Common motifs in protein self-assembly

The importance of misfolding proteins forming amyloid fibrils for the aetiology of many diseases, particularly those of old age, is well recognized. This phenomenon is now thought to be a universal property of proteins, as long as appropriate conditions for loosening the native folded structure can be found, which may be outside those of normal physiology. However, the beta-sheet-rich structure of the amyloid fibril does not need to exist in isolation. Recent work has shown that higher order assemblies of the fibrils occur into structures resembling spherulites found in common synthetic semi-crystalline polymers. In these, the fibrils grow outwards from an inner core, thought to be amorphous. Data will be presented on the kinetics of growth of these fibrils for different proteins, so that similarities and differences can be revealed, and related to subtle differences in appearance under the microscope. The in vitro assembly of amyloid fibrils is usually thought to occur well away from the isoelectric point of the protein, and these are the conditions under which they have most been studied. Around the isoelectric point, particulate self-assembly is known to occur for beta-lactoglobulin, and we can now show this is also a generic form of protein self-assembly once the net charge on the protein is close to zero. Nevertheless, the charge is not actually zero, and salt in the solution is found to have a significant effect on the growth of the particles. The use of SAXS, thioflavin T staining and FTIR also shows that within the particles there is also clear evidence for amyloid-like beta-sheet structure, particularly in the case when salt is absent, demonstrating that this particular motif underlies this very different form of protein self-assembly.