Numerous human diseases are associated with the chronic expression of misfolded and aggregation-prone proteins. The expansion of polyglutamine residues in unrelated proteins is associated with the early onset of neurodegenerative disease. To understand how the presence of misfolded proteins leads to cellular dysfunction, we employed Caenorhabditis elegans polyglutamine aggregation models. Here, we find that polyglutamine expansions disrupted the global balance of protein folding quality control, resulting in the loss of function of diverse metastable proteins with destabilizing temperature-sensitive mutations. In turn, these proteins, although innocuous under normal physiological conditions, enhanced the aggregation of polyglutamine proteins. Thus, weak folding mutations throughout the genome can function as modifiers of polyglutamine phenotypes and toxicity.

Protein damage contributes prominently to cellular aging. To address whether this occurs at a specific period during aging or accumulates gradually, we monitored the biochemical, cellular, and physiological properties of folding sensors expressed in different tissues of C. elegans. We observed the age-dependent misfolding and loss of function of diverse proteins harboring temperature-sensitive missense mutations in all somatic tissues at the permissive condition. This widespread failure in proteostasis occurs rapidly at an early stage of adulthood, and coincides with a severely reduced activation of the cytoprotective heat shock response and the unfolded protein response. Enhancing stress responsive factors HSF-1 or DAF-16 suppresses misfolding of these metastable folding sensors and restores the ability of the cell to maintain a functional proteome. This suggests that a compromise in the regulation of proteostatic stress responses occurs early in adulthood and tips the balance between the load of damaged proteins and the proteostasis machinery. We propose that the collapse of proteostasis represents an early molecular event of aging that amplifies protein damage in age-associated diseases of protein conformation.daf-16 ͉ folding sensors ͉ hsf-1 ͉ protein misfolding ͉ stress response T he stability of the proteome is challenged by conditions that cause proteotoxic stress including errors during protein synthesis, oxidant-induced covalent modifications, inherited polymorphisms, and misfolding (1-3). Consequently, all cells have highly conserved stress-inducible pathways that detect, prevent, and resolve such damage (4, 5). Accumulation of misfolded proteins titrate the negative regulation of chaperones from HSF-1 (4, 6, 7). Likewise, an E3 ubiquitin ligase was shown to regulate the levels of DAF-16, also indicating a link between stress response activation to the balance between proteostasis capacity and the load of damaged proteins (8). The activation of cellular stress responses limits the accumulation of damaged proteins by suppression of misfolding and enhancing chaperonemediated folding, promoting the enzymatic removal of covalent modifications, and stimulating clearance by ubiquitin-mediated, proteosomal, and autophagic processes (1, 4, 5, 9, 10). The dynamics of these processes during aging, however, are poorly understood.Aging can be characterized by the loss of cellular function and increased vulnerability to environmental and physiological stress that results in enhanced susceptibility to disease. Many agedependent changes have been detected at mid-to-late lifespan of C. elegans, including the accumulation of damaged macromolecules, cell and organellar degeneration, and physiological decline (11-17). Likewise, aggregation and toxicity of diseaseassociated aggregation-prone proteins, such as huntingtin and A␤ peptide, is enhanced during aging in C. elegans and other model systems (18-21).The regulation of aging and the maintenance of proteostasis has been recently established in C. elegans. Downregulatio...

A major activity of molecular chaperones is to prevent aggregation and refold misfolded proteins. However, when allowed to form, protein aggregates are refolded poorly by most chaperones. We show here that the sequential action of two Escherichia coli chaperone systems, ClpB and DnaK-DnaJ-GrpE, can efficiently solubilize excess amounts of protein aggregates and refold them into active proteins. Measurements of aggregate turbidity, Congo red, and 4,4-dianilino-1,1-binaphthyl-5,5-disulfonic acid binding, and of the disaggregation͞refolding kinetics by using a specific ClpB inhibitor, suggest a mechanism where (i) ClpB directly binds protein aggregates, ATP induces structural changes in ClpB, which (ii) increase hydrophobic exposure of the aggregates and (iii) allow DnaK-DnaJ-GrpE to bind and mediate dissociation and refolding of solubilized polypeptides into native proteins. This efficient mechanism, whereby chaperones can catalytically solubilize and refold a wide variety of large and stable protein aggregates, is a major addition to the molecular arsenal of the cell to cope with protein damage induced by stress or pathological states.Hsp104 ͉ ClpB ͉ DnaK ͉ Congo red ͉ protein disaggregation