Degree Name

Doctor of Philosophy


School of Biological Sciences - Faculty of Science


Individual proteins have a specific three-dimensional structure that gives them their unique function. However, a protein must be folded from a linear string of amino acids in order to gain this native conformation and thus function. There are many hurdles to a protein attaining and maintaining its native conformation. Stresses that are encountered in the life of a protein, such as changes in pH, temperature and oxidative stress, can promote protein misfolding or unfolding. In addition, some genetic mutations can modify a protein such that a non-native conformation is more energetically favourable than the native state. The unfolding or misfolding of a protein makes it more likely that it will aggregate with itself. There are more than 40 human diseases associated with the inappropriate deposition of aggregated protein. It is well known that there is a welldefined and efficient quality control system to deal with intracellular proteins that have either unfolded or are misfolded. Cells have a range of molecular chaperones to inhibit inappropriate aggregation and if this fails the cell labels the proteins with ubiquitin for degradation via the proteasome. However, many proteins are secreted from cells into the extracellular environment and there are many protein deposition disorders associated with extracellular protein deposits, outside the reach of the well-characterised intracellular quality control system. This thesis reports that the secreted proteins haptoglobin and a2-macroglobulin have small heat shock protein-like chaperone activity. Both haptoglobin and a2- macroglobulin specifically inhibited the precipitation of a variety of proteins induced by either heat or oxidation, including proteins in unfractionated human serum. In addition, it was demonstrated that haptoglobin and a2-macroglobulin inhibit the precipitation of stressed proteins by forming solubilized complexes with them, cannot protect enzymes from heat-induced loss of function, and lack ATPase activity and the ability to independently refold proteins following stresses. In addition, data presented here shows that clusterin, haptoglobin and a2-macroglobulin exert potent effects on amyloid formation, and provide evidence to suggest that these effects are exerted via interactions with pre-fibrillar species. These findings suggest that clusterin, haptoglobin and a2- macroglobulin are an important element in the control of extracellular protein misfolding.

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