Chaperone-amyloid interactions

National Institute Of Diabetes And Digestive And Kidney Diseases

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Abstract

Expansion beyond 40 residues of a polyglutamine (polyQ) stretch in Huntingtin protein underlies Huntington's disease. Age of onset and progression of the disease correlates with the extent of expansion. A prominent feature of disease pathology is the accumulation of amyloid forms of misfolded polyQ. Amyloid is a highly ordered fibrous aggregate of a single type of protein that grows by recruiting the soluble form of the protein and converting it to the amyloid form as it joins an end of the fiber. The yeast system has been used for decades to study polyQ aggregation and its toxic effects in vivo. Altering function or abundance of many different chaperones and co-chaperones or disrupting various protein quality control (PQC) processes can disrupt propagation of amyloids formed of several different proteins in a variety of ways and to varying degrees. Central among these chaperones are Hsp70 and Hsp104, whose activities are regulated by many co-chaperones. Sis1 and Ydj1 are essential Hsp40 members of the J-protein family of Hsp70 co-chaperones that can direct activity of the Hsp104 machinery. Hsp42, Btn2 and Cur1 also comprise systems that sequester misfolded proteins and counteract amyloid propagation by collecting disperse amyloid aggregates into cytosolic deposition sites. Expression of polyQ in yeast is toxic and the gathering of polyQ aggregates to a single site can be associated with moderation of this toxicity. We are investigating interactions of chaperones and other PQC factors with each other and with amyloid-forming proteins, such as Huntingtin, to understand mechanisms of how cellular PQC systems detoxify amyloid. Our earlier work showed that that the human Hsp70 co-chaperone DnaJB6b (related to yeast Sis1) can protect cells from expression of a toxic Huntingtin related polyQ-GFP fusion. We found DnaJB6b bound to polyQ aggregates and sequestered them into large cytoplasmic foci in a reaction that depended on Hsp70 and actin. Such spatial segregation of aggregates is recognized in the field as a broadly conserved cellular response that reduces toxicity of small disperse protein aggregates. Although the protection correlated with collection of disperse polyQ aggregates into a single site, we found modifed versions of DnaJB6b that failed to counteract toxicity yet still sequestered the disperse aggregates. We further found DnaJB6b had a separate activity, independent of its interaction with Hsp70, that protects cells from the toxicity even when aggregates remained disperse. Thus, sequestration alone is not protective, and the protection and sequestration are separable. Overall these findings are consistent with a generally held idea that the smaller aggregates are the toxic species, show that something separate from spatial segregation can provide protection from toxicity, and suggest DnaJB6b can bind and detoxify these aggregates independently of Hsp70. Continued work identified a role of Hsp104 in the toxicity of polyQ. Propagation of the small disperse aggregates of polyQ needs support of the normal action of Hsp104, which helps maintain cell health by resolubilizing proteins from aggregates. In attempting to recover protein from the amyloid aggregates by extracting monomers, Hsp104 causes the amyloid to divide into smaller pieces that each can continue to propagate as amyloid. This default "replication" of amyloid by normal Hsp104 disaggregation activity promotes and is essential for continued propagation of amyloid in yeast. Our findings are again consistent with smaller polyQ aggregates being more toxic and show that cellular PQC factors that normally protect cells from protein misfolding and aggregation can strongly influence amyloid-formation and toxicity.