Your Cells Hold Thousands of Giant Hollow Vaults and No One Knows Why (Video)

0:02 Vault particles in cells

In this video, we will learn about vaults, some of the largest particles inside our cells. Vaults are large hollow barrel-shaped ribonucleoprotein particles. They sit in the cytoplasm of many eukaryotic cells, including human cells. A ribonucleoprotein particle is a complex built from protein and RNA, like the ribosome. A eukaryotic cell is a cell with a nucleus. We begin with how researchers found vaults, where they occur, and their shape, and then turn to the four components: the major vault protein, the vault poly(ADP-ribose) polymerase, the telomerase-associated protein 1, and vault RNA.

0:44 How vaults were discovered

Nancy Kedersha and Leonard Rome found vaults in 1986. They were working at UCLA and purifying coated vesicles from rat liver. A coated vesicle is a small membrane sac with a protein coat on its outer surface. The researchers were looking for these vesicles in a liver extract. The vault particles appeared in electron micrographs as a contaminant. An electron micrograph is an image formed with an electron microscope. The contaminant had a uniform size and shape. The uniformity suggested a defined particle rather than random debris. The arched, ribbed outline reminded them of the vaulted ceilings of cathedrals. A vaulted ceiling is an arched ceiling made of stone or brick. The resemblance gave the particle its name. The word vault refers to the arched ceiling, and the particle became the vault.

1:40 Where vaults occur

A typical mammalian cell contains roughly 10,000 to 100,000 vaults. Most vaults sit in the cytoplasm. They are absent from baker's yeast, the fruit fly Drosophila, the nematode C. elegans and the plant Arabidopsis. The pattern indicates that vaults were lost in several lineages.

2:01 Shape and size of a vault

A vault is a hollow barrel about 70 nanometers long and 40 nanometers wide. A nanometer is one billionth of a meter. The barrel has a narrow waist and a cap at each end. The two ends look the same in the electron microscope. Its mass is about 13 megadaltons. A dalton is the mass of one hydrogen atom, and a megadalton is one million daltons. The mass is roughly three times the mass of a eukaryotic ribosome. The vault is the largest known ribonucleoprotein particle in the cell. A vault is built as two identical halves that meet at the waist. The interior is a large enclosed space, big enough to hold hundreds of average-sized proteins.

2:45 The major vault protein (MVP)

The major vault protein, or MVP, is a protein of about 100 kilodaltons. A kilodalton is one thousand daltons. MVP makes up about 70 percent of the vault's mass. The shell contains 78 copies of MVP, with 39 in each half. The copies sit side by side like the staves of a barrel. A stave is one of the curved boards that form the side of a barrel. Each MVP chain starts at the waist, where its N-terminal region meets the matching chain from the other half. The N-terminus is the end of a protein chain that carries the free amino group. The chain runs up the wall to the cap. The wall is built from nine repeat domains, then a shoulder domain, then a long cap helix. A domain is a compact part of a protein that folds on its own. A helix is a coiled protein structure. The cap helices of 39 chains pack together to close the cap. The two halves are held together at the waist by relatively weak contacts. The weak contacts allow a vault to separate into halves and reassemble. MVP alone, expressed in insect cells, assembles into empty vault-like particles. The shell therefore needs no other component to form. The empty particles have the same barrel shape as a vault.

4:05 vPARP

The vault poly(ADP-ribose) polymerase, or vPARP, is also called PARP4. It is a protein of about 190 kilodaltons. vPARP binds to the inner surface of the MVP shell. The binding uses a short region near its C-terminus, called the MVP-interaction domain, or INT domain. The C-terminus is the end of a protein chain that carries the free carboxyl group. The INT domain provides the attachment point for cargo in engineered vaults.

4:35 TEP1

Telomerase-associated protein 1, or TEP1, is a large protein of about 290 kilodaltons. Researchers first found TEP1 in the telomerase complex. TEP1 holds the RNA in the particle. In mice that lack TEP1, vault RNA is unstable and is not found in vaults, while the MVP shell still forms.

4:57 Vault RNAs

Vault RNAs are small non-coding RNAs about 88 to 100 nucleotides long. A nucleotide is one building block of RNA, and a non-coding RNA is not translated into protein. Humans have four vault RNA genes, named vtRNA1-1, vtRNA1-2, vtRNA1-3 and vtRNA2-1. The four genes cluster on chromosome 5. Vault RNA makes up only about 5 percent of the vault's mass. The RNA sits near the caps. Less than about 5 percent of the vault RNA in a cell sits inside vaults. Most vault RNA is free in the cytoplasm. Free vault RNAs have roles of their own. vtRNA1-1 binds the autophagy receptor p62 and limits autophagy. Autophagy is the process by which a cell degrades its own components. vtRNA2-1, also called nc886, binds and inhibits the kinase PKR. A kinase is an enzyme that adds phosphate groups to proteins. The free vault RNAs therefore act outside the vault.

6:03 Proposed functions of vaults

Researchers have not established the function of vaults. Several hypotheses remain, and each has supporting evidence. The first hypothesis is drug resistance. In 1995, researchers identified the lung resistance-related protein, or LRP, as MVP. LRP was overexpressed in multidrug-resistant cancer cell lines. Multidrug-resistant means that the cells resist several anticancer drugs. The finding suggested that vaults help cells remove drugs. Mice that lack MVP have no vaults, are viable and healthy, and their cells show no change in sensitivity to anticancer drugs. Vaults are therefore not required for drug resistance. The drug resistance hypothesis does not explain the normal role of vaults. The second hypothesis is transport between the nucleus and the cytoplasm. Some vaults sit at nuclear pore complexes. The size and hollow shape suggest a carrier. No cargo has been shown to move this way under normal conditions. The third hypothesis is innate immunity. Lung epithelial cells need MVP to take up and clear Pseudomonas aeruginosa. Pseudomonas aeruginosa is a bacterium that can infect the lung. Mice without MVP suffer more severe lung infection by this bacterium. The immunity result links MVP to bacterial clearance in the lung. The knockout mice show that vaults are not essential for life in laboratory conditions. The normal role of vaults remains open.

7:37 Vaults as delivery vehicles

Four features make vaults suitable for drug and vaccine delivery. They have a uniform size, a large interior, a natural presence in the body, and production from a single recombinant protein. The uniform size and large interior allow cargo to fit inside. The natural presence means that vaults cause little immune reaction. Researchers produce recombinant vaults from one protein, MVP. A cargo protein fused to the INT domain from vPARP binds inside the vault. Fused means that the cargo and the INT domain form one continuous protein chain. The INT domain anchors the cargo to the inner shell. Peptides added to the C-terminus of MVP appear on the outside of the caps. A peptide is a short chain of amino acids. These outer peptides can target vaults to specific cells. Peptides at the N-terminus sit inside at the waist. Vaults packaged with the chemokine CCL21 slowed lung tumor growth in mice. A chemokine is a signaling protein that attracts immune cells. The effect came from attracting immune cells. Researchers have prepared these vaults for clinical testing. The cargo stays inside the shell, and the outer peptides control where the vault goes.

8:56 Vault-like particles in bacteria and phages

Vaults were long thought to occur only in eukaryotes. A eukaryote is an organism whose cells have a nucleus. A 2026 search of predicted protein sequences, which has not yet been peer reviewed, reported MVP-like proteins in some bacteria, archaea and bacteriophages. Archaea are single-celled organisms distinct from bacteria. A bacteriophage is a virus that infects bacteria. The search found these proteins in less than 1 percent of sequenced species. Several of the proteins formed vault-like particles when produced in E. coli. One phage protein built a shell of 96 chains instead of 78. The shell shape was similar, but the chain count differed. If confirmed, vaults are older and more widespread than once thought. The function of these bacterial and phage vaults is also unknown. Peer review means that other scientists evaluate the work before publication. E. coli is a bacterium used in the laboratory.

9:59 Recap and question

A vault is a hollow barrel built from 78 MVP chains in two halves. It carries vPARP, TEP1 and vault RNA inside. Vaults occur in most animal cells. Their function remains unresolved, and researchers use them as engineered carriers. One question remains to answer. Which vault component can form vault-like particles by itself? The answer is the major vault protein. How do researchers load cargo inside? They fuse the cargo to the INT domain of vPARP. The major vault protein forms the shell without vPARP, TEP1 or vault RNA. The INT domain binds the inner surface of the shell, so the fused cargo stays inside.