Thevault orvault cytoplasmic ribonucleoprotein is aeukaryoticorganelle (a structure in the cells of multicellular organisms) whose function is not yet fully understood. Discovered and isolated byNancy Kedersha andLeonard Rome in 1986,[2] vaults arecytoplasmic structures (outside thenucleus) which, whennegative-stained and viewed under an electron microscope, resemble the arches of a cathedral's vaulted ceiling, with 39-fold (orD39d)symmetry.[1] They are present in many types of eukaryotic cells and appear to be highlyconserved amongeukaryotes.[3]
Vaults are largeribonucleoprotein particles. About 3 times the size of aribosome and weighing approximately 13MDa, they are found in most eukaryoticcells and all higher eukaryotes. They measure 34nm by 60 nm from anegative stain, 26 nm by 49 nm fromcryo-electron microscopy, and 35 nm by 59 nm fromSTEM.[4] The vault consists primarily ofproteins, making it difficult to stain with conventional techniques.
The protein structure consists of an outer shell composed of 78 copies of the ~100 kDamajor vault protein (MVP). Inside are two associated vault proteins,TEP1 andVPARP. TEP1, also known as the telomerase-associated protein 1,[5] is 290 kDa and VPARP (also known as PARP4) is related topoly (ADP-ribose) polymerase (PARP) and is 193 kDa.[6] Vaults from higher eukaryotes also contain one or several smallvault RNAs (vRNAs, also known as vtRNAs) of 86–141 bases within.[7]
The MVP subunits are composed head-to-head, with theN-termini of each half-vault facing each other. From the N-terminal to theC-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 theWD40 repeat, binds to the cap domain, with one particular type of vRNA plugging the cap.[8]
Despite not being fully elucidated, vaults have been associated with thenuclear pore complexes and their octagonal shape appears to support this.[9][10] Vaults have been implicated in a broad range of cellular functions including nuclear-cytoplasmic transport, mRNA localization, drug resistance, cell signaling, nuclear pore assembly, and innate immunity.[11] The three vault proteins (MVP, VPARP, and TEP1) have each been knocked out individually and in combination (VPARP and TEP1) in mice.[12][13][14] All of the knockout mice are viable and no major phenotypic alterations have been observed.Dictyostelium encode three different MVPs, two of which have been knocked out singly and in combination.[15] The only phenotype seen in theDictyostelium double knockout was growth retardation under nutritional stress.[16] If vaults are involved in essential cellular functions, it seems likely that redundant systems exist that can ameliorate their loss.
In the late 1990s, researchers found that vaults (especially the MVP) were over-expressed incancer patients who were diagnosed withmultidrug resistance, that is the resistance against manychemotherapy treatments.[17] 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.[15]
These four species aremodel 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.[3]
Homologs of the major vault protein have been computationally found in bacteria.Cyanobacterial sequences appear most similar.[20][21] Pfam is also able to identify some such homologs.[18]
TheRome lab at UCLA has collaborated with a number of groups to use thebaculovirus system to produce large quantities of vaults. When themajor vault protein (MVP) is expressed in insect cells, vault particles are assembled onpolyribosomes in the cytoplasm.[22] By using molecular genetic techniques to modify the gene encoding the major vault protein, vault particles have been produced with chemically activepeptides attached to their sequence. These modified proteins are incorporated into the inside of the vault particle without altering its basic structure. Proteins and peptides can also be packaged into vaults by attachment of a packaging domain derived from the VPARP protein.[16] A number of modified vault particles have been produced in order to test the concept that vaults can be bio-engineered to allow their use in a wide variety of biological applications includingdrug delivery, biological sensors, enzyme delivery,controlled release, andenvironmental remediation.
A vault has been packaged with achemokine for potential use to activate the immune system to attack lung cancer.[23][24]
^Mossink MH, van Zon A, Fränzel-Luiten E, Schoester M, Kickhoefer VA, Scheffer GL, Scheper RJ, Sonneveld P, Wiemer EA (December 2002). "Disruption of the murine major vault protein (MVP/LRP) gene does not induce hypersensitivity to cytostatics".Cancer Research.62 (24):7298–304.PMID12499273.
^abRome LH, Kickhoefer VA (February 2013). "Development of the vault particle as a platform technology".ACS Nano.7 (2):889–902.doi:10.1021/nn3052082.PMID23267674.
^Rome L, Kedersha N, Chugani D (August 1991). "Unlocking vaults: organelles in search of a function".Trends in Cell Biology.1 (2–3):47–50.doi:10.1016/0962-8924(91)90088-Q.PMID14731565.
^Sokolskyi, Tymofii (December 12, 2019). "Bacterial Major Vault Protein homologs shed new light on origins of the enigmatic organelle".bioRxiv10.1101/872010.
^Sharma S, Zhu L, Srivastava MK, Harris-White M, Huang M, Lee JM, Rosen F, Lee G, Wang G, Kickhoefer V, Rome LH, Baratelli F, St John M, Reckamp K, Chul-Yang S, Hillinger S, Strieter R, Dubinett S (January 2013)."CCL21 Chemokine Therapy for Lung Cancer".International Trends in Immunity.1 (1):10–15.PMC4175527.PMID25264541.
