Identification of the Core Structure of Lysozyme Amyloid Fibrils by Proteolysis

Human lysozyme variants form amyloid fibrils in individuals suffering
from a familial non-neuropathic systemic amyloidosis. In vitro, wild-type
human and hen lysozyme, and the amyloidogenic mutants can be induced
to form amyloid fibrils when incubated under appropriate conditions. In
this study, fibrils of wild-type human lysozyme formed at low pH have
been analyzed by a combination of limited proteolysis and Fouriertransform
infrared (FTIR) spectroscopy, in order to map conformational
features of the 130 residue chain of lysozyme when embedded in the
amyloid aggregates. After digestion with pepsin at low pH, the lysozyme
fibrils were found to be composed primarily of N and C-terminally
truncated protein species encompassing residues 26–123 and 32–108,
although a significant minority of molecules was found to be completely
resistant to proteolysis under these conditions. FTIR spectra provide
evidence that lysozyme fibrils contain extensive β-sheet structure and a
substantial element of non β-sheet or random structure that is reduced
significantly in the fibrils after digestion. The sequence 32–108 includes the
β-sheet and helix C of the native protein, previously found to be prone to
unfold locally in human lysozyme and its pathogenic variants. Moreover,
this core structure of the lysozyme fibrils encompasses the highly
aggregation-prone region of the sequence recently identified in hen
lysozyme. The present proteolytic data indicate that the region of the
lysozyme molecule that unfolds and aggregates most readily corresponds
to the most highly protease-resistant and thus highly structured region of
the majority of mature amyloid fibrils. Overall, the data show that amyloid
formation does not require the participation of the entire lysozyme chain.
The majority of amyloid fibrils formed from lysozyme under the conditions
used here contain a core structure involving some 50% of the polypeptide
chain that is flanked by proteolytically accessible N and C-terminal regions.