Sis1 potentiates the stress response to protein aggregation and elevated temperature

Cells adapt to conditions that compromise protein conformational stability by activating various stress response pathways, but the mechanisms used in sensing misfolded proteins remain unclear. Moreover, aggregates of disease proteins often fail to induce a productive stress response. Here, using a yeast model of polyQ protein aggregation, we identified Sis1, an essential Hsp40 co-chaperone of Hsp70, as a critical sensor of proteotoxic stress. At elevated levels, Sis1 prevented the formation of dense polyQ inclusions and directed soluble polyQ oligomers towards the formation of permeable condensates. Hsp70 accumulated in a liquid-like state within this polyQ meshwork, resulting in a potent activation of the HSF1 dependent stress response. Sis1, and the homologous DnaJB6 in mammalian cells, also regulated the magnitude of the cellular heat stress response, suggesting a general role in sensing protein misfolding. Sis1/DnaJB6 functions as a limiting regulator to enable a dynamic stress response and avoid hypersensitivity to environmental changes. Identifying factors that enable cells to induce a potent stress response to amyloid-like aggregation can provide further insight into the mechanism of stress regulation. Here, the authors express polyglutamine-expanded Huntingtin as a model disease protein in yeast cells and perform a genetic screen for chaperone factors that allow yeast cells to activate a potent stress response. They identify Sis1, an essential Hsp40 co-chaperone of Hsp70, as a critical sensor of proteotoxic stress and further show that both Sis1 and its mammalian homolog DnaJB6 regulate the magnitude of the cellular heat stress response, indicating that this mechanism is conserved.FRAP experiments were performed at the Max Planck Institute of Biochemistry Imaging Core Facility

Formation of toxic oligomers of polyQ-expanded Huntingtin by prion-mediated cross-seeding

Manifestation of aggregate pathology in Huntington's disease is thought to be facilitated by a preferential vulnerability of affected brain cells to age-dependent proteostatic decline. To understand how specific cellular backgrounds may facilitate pathologic aggregation, we utilized the yeast model in which polyQ-expanded Huntingtin forms aggregates only when the endogenous prion-forming protein Rnq1 is in its amyloid-like prion [PIN+] conformation. We employed optogenetic clustering of polyQ protein as an orthog-onal method to induce polyQ aggregation in prion-free [pin-] cells. Optogenetic aggregation circumvented the prion requirement for the formation of detergent-resistant polyQ inclusions but bypassed the formation of toxic polyQ oligomers, which accumulated specifically in [PIN+] cells. Reconstitution of aggregation in vitro suggested that these polyQ oligomers formed through direct templating on Rnq1 prions. These findings shed light on the mechanism of prion-mediated formation of oligomers, which may play a role in triggering polyQ pathology in the patient brain

Sis1 potentiates the stress response to protein aggregation and elevated temperature

Cells adapt to conditions that compromise protein conformational stability by activating various stress response pathways, but the mechanisms used in sensing misfolded proteins remain unclear. Moreover, aggregates of disease proteins often fail to induce a productive stress response. Here, using a yeast model of polyQ protein aggregation, we identified Sis1, an essential Hsp40 co-chaperone of Hsp70, as a critical sensor of proteotoxic stress. At elevated levels, Sis1 prevented the formation of dense polyQ inclusions and directed soluble polyQ oligomers towards the formation of permeable condensates. Hsp70 accumulated in a liquid-like state within this polyQ meshwork, resulting in a potent activation of the HSF1 dependent stress response. Sis1, and the homologous DnaJB6 in mammalian cells, also regulated the magnitude of the cellular heat stress response, suggesting a general role in sensing protein misfolding. Sis1/DnaJB6 functions as a limiting regulator to enable a dynamic stress response and avoid hypersensitivity to environmental changes. Identifying factors that enable cells to induce a potent stress response to amyloid-like aggregation can provide further insight into the mechanism of stress regulation. Here, the authors express polyglutamine-expanded Huntingtin as a model disease protein in yeast cells and perform a genetic screen for chaperone factors that allow yeast cells to activate a potent stress response. They identify Sis1, an essential Hsp40 co-chaperone of Hsp70, as a critical sensor of proteotoxic stress and further show that both Sis1 and its mammalian homolog DnaJB6 regulate the magnitude of the cellular heat stress response, indicating that this mechanism is conserved

Protein instability associated with AARS1 and MARS1 mutations causes trichothiodystrophy

Trichothiodystrophy (TTD) is a rare hereditary neurodevelopmental disorder defined by sulfur-deficient brittle hair and nails and scaly skin, but with otherwise remarkably variable clinical features. The photosensitive TTD (PS-TTD) forms exhibits in addition to progressive neuropathy and other features of segmental accelerated aging and is associated with impaired genome maintenance and transcription. New factors involved in various steps of gene expression have been identified for the different non-photosensitive forms of TTD (NPS-TTD), which do not appear to show features of premature aging. Here, we identify alanyl-tRNA synthetase 1 and methionyl-tRNA synthetase 1 variants as new gene defects that cause NPS-TTD. These variants result in the instability of the respective gene products alanyl- and methionyl-tRNA synthetase. These findings extend our previous observations that TTD mutations affect the stability of the corresponding proteins and emphasize this phenomenon as a common feature of TTD. Functional studies in skin fibroblasts from affected individuals demonstrate that these new variants also impact on the rate of tRNA charging, which is the first step in protein translation. The extension of reduced abundance of TTD factors to translation as well as transcription redefines TTD as a syndrome in which proteins involved in gene expression are unstable

Pathways of cellular proteostasis in aging and disease

Ensuring cellular protein homeostasis, or proteostasis, requires precise control of protein synthesis, folding, conformational maintenance, and degradation. A complex and adaptive proteostasis network coordinates these processes with molecular chaperones of different classes and their regulators functioning as major players. This network serves to ensure that cells have the proteins they need while minimizing misfolding or aggregation events that are hallmarks of age-associated proteinopathies, including neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. It is now clear that the capacity of cells to maintain proteostasis undergoes a decline during aging, rendering the organism susceptible to these pathologies. Here we discuss the major proteostasis pathways in light of recent research suggesting that their age-dependent failure can both contribute to and result from disease. We consider different strategies to modulate proteostasis capacity, which may help develop urgently needed therapies for neurodegeneration and other age-dependent pathologies

A Cytotoxin-Producing Strain of Vibrio cholerae Non-O1, Non-O139 as a Cause of Cholera and Bacteremia after Consumption of Raw Clams

We report a case of a cholera-like gastroenteritis subsequent with bacteremia in a healthy man following consumption of raw clams. Although we failed to recover the organism from the patient's stool culture, his blood culture was positive for a non-cholera toxin-producing yet cytotoxin-producing non-O1 and non-O139 Vibrio cholerae

Formation of toxic oligomers of polyQ-expanded Huntingtin by prion-mediated cross-seeding

Manifestation of aggregate pathology in Huntington's disease is thought to be facilitated by a preferential vulnerability of affected brain cells to age-dependent proteostatic decline. To understand how specific cellular backgrounds may facilitate pathologic aggregation, we utilized the yeast model in which polyQ-expanded Huntingtin forms aggregates only when the endogenous prion-forming protein Rnq1 is in its amyloid-like prion [PIN+] conformation. We employed optogenetic clustering of polyQ protein as an orthogonal method to induce polyQ aggregation in prion-free [pin−] cells. Optogenetic aggregation circumvented the prion requirement for the formation of detergent-resistant polyQ inclusions but bypassed the formation of toxic polyQ oligomers, which accumulated specifically in [PIN+] cells. Reconstitution of aggregation in vitro suggested that these polyQ oligomers formed through direct templating on Rnq1 prions. These findings shed light on the mechanism of prion-mediated formation of oligomers, which may play a role in triggering polyQ pathology in the patient brain

Chaperone Function of Hgh1 in the Biogenesis of Eukaryotic Elongation Factor 2

Eukaryotic elongation factor 2 (eEF2) is an abundant and essential component of the translation machinery. The biogenesis of this 93 kDa multi-domain protein is assisted by the chaperonin TRiC/CCT. Here, we show in yeast cells that the highly conserved protein Hgh1 (FAM203 in humans) is a chaperone that cooperates with TRiC in eEF2 folding. In the absence of Hgh1, a substantial fraction of newly synthesized eEF2 is degraded or aggregates. We solved the crystal structure of Hgh1 and analyzed the interaction of wild-type and mutant Hgh1 with eEF2. These experiments revealed that Hgh1 is an armadillo repeat protein that binds to the dynamic central domain III of eEF2 via a bipartite interface. Hgh1 binding recruits TRiC to the C-terminal eEF2 module and prevents unproductive interactions of domain III, allowing efficient folding of the N-terminal GTPase module. eEF2 folding is completed upon dissociation of TRiC and Hgh1