A mechanism of ferritin crystallization revealed by cryo-STEM tomography

Protein crystallization is important in structural biology, disease research and pharmaceuticals. It has recently been recognized that nonclassical crystallization involving initial formation of an amorphous precursor phase-occurs often in protein, organic and inorganic crystallization processes(1-5). A two-step nucleation theory has thus been proposed, in which initial low-density, solvated amorphous aggregates subsequently densify, leading to nucleation(4,6,7). This view differs from classical nucleation theory, which implies that crystalline nuclei forming in solution have the same density and structure as does the final crystalline state(1). A protein crystallization mechanism involving this classical pathway has recently been observed directly(8). However, a molecular mechanism of nonclassical protein crystallization(9-15) has not been established(9,11,14). To determine the nature of the amorphous precursors and whether crystallization takes place within them (and if so, how order develops at the molecular level), three-dimensional (3D) molecular-level imaging of a crystallization process is required. Here we report cryogenic scanning transmission microscopy tomography of ferritin aggregates at various stages of crystallization, followed by 3D reconstruction using simultaneous iterative reconstruction techniques to provide a 3D picture of crystallization with molecular resolution. As crystalline order gradually increased in the studied aggregates, they exhibited an increase in both order and density from their surface towards their interior. We observed no highly ordered small structures typical of a classical nucleation process, and occasionally we observed several ordered domains emerging within one amorphous aggregate, a phenomenon not predicted by either classical or two-step nucleation theories. Our molecular-level analysis hints at desolvation as the driver of the continuous order-evolution mechanism, a view that goes beyond current nucleation models, yet is consistent with a broad spectrum of protein crystallization mechanisms.