Protein aggregates are the hallmark of stressed and ageing cells, and characterize several pathophysiological states1,2. Healthy metazoan cells effectively eliminate intracellular protein aggregates3,4, indicating that efficient disaggregation and/or degradation mechanisms exist. However, metazoans lack the key heat-shock protein disaggregase HSP100 of non-metazoan HSP70-dependent protein disaggregation systems5,6, and the human HSP70 system alone, even with the crucial HSP110 nucleotide exchange factor, has poor disaggregation activity in vitro4,7. This unresolved conundrum is central to protein quality control biology. Here we show that synergic cooperation between complexed J-protein co-chaperones of classes A and B unleashes highly efficient protein disaggregation activity in human and nematode HSP70 systems. Metazoan mixed-class J-protein complexes are transient, involve complementary charged regions conserved in the J-domains and carboxy-terminal domains of each J-protein class, and are flexible with respect to subunit composition. Complex formation allows J-proteins to initiate transient higher order chaperone structures involving HSP70 and interacting nucleotide exchange factors. A network of cooperative class A and B J-protein interactions therefore provides the metazoan HSP70 machinery with powerful, flexible, and finely regulatable disaggregase activity and a further level of regulation crucial for cellular protein quality control.

In addition to assisting with protein folding, SSB and NAC also regulate ribosome biogenesis (see also companion paper from Albanèse et al. in this issue).

Huntington's disease (HD) is a neurodegenerative disorder caused by an expanded CAG trinucleotide repeat in the huntingtin gene (). Molecular chaperones have been implicated in suppressing or delaying the aggregation of mutant Htt. Using and assays, we have identified a trimeric chaperone complex (Hsc70, Hsp110, and J-protein) that completely suppresses fibrilization of HttExon1Q The composition of this chaperone complex is variable as recruitment of different chaperone family members forms distinct functional complexes. The trimeric chaperone complex is also able to resolubilize Htt fibrils. We confirmed the biological significance of these findings in HD patient-derived neural cells and on an organismal level in Among the proteins in this chaperone complex, the J-protein is the concentration-limiting factor. The single overexpression of DNAJB1 in HEK293T cells is sufficient to profoundly reduce HttExon1Q aggregation and represents a target of future therapeutic avenues for HD.

The adaptation of protein synthesis to environmental and physiological challenges is essential for cell viability. Here, we show that translation is tightly linked to the proteinfolding environment of the cell through the functional properties of the ribosome bound chaperone NAC (nascent polypeptide-associated complex). Under non-stress conditions, NAC associates with ribosomes to promote translation and protein folding. When proteostasis is imbalanced, NAC relocalizes from a ribosome-associated state to protein aggregates in its role as a chaperone. This results in a functional depletion of NAC from the ribosome that diminishes translational capacity and the flux of nascent proteins. Depletion of NAC from polysomes and re-localisation to protein aggregates is observed during ageing, in response to heat shock and upon expression of the highly aggregation-prone polyglutamine-expansion proteins and Ab-peptide. These results demonstrate that NAC has a central role as a proteostasis sensor to provide the cell with a regulatory feedback mechanism in which translational activity is also controlled by the folding state of the cellular proteome and the cellular response to stress.

SummaryProtein aggregation is enhanced upon exposure to various stress conditions and aging, which suggests that the quality control machinery regulating protein homeostasis could exhibit varied capacities in different stages of organismal lifespan. Recently, an efficient metazoan disaggregase activity was identified in vitro, which requires the Hsp70 chaperone and Hsp110 nucleotide exchange factor, together with single or cooperating J‐protein co‐chaperones of classes A and B. Here, we describe how the orthologous Hsp70s and J‐protein of Caenorhabditis elegans work together to resolve protein aggregates both in vivo and in vitro to benefit organismal health. Using an RNAi knockdown approach, we show that class A and B J‐proteins cooperate to form an interactive flexible network that relocalizes to protein aggregates upon heat shock and preferentially recruits constitutive Hsc70 to disaggregate heat‐induced protein aggregates and polyQ aggregates that form in an age‐dependent manner. Cooperation between class A and B J‐proteins is also required for organismal health and promotes thermotolerance, maintenance of fecundity, and extended viability after heat stress. This disaggregase function of J‐proteins and Hsc70 therefore constitutes a powerful regulatory network that is key to Hsc70‐based protein quality control mechanisms in metazoa with a central role in the clearance of aggregates, stress recovery, and organismal fitness in aging.