Proteins misfold because of random fluctuations in protein conformations, the presence of destabilizing mutations, stress conditions, or global alterations in cellular physiology. Protein misfolding is also emerging as a major cause of human disease. Diseases that are caused or exacerbated by misfolding events include lysosomal storage disorders, cystic fibrosis, cancer and a range of neurodegenerative disorders, such as Alzheimer, Huntington or Parkinson disease. Finally, protein misfolding and aggregation are hallmarks of aging. Together, these examples illustrate that protein homeostasis is a key requirement for cellular and organismal health and survival.
Organisms have evolved an elaborate PQC machinery to preserve the functionality of their proteomes. Three different strategies are employed to counteract protein damage: refolding of misfolded proteins by molecular chaperones, removal of misfolded proteins by energy-dependent proteases or degradation by autophagy, a process of selective uptake and degradation of damaged proteins in membrane-enclosed compartments.1
As a result misfolded proteins are either repaired or eliminated.
In recent years, evidence of yet another PQC mechanism has been accumulating.2
Organisms as diverse as bacteria and humans have the ability to reversibly constrain misfolded proteins in specialized membrane-free compartments or inclusion bodies. Not surprisingly, the formation of these compartments is strongly enhanced by environmental stress. Inclusions are also commonly associated with protein misfolding diseases. In addition to disease-associated proteins, they can contain ubiquitylated proteins, defective ribosomal products or oxidatively damaged proteins. However, whether these proteins reside in the same compartment or are targeted to different compartments had long remained undetermined.
This situation changed in 2008 when a seminal report described two distinct stress-inducible compartments for misfolded proteins termed IPOD (insoluble protein deposit) and JUNQ (juxtanuclear quality control) in eukaryotic cells.3
The juxtanuclear compartment was associated with the nuclear envelope and contained ubiquitylated proteins that rapidly exchanged with the surrounding cytosol. Additional constituents were identified as proteasomes and chaperones. Consequently, the JUNQ was proposed to provide a specialized environment for chaperone-mediated refolding or protein degradation.3,4
The IPOD on the other hand was identified in the perivacuolar region of yeast cells. It contained largely immobile aggregates of amyloid-forming proteins. Sequestration in the IPOD may reduce aberrant interactions between misfolded proteins and essential cellular components. This led to the proposal that the IPOD could serve a cytoprotective function.3,4
Despite these advances, however, the molecular mechanisms that govern the formation of aggregate deposition sites have so far remained largely elusive.
We used a yeast prion reporter assay to identify cellular factors involved in protein aggregation.5
Our study identified Btn2 and Cur1, two members of the Hook family of transport factors, as potent modifiers of a synthetic yeast prion. Surprisingly, we found that the effects of Btn2 and Cur1 were not direct—as previously suggested for a different prion6
—but largely mediated by nuclear targeting of Sis1, a chaperone of the Hsp40 family that is required for prion maintenance. Further investigations revealed that Btn2 and Cur1 are important protein-sorting factors that physically and functionally interact with Sis1 or the small heat shock protein Hsp42 to coordinate the accumulation of aggregates in deposition sites in times of acute stress. In the following, I shall describe how these findings have advanced our understanding of spatial PQC in yeast.