Amyloid-Like Fibrils from a Peptide-Analogue of the Central Domain of Silkmoth Chorion Proteins

Abstract

The structure and self-assembly properties ofa 51-residue peptide- analogue of the central conservative domain of silkmoth chorion (eggshell) proteins were studied in detail. This peptide (cA peptide) forms amyloid-like fibrils, under a variety of conditions.

Thefibrils bind Congo red and thioflavin T. Negative staining and shadowing showed that the fibrils are twisted, forming double helices. The average width of the basic double helical unit is ∼90 Å and the helical pitch of thedouble helix ∼920 Å. Individual fibrils (protofilaments) which super-coil to form the double helix have a diameter of ∼30—40 Å. FT-IR and FT-Raman spectroscopy indicated an antiparallel β-sheet type of structure. X-ray diffraction patterns from fibres of the cA-peptide, taken with a double-mirror camera, clearly indicate a “rich”, oriented cross-β fibre pattern characterized by a meridional reflection at ∼4.66 Å and an equatorial reflection at ∼10.12 Å.

Modeling studies suggest that a twisted β-sheet of 4-residue β-strands alternating with β-turns is the basic structural motif of the fibrils. The models are similar to the cross-βstructure proposed a decade and a half ago for silkmoth chorion proteins to dictate the helicoidal architecture of intact, native chorions. Thus, it appears that silkmoth chorion is the first well documented case, where amyloid plaques formed from self-assembly of chorion proteins, in vivo protect the oocyte and the developing embryo from a wide range of environmental hazards.