Two sites in the C-terminal β-chain tail mediate interactions of the chaperone clusterin with amyloid beta and other misfolded client proteins

Clusterin (CLU) is a constitutively secreted mammalian chaperone that binds in extracellular body fluids to misfolded client proteins to neutralise their toxicity and mediate their safe disposal by cell uptake and intracellular degradation. However, the regions of CLU critical for its interactions with misfolded proteins remain still largely unknown. To identify binding sites, we expressed a panel of CLU deletion and alanine-stretch mutants in a recently developed mammalian expression system. Mutant CLU molecules lacking detectable structural aberrations were subjected to functional analyses to compare their abilities with that of wild type CLU to bind to misfolded proteins and to inhibit protein aggregation. These analyses implicated two regions in the flexible {beta}-chain C-terminal tail of CLU as being important in the interactions of the chaperone with misfolded proteins, including aggregating the Alzheimers amyloid {beta}-peptide (A{beta}). We then designed in silico sequence-specific single-domain camelid nanobodies to confirm the function of the two putative client protein binding sites. Based on our experimental results and in silico binding site predictions, we suggest that the potent ability of CLU to promiscuously interact with many different misfolded proteins, regardless of their size or structure, arises from the location of multiple client protein binding sites in its flexible tail region.