Molecular basis for chaperone activities of the BRICHOS domain against different types of clumpy clients : a route to prevent amyloid toxicity

Protein aggregation is a hallmark of a wide range of human disorders, including Alzheimer’s disease and type II diabetes, and are often associated with imbalances in the cellular protein homeostasis. Molecular chaperones play an important role in modulating proteostasis and thereby counteract toxic consequences of misfolded or aggregated proteins. In this thesis, we investigated the molecular chaperone functions of several isolated BRICHOS domains against amyloid fibril formation and non-fibrillar protein aggregation. We propose that the ability of the BRICHOS domain to chaperone substrates with structurally distinct aggregation pathways is encoded in its ability to form different assembly states. BRICHOS domains are found in about ten distantly related protein families. It was proposed that they have an intramolecular chaperone-like function, preventing misfolding of a b-sheet prone region within their respective precursor proteins. Surprisingly, the activity of the Bri2 BRICHOS and proSP-C BRICHOS domain can extend to other aggregation-prone peptides and proteins. However, the molecular mechanisms of this diverse substrate spectrum remained unclear. Here we show that the Bri2 BRICHOS domain forms polydisperse assembly states ranging from monomers, that efficiently reduce amyloid-associated neurotoxicity in hippocampal mouse brain slices, to large oligomers that exclusively exhibit activities against non-fibrillar protein aggregation (paper I). Based on these findings, we designed a stable Bri2 BRICHOS monomer mutant that specifically blocks the formation of toxic species during amyloid fibril formation and partly disassembles wild-type Bri2 BRICHOS oligomers into monomers (paper II). Furthermore, we show that the conversion from Bri2 BRICHOS monomers towards large oligomers and hence the generation of activities against non-fibrillar protein aggregation is triggered by reducing conditions and is mediated through distinct thiol reactivities (paper III). The ability to adopt polydisperse assembly states together with activities against fibrillar and non-fibrillar protein aggregation are not only limited to Bri2 BRICHOS but similarly apply to Bri3 BRICHOS (paper IV). In contrast to Bri2 BRICHOS and Bri3 BRICHOS, proSP-C BRICHOS exists mostly as trimers in solution but a mutation at the homologous position in Bri2 BRICHOS (as shown in paper II) similarly resulted in a stable proSP-C BRICHOS monomer variant. This monomer mutant enabled us to investigate in detail the binding spectrum of the proSP-C BRICHOS domain towards different aggregates during amyloid fibril formation (paper V). This thesis gives new insights into the structure and function relationship of the molecular chaperone domain BRICHOS