The vault complex is a ubiquitous 13-MDa ribonucleoprotein
assembly, composed of three proteins (TEP1, 240 kDa; VPARP,
193 kDa; and MVP, 100 kDa) that are highly conserved in
eukaryotes and an untranslated RNA (vRNA). The vault has
been shown to affect multidrug resistance in cancer cells,
and one particular component, MVP, is thought to play a
role in the transport of drug from the nucleus. To locate
the position of the vRNA, vaults were treated with RNases,
and cryo-electron microscopy (cryo-EM) was performed on
the resulting complexes. Using single-particle reconstruction
techniques, 3,476 particle images were combined to generate
a 22-Å-resolution structure. Difference mapping between
the RNase-treated vault and the previously calculated intact
vault reconstructions reveals the vRNA to be at the ends
of the vault caps. In this position, the vRNA may interact
with both the interior and exterior environments of the
vault. The finding of a 16-fold density ring at the top
of the cap has allowed modeling of the WD40 repeat domain
of the vault TEP1 protein within the cryo-EM vault density.
Both stoichiometric considerations and the finding of higher
resolution for the computationally selected and refined
“barrel only” images indicate a possible symmetry
mismatch between the barrel and the caps. The molecular
architecture of the complex is emerging, with 96 copies
of MVP composing the eightfold symmetric barrel, and the
vRNA together with one copy of TEP1 and four predicted
copies of VPARP comprising each cap.