Self-Assembly of Electrostatic Cocrystals from Supercharged Fusion Peptides and Protein Cages

Self-assembly is a convenient process to arrange complex biomolecules into large hierarchically ordered structures. Electrostatic attraction between the building blocks is a particularly interesting driving force for the assembly process, as it is easily tunable and reversible. Large biomolecules with high surface charge density, such as proteins and protein cages, are very promising building blocks due to their uniform size and shape. Assemblies of functional molecules with well-defined nanostructures have wide-ranging applications but are difficult to produce precisely by synthetic methods. Furthermore, obtaining highly ordered structures is an important prerequisite for X-ray structure analysis. Here we show how negatively charged ferritin and viral protein cages can adopt specific cocrystal structures with supercharged cationic polypeptides (SUPs, K72) and their recombinant fusions with green fluorescent protein (GFP-K72). The cage structures and recombinant proteins self-assemble in aqueous solution to large ordered structures, where the structure morphology and size are controlled by the ratio of oppositely charged building blocks and the electrolyte concentration. Both ferritin and viral cages form cocrystals with face centered cubic structure and lattice constants of 14.0 and 28.5 nm, respectively. The crystals are porous and the cationic recombinant proteins occupy the voids between the cages. Such systems resemble naturally occurring occlusion bodies and may serve as protecting agents as well as aid the structure determination of biomolecules by X-ray scattering.