vaults: novel cell particles
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Q: Vaults are so large, Why weren't they discovered sooner?

A: To understand how particles nearly three times the size of ribosomes, present in thousands of copies per cell, could have gone undetected until just a few years ago, one must consider that conventional stains for EM, such as the heavy metal salts osmium tetroxide and uranyl acetate, have a high affinity for the charged components of membranes and nucleic acids but are poorly attracted to highly proteinaceous materials. Therefore, particles with high protein content such as vaults are nearly invisible when sections of cells are examined by TEM using a positive stain (they would appear as just another undefined cytoplasmic granule). Vaults were only visualized after isolation and negative staining, a method that reveals the structure of an object by virtue of the exclusion of a stain. With this method, the purified vault particle is covered with a stain that surrounds it and settles in surface depressions to reveal its unusual outline and contours.

Q: What is the function of vaults?

A: The precise function of vaults is still unknown. Their size, shape and localization to nuclear pores suggests that they may be a plug of the pore complex. Their hollow interior and role in multidrug resistance indicates that they may be carriers. The fact that one of the high molecular weight vault proteins (p240 or TEP1) is a component of telomerase may indicate a role in RNP assembly, transport or regulation.

Q: Do vaults have telomerase activity?

A: No telomerase activity has been detected in highly purified vaults.

Q: Why engineer vaults?

A: As vaults are highly conserved throughout eukaryotic evolution and abundantly expressed in all cells, it is likely that they will not be immunogenic. Like viruses, vaults self assemble and have a large internal cavity. If vaults can be engineered to contain exogenous materials (proteins, nucleic acids, drugs etc.). they may be suitable for use as delivery vehicles.

Q: Do vaults open and close in the cell?

A: Although there is no direct evidence that vaults open and close in cells, recent experiments by Dr. Michael Poderycki suggest the vaults are dynamic structures likely able to open and close in solution.

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