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.
Konstantoulea, K., Kunach, P., Tagad, H., Diamond, M. I., Louros, N. N.
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