Evidence that clusterin has discrete chaperone and ligand binding sites
Clusterin is the first identified extracellular mammalian chaperone and binds to a wide variety of partly unfolded, stressed proteins. Clusterin also binds to many different unstressed ligands including the cell surface receptor low density lipoprotein receptor-related protein-2 (LRP-2). It is unknown whether clusterin binds to all of these many ligands via one or more binding sites. Furthermore, the region(s) of clusterin involved in these many binding interactions remain to be identified. As part of an investigation of these issues, we expressed recombinant human clusterin in the yeast Pichia pastoris. The resultant protein had variable proteolytic truncations of the C-terminal region of the a-chain and the N-terminal region of the b-chain. We compared the chaperone and ligand binding activities of this recombinant product with those of clusterin purified from human serum. We also tested whether the binding of clusterin to ligands could be inhibited by competitive binding with other clusterin ligands or by anti-clusterin monoclonal antibodies. Collectively, our results indicate that (i) clusterin has three independent classes of binding sites for LRP-2, stressed proteins and unstressed ligands, respectively, and (ii) the binding sites for LRP-2 and stressed proteins are likely to be in parts of the molecule other than the C-terminal region of the alpha-chain or the N-terminal region of the b-chain. It has been suggested that, in vivo, clusterin binds to toxic molecules in the extracellular environment and carries these to cells expressing LRP-2 for uptake and degradation. This hypothesis is supported by our demonstration that clusterin has discrete binding sites for LRP-2 and other (potentially toxic) molecules.