PH‐Dependent structural ensembles of haptoglobin suppress β2‐microglobulin amyloidosis: role in lysosomal localization and extracellular protein quality control (752.7)

Abstract

Several diseases involve extracellular deposition of misfolded proteins, which is associated with inflammation and local acidosis. We have investigated the chaperone‐like activity of haptoglobin towards the amyloid aggregation of β2‐microglobulin (β2m) and of α‐synuclein and its role in clearing the amyloid deposits from the extracellular space. At neutral pH, haptoglobin prevents the formation of β2m amyloid fibrils but not that of α‐synuclein. Under conditions of local acidosis, pH 5.5‐6.5, it prevents β2m fibril formation with relatively decreased efficiency. We have investigated the structural and stability aspects of haptoglobin as a function of pH. Between pH 5.5‐8.0, it exists in a continuum of conformational states with subtle perturbation in tertiary structure, moderately exposed hydrophobic surfaces and distinct latent heat of unfolding. Below pH 5.0 it exists in a molten‐globule‐like conformation with no chaperone activity. Live cell imaging and flow cytometry studies show that the species of β2m formed in the presence of haptoglobin are not cytotoxic and readily undergo lysosomal degradation. Analytical ultracentrifugation and dot blot studies reveal that haptoglobin interacts with prefibrillar oligomeric species formed during β2m fibril formation. Thus, haptoglobin is an extracellular chaperone with promising therapeutic implications in extracellular amyloid diseases.