(373a) Opening of Self-Assembled Protein Nanocapsules Triggered by Low pH

Vaults are nanoscale, ribonucleoprotein capsules (41 nm x 72.5 nm) comprised primarily of 96 self-assembled copies of one 104 kDa protein. The biological function for these nanocapsules, which are ubiquitous intracellular components of eukaryotes, is unknown; yet they may prove useful for drug delivery and for compartmentalized materials synthesis. Our aim is to design mechanisms for reversible vault assembly/disassembly in order to control the encapsulation and release of materials. By monitoring the intrinsic fluorescence of tryptophan residues of vault proteins as a function of pH, evidence was gathered that suggests a partially open vault conformation at pH 3.4. Surface studies of adsorbed vaults have been carried out to further elucidate the nature of vault conformation change at reduced pH. Vault monolayers adsorbed onto self-assembled monolayers (SAMs) have been investigated using the quartz crystal microbalance (QCM) and surface plasmon resonance (SPR). Evidence was obtained that vaults adsorb in an end-on configuration, and that at low pH, the vaults dissociate along their equator. Half of the vault particle is released into solution and the remainder tends to flatten on the surface. If the pH is raised to 6.5, the adsorbed vault half assumes a more upright, cup-like conformation. Negative-stain transmission electron microscopy (TEM) images of vaults exposed to low pH support our interpretation of QCM and SPR data. However, significant vault conformation change appears to occur even at near neutral pH, as QCM studies show that relatively large antibody molecules can gain access to antigen that is known to reside in the vault interior. All vault components have been cloned and overexpressed, thereby providing an attractive system for study of biological self-assembly and a potentially versatile platform for biomaterials design.