Vaults are hollow, ribonucleoprotein capsules (72.5 x 41 x 41 nm) comprised primarily of 96 self-assembled copies of one 97 kDa protein. The biological function for these nanocapsules, which are ubiquitous intracellular components of eukaryotes, is unknown; yet they may prove useful for material entrapment and drug delivery. If vaults are to serve as protective compartments for the transport of drugs, it is necessary to target material to the vault nanocapsule interior in order to optimize drug loading. Current experiments aim to attach chemical species to the mINT (minimal interaction domain) protein, which is known to enter and associate with the vault interior via vault “breathing,” therefore “shuttling” the attached molecules inside the vaults.

As proof of concept, we have attached functionalized Nanogold clusters (Nanoprobes) to the 6-His tag of mINT via coordination with Ni2+ ions in solution. After mixing this complex with vaults, loaded vaults are separated from unbound probes by immunoprecipitation with antibody-agarose beads. Successful encapsulation of the gold-mINT complex within vaults has been verified with parallel western blot and silver development (to detect gold) analyses, and by TEM imaging after Nanogold enhancement. Further verification of gold encapsulation may be provided by studies of the KCl quenching of encapsulated mINT-GFP (green fluorescent protein) fusion protein fluorescence.

Future studies with gold-loaded vaults will focus on probe release by thrombin cleavage (of the mINT protein) or low pH environments. Ideally, gold-loaded vaults would be useful for photothermal treatment of cancer. After being targeted specifically to tumor cells (via tags localized on the exterior of the vault ends), the encapsulated gold would preferentially absorb x-rays or near-infrared light and heat up, resulting in bubble formation and subsequent rupture of the surrounding tumor cells.