Further support was provided by Burga et al. 2002), the yeast Saccharomyces cerevisae ( Cowen and Lindquist 2005) and the fish Astyanax mexicanus ( Rohner et al. Subsequently, similar observations have been made in the plant Arabidopsis thaliana ( Queitsch et al. Pioneering work carried out by Rutherford and Lindquist (1998) showed that inhibition of the chaperone Hsp90 can unveil cryptic genetic variation-genotypic variation without phenotypic variation-in the fruit fly Drosophila melanogaster. A recent study has shown that RNA chaperones-they help RNA molecules to fold properly, and comprise a class of chaperones different from these three systems-can also buffer deleterious mutations in Escherichia coli ( Rudan et al. Overwhelming evidence shows that Hsp90 and GroEL can buffer mutations ( Bogumil and Dagan 2012), but whether the same holds for any major chaperone from the Hsp70 system is to our knowledge unknown. There are three main chaperone systems, which are the Hsp90 system, the Hsp70 system, and the Hsp60 system (or chaperonins), of which the bacterial GroEL is a prominent member ( Hartl et al. Specifically, they increase the number of amino acid sequences that fold into the same structure and that can perform the function associated with this structure. In the context of protein evolution, chaperones are able to increase a protein’s mutational robustness because they alter the mapping from protein genotypes into protein phenotypes, that is, into the structures that proteins form ( Rutherford 2003). Because some chaperones can buffer the deleterious effects of mutations that affect protein folding, they are also a source of mutational robustness. Thanks to these roles, chaperones can restore the native conformation of proteins destabilized by environmental perturbations, thus providing environmental robustness to organisms coping with stressful conditions. 2004 Hartl and Hayer-Hartl 2009 Hartl et al. Chaperones, also called heat-shock proteins, assist proteins in reaching their native conformations, prevent protein aggregation, and refold misfolded proteins ( Young et al. Molecular chaperones ( Ellis 1987) are one of the best-known sources of both types of robustness ( Fares 2015). The robustness of a system against perturbations that are environmental (e.g., a change in temperature) is referred to as environmental robustness, whereas robustness against perturbations caused by genetic mutations receives the name of mutational or genetic robustness. This property describes the ability of a biological system to preserve its phenotype in a particular environment despite perturbations that it encounters. Robustness is one of the fundamental properties of living systems ( de Visser et al. Molecular chaperones, mutational robustness, experimental evolution, protein evolution, Escherichia coli, DnaK Introduction They illustrate how an individual protein like a chaperone can have a disproportionate effect on the evolution of a proteome. Our results imply that all three major chaperone classes can buffer mutations and affect protein evolution. We find that clients that interact strongly with DnaK evolve faster than weakly interacting clients. coli, Salmonella enterica, and 83 other gamma-proteobacteria. Specifically, we studied the evolutionary rates of DnaK clients using the genomes of E. We also show that this elevated mutational buffering translates into differences in evolutionary rates on intermediate and long evolutionary time scales. Additionally, our sequence data show that DnaK overexpression increases mutational robustness, the tolerance of its clients to nonsynonymous nucleotide substitutions. Overexpression of the Hsp70 chaperone DnaK helps cells cope with mutational load and completely avoid the extinctions we observe in lineages evolving without chaperone overproduction. To this end, we performed a mutation accumulation experiment in Escherichia coli, followed by whole-genome resequencing. Hsp70 chaperones use a chaperoning mechanism different from that of Hsp90 and GroEL, and it is not known whether they can also buffer mutations. Thanks to these abilities, some chaperones, such as the Hsp90 protein or the chaperonin GroEL, can buffer the deleterious phenotypic effects of mutations that alter protein structure and function. Molecular chaperones, also known as heat-shock proteins, refold misfolded proteins and help other proteins reach their native conformation.
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