αB-Crystallin (HSPB5) is a small heat-shock protein that is composed of dimers that then assemble into a polydisperse ensemble of oligomers. Oligomerisation is mediated by heterologous interactions between the C-terminal tail of one dimer and the core "α-crystallin" domain of another and stabilised by interactions made by the N-terminal region. Comparatively little is known about the latter contribution， but previous studies have suggested that residues in the region 54-60 form contacts that stabilise the assembly. We have generated mutations in this region (P58A， S59A， S59K， R56S/S59R and an inversion of residues 54-60) to examine their impact on oligomerisation and chaperone activity in vitro. By us... More
αB-Crystallin (HSPB5) is a small heat-shock protein that is composed of dimers that then assemble into a polydisperse ensemble of oligomers. Oligomerisation is mediated by heterologous interactions between the C-terminal tail of one dimer and the core "α-crystallin" domain of another and stabilised by interactions made by the N-terminal region. Comparatively little is known about the latter contribution， but previous studies have suggested that residues in the region 54-60 form contacts that stabilise the assembly. We have generated mutations in this region (P58A， S59A， S59K， R56S/S59R and an inversion of residues 54-60) to examine their impact on oligomerisation and chaperone activity in vitro. By using native mass spectrometry， we found that all the αB-crystallin mutants were assembly competent， populating similar oligomeric distributions to wild-type， ranging from 16-mers to 30-mers. However， circular dichroism spectroscopy， intrinsic tryptophan and bis-ANS fluorescence studies demonstrated that the secondary structure differs to wild type， the 54-60 inversion mutation having the greatest impact. All the mutants exhibited a dramatic decrease in exposed hydrophobicity. We also found that the mutants in general were equally active as the wild-type protein in inhibiting the amorphous aggregation of insulin and seeded amyloid fibrillation of α-synuclein in vitro， except for the 54-60 inversion mutant， which was significantly less effective at inhibiting insulin aggregation. Our data indicate that alterations in the part of the N-terminal region proximal to the core domain do not drastically affect the oligomerisation of αB-crystallin， reinforcing the robustness of αB-crystallin in functioning as a molecular chaperone.
