Characterising the small heat shock proteins: structure and regulation

The small heat shock protein HSP27 is expressed throughout the body and involved in a variety of cellular processes. Its multiple roles may all be mediated by its ability to chaperone partially folded protein substrates, and its malfunction is implicated in a range of diseases such as motor neuropathy and neurodegeneration. Here I describe structural, dynamical and functional investigations into this protein, particularly regarding its regulation by oxidation and phosphorylation. The foundations of this work were a protein crystal structure obtained on the oxidised core domain of the wild-type protein, which forms a disulphide-locked dimer, and additional structures that contain neuropathy-associated mutations. Observations of a structurally variable region, namely the β2-strand at the beginning of the core domain, led us to probe the nature of this region in solution and examine whether phosphorylation at the nearby phosphoserine site led the β2 to bind more strongly to the core domain, as phosphorylation elicits HSP27 oligomer disruption and activity modulation. I am able to postulate on the structural effect of mutation using the mutant crystal structures, and report the first structure showing the interface formed by the HSP27 core domain dimer in the ‘antiparallel III’ register. This finding, in tandem with the unusual nature of the intact intra-subunit disulphide bond reported here, motivated studies into the role of this disulphide in the redox response of the HSP27 chaperone. These include dynamical studies on the subunit exchange of the protein and functional investigations showing that the disulphide is formed in the cell and that chaperone behaviour depends on the oxidation state. Moreover, I show that the redox potential of the disulphide is affected by mutation, as is the change in chaperone response on oxidation. Taken together, I suggest that HSP27 is regulated by oxidation through the intra-dimer disulphide, which allows it to respond to oxidative stress within the cell.