Vault (organelle)

The vault or vault cytoplasmic ribonucleoprotein is a eukaryotic organelle whose function is not yet fully understood. Discovered and isolated by Nancy Kedersha and Leonard Rome in 1986, vaults are cytoplasmic organelles which, when negative-stained and viewed under an electron microscope, resemble the arches of a cathedral's vaulted ceiling, with 39-fold (or D39d) symmetry. They are present in many types of eukaryotic cells, and appear to be highly conserved among eukaryotes. Vaults are large ribonucleoprotein particles. About 3 times the size of a ribosome and weighing approximately 13 MDa, they are found in most eukaryotic cells and all higher eukaryotes. 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 an outer shell composed of 78 copies of the ~100 kDa major vault protein (MVP). Inside are two associated vault proteins, TEP1 and VPARP. TEP1, also known as the telomerase-associated protein 1, is 290 kDa and VPARP (also known as PARP4) is related to poly (ADP-ribose) polymerase (PARP) and is 193 kDa. Vaults from higher eukaryotes also contain one or several small vault RNAs (vRNAs, also known as vtRNAs) of 86–141 bases within. The MVP subunits are composed head-to-head, with the N-termini of each half-vault facing each other. From the N-terminal to the C-terminal, a MVP subunit folds into 9 repeat domains, 1 band7-like shoulder domain, 1 cap-helix domain, and 1 cap-ring domain, corresponding to the shape of the vault shell. VPARP binds to repeat domain #4. TEP1, itself a ring due to the WD40 repeat, binds to the cap domain, with one particular type of vRNA plugging the cap. Despite not being fully elucidated, vaults have been associated with the nuclear pore complexes and their octagonal shape appears to support this.