On Possible Function and Toxicity of Multiple Oligomeric/Conformational States of a Globular Protein – Human Stefin B

Our aim was to follow the differences in stability, folding rates and, more recently, the differences in aggregation propensity of the two human stefins, their chimeras and site mutations. The structure of the stefin B tetramer revealed two domain-swapped dimers intertwined by loop-exchange. Only the tetramers and domain-swapped dimers interacted with Aβ and inhibited its fibril growth. In the mechanism of amyloid fibril formation by stefin B in vitro, the tetramers may be off-pathway to prefibrillar aggregates, protofibrils and fibrils. In cells, aggregates of stefin B accumulated in aggresome-like bodies. The conformation of the higher oligomers of stefin B is still ‘native-like’. However, they show some ANS dye binding, characteristic of exposure of the hydrophobic residues. The higher order oligomers (6–12 mers) proved more toxic to cells than lower oligomers: dimers and tetramers, which practically did not differ from the monomers and were not toxic. Accordingly, the higher oligomers inserted into membranes best. A mutant that formed a molten globule state was also toxic.