Final answer:
Amyloid fibrils in polyQ diseases form due to the misfolding of proteins which transition from α-helix to β-pleated sheets, aggregating into toxic plaques.
Step-by-step explanation:
In polyQ diseases, proteins that normally fold into soluble α-helix forms misfold into β-pleated sheets, which have a sticky nature, leading to their aggregation into amyloid fibrils and plaques. These misfolded proteins can accumulate because their altered three-dimensional shapes no longer function correctly and instead become toxic to the cells. An example of a polyQ disease is Huntington's disease, whereby the polyglutamine expansion within the huntingtin protein leads to its misfolding and subsequent formation of amyloid-like aggregates. In addition to the misfolding, there are cellular processes involved in trying to clear these proteins; however, when the systems are overwhelmed or malfunction, it leads to the toxic accumulation that is characteristic of proteopathies like Alzheimer's, Parkinson's, and Huntington's disease.