Higher
order structure of vaults has been elucidated in collaboration
with a number of excellent scientists (including Dr. John Heuser
at Wash U, Dr. Tim Baker at Purdue, Dr. Joe Wall and colleagues
a Brookhaven, and most recently Dr.
Phoebe
Stewart at UCLA) using various EM techniques. John Heuser
observed two very different vault conformations (an open and a
closed form) when he deposited the particles onto a charged surface.
These
forms as well as intermediate structures suggest that inside cells,
vaults may be able to open and close. This process was depicted
by a model which showed the closed form as an ovoid hollow barrel
which can open into two eight-petalled flowers surrounding and
connected to a central ring.
We
have attempted to correlate vault structural components (16 petals
and hooks, 2 central rings) with its protein and RNA composition.
Since the petals represent the largest part of the flowers, it
seems likely that they are composed of the MVP, which constitutes
more than 70% of the protein mass. Mathematical analysis indicate
that there are 96 copies of the MVP per vault or 6 copies per
petal. This is supported by recent cryo-EM data of Lawerence
Kong, Phoebe Stewart
and Amara Siva who have
carried out a cryo-reconstruction (see
cryo-EM).
In light of the tendency of the petal to split in thirds, it seems
likely that each third is made up of two MVPs. The vault RNA comprises
less than 5% of the total mass of a vault particle and calculations
on data from rat liver vaults suggest that each vRNA is present
in approximately 16 copies per particle. This would therefore
suggest that one RNA is associated with each petal. Since digestion
of the vRNA within vaults with an enzyme specific for RNA caused
no change in the vault, it seems unlikely that the RNA plays a
structural role.
CryoEM Structure
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