Quaternary dynamics underlie the chaperone function of Hsp27 and αB-crystallin in maintaining proteostasis

The pathogenesis of neurodegenerative diseases, such as Alzheimer’s disease and amyotrophic lateral sclerosis, is believed to be caused by the aggregation of non-native proteins. The small heat shock proteins (sHsps) are a class of molecular chaperones which act as the first line of defence against intracellular protein aggregation. Defining the structure-function relationship of sHsps is critical to understanding the molecular mechanisms by which they inhibit protein aggregation. This thesis primarily utilised native mass spectrometry (MS) to study the structure and dynamics of the human sHsps Hsp27 (HSPB1) and αB-crystallin (αB-c, HSPB5). Post-translational modifications (PTM) regulate the function of sHsps by inducing a range of structural changes from the secondary to the quaternary level. Serine phosphorylation of Hsp27 occurs at residues 15, 78 and 82. However, the site-specific effect of phosphorylation at each site and how the degree of phosphorylation affects Hsp27 structure and function had not been extensively characterised. One aspect of this thesis was to explore how phosphorylation affects the structure and function of Hsp27 by using mutations that mimic phosphorylation (MMP), where serine residues were substituted for aspartic acid. Utilising native MS and other biophysical techniques, this work shows that increasing the number of MMP alters the dimer-oligomer equilibrium of Hsp27, such that the proportion of dimer increases. The increase in dimer abundance correlates with an enhanced capacity of Hsp27 to inhibit amorphous and fibrillar aggregation of client proteins. Thus, based on this work it is concluded that phosphorylation of Hsp27 in vivo induces dissociation of large oligomers into chaperone switch’; during periods of cellular stress phosphorylation of Hsp27 occurs in order to help maintain intracellular proteostasis