Fig. 2. Proteinaceous nanocompartments as multiscale contrast agents.

Schematic summarizing work on metalloproteins for molecular imaging applications. (a) Genetic constructs for expression of the M. xanthus encapsulin system in mammalian systems consisting of its shell forming monomer MxEncA and a multigene expression cassette for co-expression of its endogenous cargo proteins (MxEncBCD) or engineered cargos such as a soluble bacterial tyrosinase (BmTyr) with a C-terminal encapsulation signal. Cutaway view of the MxEnc nanocompartment (T=3) schematically showing internal cargo proteins either yielding iron oxides for detection in MRI or cryoET or melanin pigments that afford (b) detection by MRI, optoacoustics, and cryo-electron tomography. (c) Genetic constructs for expression of the Q. thermotolerans encapsulin system in mammalian systems consisting of its shell forming monomer QtEnc and its iron-mineralizing cargo protein QtIMEF, or other engineered cargos such as fluorescent proteins. Cutaway view of the larger QtEnc nanocompartment (T=4 icosahedral symmetry) showing a zoom-in onto the pore region at the fivefold symmetry center and docked QtIMEF cargo yielding effective iron biomineralization affording contrast in TEM images of (d) HEK293T cells and T4/5 Drosophila neurons. Structures of BM3h (PDB: 4DU2), ferritin (EMD-2788), Mx Encapsulin (EMD-5917), BmTyr (PDB: 3NM8), Qt Encapsulin (EMD-4879) and QtIMEF (PDB: 6N63) were visualized using ChimeraX³³. Adapted from Ref.⁵⁰, Springer Nature Ltd (b) and Ref.⁵¹, American Chemical Society (d).