Abstract
Prior crystal structures of the vault have provided clues of its structural variability but are non-conclusive due to crystal packing. Here, we obtained vaults by engineering at the N terminus of rat major vault protein (MVP) an HIV-1 Gag protein segment and determined their near-atomic resolution (∼4.8 Å) structures in a solution/non-crystalline environment. The barrel-shaped vaults in solution adopt two conformations, 1 and 2, both with D39 symmetry. From the N to C termini, each MVP monomer has three regions: body, shoulder, and cap. While conformation 1 is identical to one of the crystal structures, the shoulder in conformation 2 is translocated longitudinally up to 10 Å, resulting in an outward-projected cap. Our structures clarify the structural discrepancies in the body region in the prior crystallography models. The vault's drug-delivery potential is highlighted by the internal disposition and structural flexibility of its Gag-loaded N-terminal extension at the barrel waist of the engineered vault.