A functional amyloid scaffold shapes insect egg coats

Functional amyloids serve as structural scaffolds across biology, yet the molecular architecture and assembly principles of many remain unresolved. The lepidopteran egg coat, or chorion, presents a striking example: hundreds of paralogous proteins sharing a conserved central domain assemble into a mechanically resilient amyloid matrix essential for embryo protection. Here, combining evolutionary analysis of more than 500 sequences with cryo-electron microscopy and biophysical assays, we determine the atomic structure of chorion amyloid filaments and uncover the principles governing their assembly. Contrary to previous structural predictions, chorion filaments adopt a {beta}-serpentine fold stabilized by a short hexapeptide motif that forms homotypic steric-zipper interfaces, self-assembles autonomously, and seeds full-length filament growth. Assembly proceeds through secondary nucleation, while thermodynamic and structural analyses support a hierarchical assembly mechanism in which motif-driven interactions nucleate filament formation prior to consolidation of the mature protofilament core. These findings establish the molecular basis of insect egg-coat assembly and demonstrate that assembly mechanisms commonly associated with pathological aggregation can also operate within a biologically regulated functional amyloid framework.