title: Misfolded proteins impose a dosage-dependent fitness cost and trigger a cytosolic unfolded protein response in yeast.
abstract: Evolving lineages face a constant intracellular threat: most new coding sequence mutations destabilize the folding of the encoded protein. Misfolded proteins form insoluble aggregates and are hypothesized to be intrinsically cytotoxic. Here, we experimentally isolate a fitness cost caused by toxicity of misfolded proteins. We exclude other costs of protein misfolding, such as loss of functional protein or attenuation of growth-limiting protein synthesis resources, by comparing growth rates of budding yeast expressing folded or misfolded variants of a gratuitous protein, YFP, at equal levels. We quantify a fitness cost that increases with misfolded protein abundance, up to as much as a 3.2% growth rate reduction when misfolded YFP represents less than 0.1% of total cellular protein. Comparable experiments on variants of the yeast gene orotidine-5'-phosphate decarboxylase (URA3) produce similar results. Quantitative proteomic measurements reveal that, within the cell, misfolded YFP induces coordinated synthesis of interacting cytosolic chaperone proteins in the absence of a wider stress response, providing evidence for an evolved modular response to misfolded proteins in the cytosol. These results underscore the distinct and evolutionarily relevant molecular threat of protein misfolding, independent of protein function. Assuming that most misfolded proteins impose similar costs, yeast cells express almost all proteins at steady-state levels sufficient to expose their encoding genes to selection against misfolding, lending credibility to the recent suggestion that such selection imposes a global constraint on molecular evolution.
authors.affiliation: Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA.
authors.forename: Kerry A
authors.initials: KA
authors.lastname: Geiler-Samerotte
authors.forename: Michael F
authors.initials: MF
authors.lastname: Dion
authors.forename: Bogdan A
authors.initials: BA
authors.lastname: Budnik
authors.forename: Stephanie M
authors.initials: SM
authors.lastname: Wang
authors.forename: Daniel L
authors.initials: DL
authors.lastname: Hartl
authors.forename: D Allan
authors.initials: DA
authors.lastname: Drummond
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry; D003600:Cytosol / Q000737:chemistry* / Q000378:metabolism; D019143:Evolution, Molecular; D005656:Fungal Proteins / Q000737:chemistry*; D006358:Hot Temperature; D008164:Luminescent Proteins / Q000737:chemistry; D018832:Molecular Chaperones / Q000737:chemistry; D011489:Protein Denaturation; D017510:Protein Folding; D011506:Proteins / Q000737:chemistry; D040901:Proteomics / Q000379:methods; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry; D014158:Transcription, Genetic
chemical_list: D001426:Bacterial Proteins; D005656:Fungal Proteins; D008164:Luminescent Proteins; D018832:Molecular Chaperones; D011506:Proteins; D029701:Saccharomyces cerevisiae Proteins; C554167:URA3 protein, S cerevisiae; C054562:yellow fluorescent protein, Bacteria