Structural Basis of Amyloid Fibril Assembly by Plant Seed Storage Proteins

Amyloid fibrils are highly ordered protein assemblies characterized by a cross-β architecture. A wide range of proteins can adopt amyloid states, contributing to both disease-related pathology and normal physiological function. While animal-derived amyloids have been extensively examined in atomic detail, amyloid fibrils formed by plant proteins remain relatively understudied. Here we systematically assess three major seed storage proteins-oat 12S globulin, soybean 7S globulin, and rice glutelin-under harsh cooking-like conditions (pH 2, 85 °C). Oat globulin and rice glutelin readily form fibrils in both purified preparations and whole-seed extracts, whereas soybean globulin forms fibrils only in purified preparations and remains largely amorphous in whole-seed extracts. Using cryo-electron microscopy, we determine the structure of oat globulin fibrils at 3.9 Å resolution. The fibril core adopts a compact triangular architecture with pseudo-threefold symmetry and is stabilized by extensive hydrophobic and aromatic packing. Our findings establish the molecular basis of amyloid formation in plant seeds and expand the structural landscape of amyloid fibrils beyond animal and microbial systems, providing a foundation for understanding amyloid formation in plant- and food-derived proteins.