High-pressure studies on protein aggregates and amyloid fibrils.

High hydrostatic pressure (HHP) modulates protein-protein and protein-solvent interactions through volume changes and thereby affects the equilibrium of protein conformational species between native and denatured forms as well as monomeric, oligomeric, and aggregated forms without the addition of chemicals or use of high temperature. Because of this unique property, HHP has provided deep insights into the thermodynamics and kinetics of protein folding and aggregation, including amyloid fibril formation. In particular, HHP is a useful tool to stabilize and populate specific folding intermediates, the characterization of which provides thorough understanding of protein folding and aggregation pathways. Furthermore, recent application of HHP for dissociation of protein aggregates, such as inclusion bodies (IBs), into native proteins in a single step facilitates protein preparation for structural and functional studies. This chapter overviews recent HHP studies on the population and characterization of folding intermediates associated with protein aggregation and protein refolding from protein aggregates of amyloid fibrils and IBs. Finally, we describe overall experimental procedures of HHP-mediated protein refolding and provide a detailed discussion of each operating parameter to optimize the refolding.