Suppression of aggregate and amyloid formation by a novel intrinsically disordered region in metazoan Hsp110 chaperones

Molecular chaperones maintain protein homeostasis (proteostasis) by ensuring the proper folding of polypeptides. Loss of proteostasis has been linked to the onset of numerous neurodegenerative disorders including Alzheimers, Parkinsons, and Huntingtons disease. Hsp110 is related to the canonical Hsp70 class of protein folding molecular chaperones and interacts with Hsp70 as a nucleotide exchange factor (NEF), promoting rapid cycling of ADP for ATP. In addition to its NEF activity, Hsp110 possesses an Hsp70-like substrate binding domain (SBD) whose biological roles remain undefined. Previous work in Drosophila melanogaster has shown that loss of the sole Hsp110 gene (Hsc70cb) accelerates the aggregation of polyglutamine (polyQ)-expanded human Huntingtin, while its overexpression protects against polyQ-mediated neuronal cell death. We hypothesize that in addition to its role as an Hsp70 NEF, Drosophila Hsp110 may function in the fly as a protective protein "holdase", preventing the aggregation of unfolded polypeptides via the SBD-{beta} subdomain. Using an in vitro protein aggregation assay we demonstrate for the first time that Drosophila Hsp110 effectively prevents aggregation of the model substrate citrate synthase. We also report the discovery of a redundant and heretofore unknown potent holdase capacity in a 138 amino-acid region of Hsp110 carboxyl-terminal to both SBD-{beta} and SBD- (henceforth called the C-terminal extension). This sequence is highly conserved in metazoan Hsp110 genes, completely absent from fungal representatives, including Saccharomyces cerevisiae SSE1, and is computationally predicted to contain an intrinsically disordered region (IDR). We demonstrate that this IDR sequence within the human Hsp110s, Apg-1 and Hsp105, inhibits the formation of amyloid A{beta}-42 and -synuclein fibrils in vitro but cannot mediate fibril disassembly. Together these findings demonstrate the existence of a second independent, passive holdase property of metazoan Hsp110 chaperones capable of suppressing both general protein aggregation and amyloidogenesis and raise the possibility of exploitation of this IDR for therapeutic benefit in combating neurodegenerative disease.