Chaperones and degradation

The role of molecular chaperones in misfolded protein degradation

Molecular chaperones assist proteins to fold to their natural state. The levels of various chaperones increase during stress such as heat shock and therefore they are also known as Heat Shock Proteins or Hsp(s). Studies over the last decade demonstrated that the same Hsp40 kDa and Hsp70 kDa chaperones that assist folding are also involved in the aggregation of misfolded proteins and in the degradation by the ubiquitin system. Clearly there is a quality control stage in which the fate of the protein must be decided: to fold, to aggregate or to be eliminated (Figure 1). How precisely this triage decision is made is currently unknown and we address this question using a model substrate in yeast cells.

Figure 1. Misfolded protein quality control: making the correct triage decision. Preferentially, the misfolded protein is refolded, keeping the active protein pool constant. However, if the protein cannot refold to its native conformation, it can be removed by Ub-dependent proteasomal degradation. If the chaperone and/or the degradation machineries become rate limiting, misfolded proteins are likely to aggregate. Both degradation and aggregation reduce the active pool of the protein. The proteasome cartoon was adapted from Nickell et al.  

Genetic studies in our laboratory indicate that the yeast Hsp70 chaperone Ssa1 and the Hsp40 chaperone sis1 are essential for the degradation of a model misfolded substrate harboring a protein quality control degrons (Furth et al., 2011; Shiber et al., 2013). However, these chaperones play different roles in substrate proteolysis. Whereas the Hsp40 Sis1 is absolutely required for ubiquitin conjugation, the Hsp70 Ssa1 is not. Our finding localizes Sis1 function to the early stage of substrate targeting to the ubiquitin system while Ssa1 likely functions downstream to the ubiquitin system, probably by escorting the ubiquitylated substrate to the proteasome.

Agregation example, a gif file



Interestingly, in the absence of Ssa1 the misfolded model substrate form cytosolic aggregates (see movie). This protein forms foci only when ubiquitylated, suggesting that besides triggering proteasomal degradation, poly-ubiquitin can also serve as a signal for aggregation in the absence of  Hsp70 in people suffering from misfolded protein diseases (also called neurodegenerative diseases), such as Alzheimer’s, Huntington’s and Parkinson’s contain ubiquitin molecules. Understanding the mechanism of ubiquitin-mediated aggregate formation in yeast may provide important information about the role of ubiquitin in the aggregates associated with neurodegenerative diseases (Shiber et al., 2013Shiber and Ravid, 2014). 

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Nickell, S., Beck, F., Scheres, S.H., Korinek, A., Forster, F., Lasker, K., Mihalache, O., Sun, N., Nagy, I., Sali, A., et al. (2009). Insights into the molecular architecture of the 26S proteasome. Proc Natl Acad Sci U S A 106, 11943-11947.