Supramolecular protein assemblies in the nucleus of human cells

Genetically encoded supramolecular protein assemblies (SMPAs) are induced to form in living cells by combination of distinct self-assembly properties. A single fusion construct contains genes encoding the heavy chain (H) of human ferritin and the citrine fluorescent protein, the latter exposing a weak dimerization interface, as well as a nuclear localization signal. Upon expression in HeLa cells, in vivo confocal fluorescence and differential interference contrast imaging revealed extended SMPA structures exclusively in the nuclei. Assemblies were typically round and took alveolar, shell-like, or hybrid structure. Transmission electron microscopy revealed a crystalline packing. Site-specific mutagenesis of the citrine dimerization interface clarified the mechanism of SMPA formation. The constituent proteins retained their activity in iron binding and fluorescence emission, thus suggesting a general strategy for formation of synthetic cellular bodies with specific biochemical function. The fluorescent protein citrine and the H subunits of human ferritin were genetically fused to combine their self-assembly properties. As a result, a three-dimensional network of interactions was established to form extended, fluorescent, and crystalline supramolecular protein assemblies (SMPAs) in live HeLa cells. With the addition of a genetically encoded nuclear localization signal, SMPA formation was targeted exclusively to the cell nucleus.