Myoglobin Amyloid Fibrils Reveal a Hierarchical Principle of Polymorphism and Electrostatic Self-Assembly.

The atomic architecture of apomyoglobin amyloid fibrils, despite the protein's dual distinction as the first structurally resolved protein and the paradigmatic nondisease amyloid, has remained a decades-long puzzle. Here, we identify electrostatic screening as the critical switch that enables the formation of highly ordered apomyoglobin fibrils, allowing us to determine the cryo-electron microscopy structures of three distinct polymorphs (PM1, PM2, and PM3) at 2.7 Å resolution. The structures reveal a conserved "hydrophobic-in, positively charged-out" architecture, where a charged surface surrounds a tightly packed core, providing a structural explanation for salt-dependent assembly. Structural comparisons reveal a hierarchical principle of amyloid organization, in which short sequence segments retain conserved local conformations dictated by their intrinsic folding propensities, while variations in supramolecular packing give rise to polymorphic diversity. These findings establish a molecular framework for understanding electrostatically controlled self-assembly and the structural basis of amyloid polymorphism.