the last major reviews, several advances have occurred. First, demonstrations of the heat-shock response outside the laboratory now include many additional taxa and environments. Many of these demonstrations are only correlative, however. More importantly, technical advances in "omic" quantification of nucleic acids and proteins, genomewide association analysis, and manipulation of genes and their expression have enabled the field to move beyond correlation. Several consequent advances are already evident: The pathway from heat-shock gene expression to stress tolerance in nature can be extremely complex, mediated through multiple biological processes and systems, and even multiple species. The underlying genes are more numerous, diverse and variable than previously appreciated, especially with respect to their regulatory variation and epigenetic changes. The impacts and limitations (e.g., due to trade-offs) of natural selection on these genes have become more obvious and better established. At last, as evolutionary capacitors, Hsps may have distinctive impacts on the evolution of other genes and ecological consequences.

Tighter Ligand Binding Can Compensate for Impaired Stability of an RNA-Binding Protein

It has been widely shown that ligand-binding residues, by virtue of their orientation, charge, and solvent exposure, often have a net destabilizing effect on proteins that is offset by stability conferring residues elsewhere in the protein. This structure-function trade-off can constrain possible adaptive evolutionary changes of function and may hamper protein engineering efforts to design proteins with new functions. Here, we present evidence from a large randomized mutant library screen that, in the case of PUF RNA-binding proteins, this structural relationship may be inverted and that active-site mutations that increase protein activity are also able to compensate for impaired stability. We show that certain mutations in RNA-protein binding residues are not necessarily destabilizing and that increased ligand-binding can rescue an insoluble, unstable PUF protein. We hypothesize that these mutations restabilize the protein via thermodynamic coupling of protein folding and RNA binding.

It's not magic - Hsp90 and its effects on genetic and epigenetic variation

Canalization, or phenotypic robustness in the face of environmental and genetic perturbation, is an emergent property of living systems. Although this phenomenon has long been recognized, its molecular underpinnings have remained enigmatic until recently. Here, we review the contributions of the molecular chaperone Hsp90, a protein that facilitates the folding of many key regulators of growth and development, to canalization of phenotype - and de-canalization in times of stress - drawing on studies in eukaryotes as diverse as baker's yeast, mouse ear cress, and blind Mexican cavefish. Hsp90 is a hub of hubs that interacts with many so-called 'client proteins,' which affect virtually every aspect of cell signaling and physiology. As Hsp90 facilitates client folding and stability, it can epistatically suppress or enable the expression of genetic variants in its clients and other proteins that acquire client status through mutation. Hsp90's vast interaction network explains the breadth of its phenotypic reach, including Hsp90-dependent de novo mutations and epigenetic effects on gene regulation. Intrinsic links between environmental stress and Hsp90 function thus endow living systems with phenotypic plasticity in fluctuating environments. As environmental perturbations alter Hsp90 function, they also alter Hsp90's interaction with its client proteins, thereby re-wiring networks that determine the genotype-to-phenotype map. Ensuing de-canalization of phenotype creates phenotypic diversity that is not simply stochastic, but often has an underlying genetic basis. Thus, extreme phenotypes can be selected, and assimilated so that they no longer require environmental stress to manifest. In addition to acting on standing genetic variation, Hsp90 perturbation has also been linked to increased frequency of de novo variation and several epigenetic phenomena, all with the potential to generate heritable phenotypic change. Here, we aim to clarify and discuss the multiple means by which Hsp90 can affect phenotype and possibly evolutionary change, and identify their underlying common feature: at its core, Hsp90 interacts epistatically through its chaperone function with many other genes and their gene products. Its influence on phenotypic diversification is thus not magic but rather a fundamental property of genetics.

Hsp90 shapes protein and RNA evolution to balance trade-offs between protein stability and aggregation

Acquisition of mutations is central to evolution; however, the detrimental effects of most mutations on protein folding and stability limit protein evolvability. Molecular chaperones, which suppress aggregation and facilitate polypeptide folding, may alleviate the effects of destabilizing mutations thus promoting sequence diversification. To illuminate how chaperones can influence protein evolution, we examined the effect of reduced activity of the chaperone Hsp90 on poliovirus evolution. We find that Hsp90 offsets evolutionary trade-offs between protein stability and aggregation. Lower chaperone levels favor variants of reduced hydrophobicity and protein aggregation propensity but at a cost to protein stability. Notably, reducing Hsp90 activity also promotes clusters of codon-deoptimized synonymous mutations at inter-domain boundaries, likely to facilitate cotranslational domain folding. Our results reveal how a chaperone can shape the sequence landscape at both the protein and RNA levels to harmonize competing constraints posed by protein stability, aggregation propensity, and translation rate on successful protein biogenesis.

Disulfide Bonds Enable Accelerated Protein Evolution

The different proteins of any proteome evolve at enormously different rates. What factors contribute to this variability, and to what extent, is still a largely open question. We hypothesized that disulfide bonds, by increasing protein stability, should make proteins' structures relatively independent of their amino acid sequences, thus acting as buffers of deleterious mutations and enabling accelerated sequence evolution. In agreement with this hypothesis, we observed that membrane proteins with disulfide bonds evolved 88% faster than those without disulfide bonds, and that extracellular proteins with disulfide bonds evolved 49% faster than those without disulfide bonds. In addition, genes encoding proteins with disulfide bonds exhibit an increased likelihood of showing signatures of positive selection. Multivariate analyses indicate that the trend is independent of a number of potentially confounding factors. The effect, however, is not observed among the longest proteins, which can become stabilized by mechanisms other than disulfide bonds.

In it for the long haul: evolutionary consequences of persistent endosymbiosis

Phylogenetically independent bacterial lineages have undergone a profound lifestyle shift: from a free-living to obligately host-associated existence. Among these lineages, intracellular bacterial mutualists of insects are among the most intimate, constrained symbioses known. These obligate endosymbionts exhibit severe gene loss and apparent genome deterioration. Evolutionary theory provides a basis to link their unusual genomic features with shifts in fundamental mechanisms - selection, genetic drift, mutation, and recombination. This mini-review highlights recent comparative and experimental research of processes shaping ongoing diversification within these ancient associations. Recent work supports clear contributions of stochastic processes, including genetic drift and exceptionally strong mutational pressure, toward degenerative evolution. Despite possible compensatory mechanisms, genome degradation may constrain how persistent endosymbionts (and their hosts) respond to environmental fluctuations.

Chance and necessity in the genome evolution of endosymbiotic bacteria of insects

An open question in evolutionary biology is how does the selection–drift balance determine the fates of biological interactions. We searched for signatures of selection and drift in genomes of five endosymbiotic bacterial groups known to evolve under strong genetic drift. Although most genes in endosymbiotic bacteria showed evidence of relaxed purifying selection, many genes in these bacteria exhibited stronger selective constraints than their orthologs in free-living bacterial relatives. Remarkably, most of these highly constrained genes had no role in the host–symbiont interactions but were involved in either buffering the deleterious consequences of drift or other host-unrelated functions, suggesting that they have either acquired new roles or their role became more central in endosymbiotic bacteria. Experimental evolution of Escherichia coli under strong genetic drift revealed remarkable similarities in the mutational spectrum, genome reduction patterns and gene losses to endosymbiotic bacteria of insects. Interestingly, the transcriptome of the experimentally evolved lines showed a generalized deregulation of the genome that affected genes encoding proteins involved in mutational buffering, regulation and amino acid biosynthesis, patterns identical to those found in endosymbiotic bacteria. Our results indicate that drift has shaped endosymbiotic associations through a change in the functional landscape of bacterial genes and that the host had only a small role in such a shift.

Bridging the physical scales in evolutionary biology: from protein sequence space to fitness of organisms and populations

Bridging the gap between the molecular properties of proteins and organismal/population fitness is essential for understanding evolutionary processes. This task requires the integration of the several physical scales of biological organization, each defined by a distinct set of mechanisms and constraints, into a single unifying model. The molecular scale is dominated by the constraints imposed by the physico-chemical properties of proteins and their substrates, which give rise to trade-offs and epistatic (non-additive) effects of mutations. At the systems scale, biological networks modulate protein expression and can either buffer or enhance the fitness effects of mutations. The population scale is influenced by the mutational input, selection regimes, and stochastic changes affecting the size and structure of populations, which eventually determine the evolutionary fate of mutations. Here, we summarize the recent advances in theory, computer simulations, and experiments that advance our understanding of the links between various physical scales in biology.