Protein condensation, phase separation, and self-assembly are different aspects of very similar phenomena. Weak but multivalent intermolecular interactions drive the growth of supramolecular protein assemblies much larger than the size of the protein unit. The physical state of such condensates may be crystalline, amorphous solid, gel, or liquid. Ferritin proves to be an ideal scaffold on which to study self-assembly. Mammalian ferritin contains 24 polypeptide subunits in a nearly-spherical shell with octagonal symmetry. The N termini are disordered and point to the exterior. Exogenous expression in cells of a hybrid ferritin with dimerizing fluorescent proteins such as Citrine resulted in self-assembly of fluorescent protein bodies1. Mutation of the hydrophobic dimerizing patch to one containing cysteine led to oxidation-sensitive self-assembly2. Addition to ferritin of intrinsically disordered domains with light-sensitive coupling, named Corelets, has helped to map phase separation within cellular compartments by means of protein condensation3. In this work we revisit these systems using a new holographic microscopy tool to map refractive index in 3D, avoiding dependence on fluorescence imaging and setting the stage for correlative electron tomography of the protein assemblies in solid or liquid phase.
1. Giuliano, B. et al, Angew. Chem. Int. Ed., 2014, 53, 1534 –1537.
2. Giuliano, B. et al, Nano Lett., 2016, 16, 6231−6235.
3. Bracha, D. et al, Cell, 2018, 175, 1467–1480.