Vault (organelle)

The vault or vault cytoplasmic ribonucleoprotein is a eukaryotic organelle whose function is not fully understood. Discovered and successfully isolated by cell biologist Nancy Kedersha and biochemist Leonard Rome of the UCLA School of Medicine in the 1980s, vaults are cytoplasmic organelles which under an electron microscope resemble the arches of a cathedral vault, with 39-fold symmetry. They are present in many types of eukaryotic cells and appear to be highly conserved amongst eukaryotes. Vaults become part of lipid rafts where they may play a role fighting pathogens.

Morphology
Vaults are large ribonucleoprotein particles. About 3 times the size of a ribosome and weighing approximately 13 MDa, they are found in many diverse eukaryotic cells. They measure 34 nm by 60 nm from a negative stain, 26 nm by 49 nm from cryo-electron microscopy, and 35 nm by 59 nm from STEM. The vaults consist primarily of proteins, making it difficult to stain with conventional techniques. The protein structure consists of many major vault proteins (MVP) bound to one of the two minor vault proteins. Two large complexes of several MVP's and a minor vault protein close together to form the barrel-like vault organelle. They also contain small vault RNAs (vRNAs) of 86–141 bases within.

Function
Despite not being fully elucidated, vaults have been associated with the nuclear pore complexes and their octagonal shape appears to support this. It has been concluded that the vault's function is the transportation of molecules, such as mRNA, from the nucleus to parts of the cytoplasm. It is also thought that vaults play a role in protein synthesis.

Association with cancer
In the late 1990s, researchers found that vaults (especially the MVP) were over-expressed in cancer patients who were diagnosed with multidrug resistance, that is the resistance against many chemotherapy treatments. Although this does not prove that increased number of vaults led to drug resistance, it does hint at some sort of involvement. This has potential in discovering the mechanisms behind drug-resistance in tumor cells and improving anticancer drugs.

Evolutionary conservation
Vaults have been identified in mammals, amphibians, avians and Dictyostelium discoideum. The Vault model used by the Pfam database identifies homologues in Paramecium tetraurelia, Kinetoplastida, many vertebrates, a cnidarian (starlet sea anemone), molluscs, Trichoplax adhaerens, flatworms, Echinococcus granulosus and Choanoflagellate.

Although vaults have been observed in many eukaryotic species, a few species do not appear to have the protein. These include:
 * Arabidopsis thaliana—a small flowering plant related to cabbage and mustard.
 * Caenorhabditis elegans—a free-living nematode that lives in soil.
 * Drosophila melanogaster—a two-winged insect also known as a fruit fly.
 * Saccharomyces cerevisiae—a species of yeast.

These four species are model organisms for plants, nematodes, animal genetics and fungi respectively. Despite these exceptions, the high degree of similarity of vaults in organisms that do have them implies some sort of evolutionary importance.