Limited Proteolysis Reveals That Amyloids from the 3D Domain-Swapping Cystatin B Have a Non-Native β-Sheet Topology.

3D domain-swapping proteins form multimers by unfolding and then sharing of secondary structure elements, often with native-like interactions. Runaway domain swapping is proposed as a mechanism for folded proteins to form amyloid fibres, with examples including serpins and cystatins. Cystatin C amyloids cause a hereditary form of cerebral amyloid angiopathy whilst cystatin B aggregates are found in cases of Unverricht-Lundborg Syndrome, a progressive form of myoclonic epilepsy. Under conditions that favour fibrillisation, cystatins populate stable 3D domain-swapped dimers both in vitro and in vivo that represent intermediates on route to the formation of fibrils. Previous work on cystatin B amyloid fibrils revealed that the α-helical region of the protein becomes disordered and identified the conservation of a continuous 20-residue elongated β-strand (residues 39-58), the latter being a salient feature of the dimeric 3D domain-swapped structure. Here we apply limited proteolysis to cystatin B amyloid fibrils and show that not only the α-helical N-terminal of the protein (residues 1-35) but also the C-terminal of the protein (residues 80-98) can be removed without disturbing the underlying fibril structure. This observation is incompatible with previous models of cystatin amyloid fibrils where the β-sheet is assumed to retain its native antiparallel arrangement. We conclude that our data favour a more generic, at least partially parallel, arrangement for cystatin β-sheet structure in mature amyloids and propose a model that remains consistent with available data for amyloids from either cystatin B or cystatin C.