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.

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.

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.

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.

Spatial quality control bypasses cell-based limitations on proteostasis to promote prion curing.

The proteostasis network has evolved to support protein folding under normal conditions and to expand this capacity in response to proteotoxic stresses. Nevertheless, many pathogenic states are associated with protein misfolding, revealing in vivo limitations on quality control mechanisms. One contributor to these limitations is the physical characteristics of misfolded proteins, as exemplified by amyloids, which are largely resistant to clearance.

Defining the limits: Protein aggregation and toxicity in vivo.

Abstract others complementary, to resolve mis-folded proteins when they arise, ranging from refolding through the action of molecular chaperones to elimination through regulated proteolytic mechanisms. These protein quality control pathways are sufficient, under normal conditions, to maintain a functioning proteome, but in response to diverse environmental, genetic and/or stochastic events, protein mis-folding exceeds the corrective capacity of these pathways, leading to the accumulation of aggregates and ultimately toxicity. Particularly devastating examples of these effects include certain neurodegenerative diseases, such as Huntington's Disease, which are associated with the expansion of polyglutamine tracks in proteins.

A size threshold limits prion transmission and establishes phenotypic diversity.

According to the prion hypothesis, atypical phenotypes arise when a prion protein adopts an alternative conformation and persist when that form assembles into self-replicating aggregates. Amyloid formation in vitro provides a model for this protein-misfolding pathway, but the mechanism by which this process interacts with the cellular environment to produce transmissible phenotypes is poorly understood. Using the yeast prion Sup35/[PSI(+)], we found that protein conformation determined the size distribution of aggregates through its interactions with a molecular chaperone.