Characterization of MVP and VPARP assembly into vault ribonucleoprotein complexes

Vaults are barrel-shaped cytoplasmic ribonucleoprotein particles composed of three proteins: the major vault protein (MVP), the vault poly(ADP-ribose)polymerase (VPARP), and the telomerase-associated protein 1, together with one or more small untranslated RNAs. To date, little is known about the process of vault assembly or about the stability of vault components. In this study, we analyzed the biosynthesis of MVP and VPARP, and their half-lives within the vault particle in human ACHN renal carcinoma cells. Using an immunoprecipitation assay, we found that it took more than 4h for newly synthesized MVPs to be incorporated into vault particles but that biosynthesized VPARPs were completely incorporated into vaults within 1.5h. Once incorporated into the vault complex, both MVP and VPARP were very stable. Expression of human MVP alone in Escherichia coli resulted in the formation of particles that had a distinct vault morphology. The C-terminal region of VPARP that lacks poly(ADP-ribose)polymerase activity co-sedimented with MVP particles. This suggests that the activity of VPARP is not essential for interaction with MVP-self-assembled vault-like particles. In conclusion, our findings provide an insight into potential mechanisms of physiological vault assembly.     

Cryoelectron microscopy imaging of recombinant and tissue derived vaults: localization of the MVP N termini and VPARP

The vault is a highly conserved ribonucleoprotein particle found in all higher eukaryotes. It has a barrel-shaped structure and is composed of the major vault protein (MVP); vault poly(ADP-ribose) polymerase (VPARP); telomerase-associated protein 1 (TEP1); and small untranslated RNA (vRNA). Although its strong conservation and high abundance indicate an important cellular role, the function of the vault is unknown. In humans, vaults have been implicated in multidrug resistance during chemotherapy. Recently, assembly of recombinant vaults has been established in insect cells expressing only MVP. Here, we demonstrate that co-expression of MVP with one or both of the other two vault proteins results in their co-assembly into regularly shaped vaults. Particles assembled from MVP with N-terminal peptide tags of various length are compared. Cryoelectron microscopy (cryoEM) and single-particle image reconstruction methods were used to determine the structure of nine recombinant vaults of various composition, as well as wild-type and TEP1-deficient mouse vaults. Recombinant vaults with MVP N-terminal peptide tags showed internal density that varied in size with the length of the tag. Reconstruction of a recombinant vault with a cysteine-rich tag revealed 48-fold rotational symmetry for the vault. A model is proposed for the organization of MVP within the vault with all of the MVP N termini interacting non-covalently at the vault midsection and 48 copies of MVP forming each half vault. CryoEM difference mapping localized VPARP to three density bands lining the inner surface of the vault. Difference maps designed to localize TEP1 showed only weak density inside of the caps, suggesting that TEP1 may interact with MVP via a small interaction region. In the absence of atomic-resolution structures for either VPARP or TEP1, fold recognition methods were applied. A total of 21 repeats were predicted for the TEP1 WD-repeat domain, suggesting an unusually large beta-propeller fold.     

The vault exterior shell is a dynamic structure that allows incorporation of vault-associated proteins into its interior

Vaults are 13 million Da ribonucleoprotein particles with a highly conserved structure. Expression and assembly by multimerization of an estimated 96 copies of a single protein, termed the major vault protein (MVP), is sufficient to form the minimal structure and entire exterior shell of the barrel-shaped vault particle. Multiple copies of two additional proteins, VPARP and TEP1, and a small untranslated vault RNA are also associated with vaults. We used the Sf9 insect cell expression system to form MVP-only recombinant vaults and performed a series of protein-mixing experiments to test whether this particle shell is able to exclude exogenous proteins from interacting with the vault interior. Surprisingly, we found that VPARP and TEP1 are able to incorporate into vaults even after the formation of the MVP vault particle shell is complete. Electrospray molecular mobility analysis and spectroscopic studies of vault-interacting proteins were used to confirm this result. Our results demonstrate that the protein shell of the recombinant vault particle is a dynamic structure and suggest a possible mechanism for in vivo assembly of vault-interacting proteins into preformed vaults. Finally, this study suggests that the vault interior may functionally interact with the cellular milieu.     

Multiple human vault RNAs. Expression and association with the vault complex

Human vaults are intracellular ribonucleoprotein particles believed to be involved in multidrug resistance. The complex consists of a major vault protein (MVP), two minor vault proteins (VPARP and TEP1), and several small untranslated RNA molecules. Three human vault RNA genes (HVG1-3) have been described, and a fourth was found in a homology search (HVG4). In the literature only the association of hvg1 with vaults was shown in vivo. However, in a yeast three-hybrid screen the association of hvg1, hvg2, and hvg4 with TEP1 was demonstrated. In this study we investigated the expression and vault association of different vault RNAs in a variety of cell lines, including pairs of drug-sensitive and drug-resistant cells. HVG1-3 are expressed in all cell lines examined, however, none of the cell lines expressed HVG4. This probably is a consequence of the absence of essential external polymerase III promoter elements. The bulk of the vault RNA associated with vaults was hvg1. Interestingly, an increased amount of hvg3 was bound to vaults isolated from multidrug-resistant cell lines. Our findings suggest that vaults bind the RNA molecules with different affinities in different situations. The ratio in which the vault RNAs are associated with vaults might be of functional importance.     

Structural domains of vault proteins: a role for the coiled coil domain in vault assembly

Vaults consist of multiple copies of three proteins (MVP, VPARP, and TEP1) and several untranslated RNAs. The function of vaults is unknown but the typical and evolutionary conserved structure indicates a role in intracellular transport. Although all vault components have been identified and characterized, not much is known about vault protein assembly. In this study we identified and analyzed structural domains involved in vault assembly with emphasis on protein-protein interactions. Using a yeast two-hybrid system, we demonstrate within MVP an intramolecular binding site and show that MVP molecules interact with each other via their coiled coil domain. We show that purified MVP is able to bind calcium, most likely at calcium-binding EF-hands. No interactions could be detected between TEP1 and other vault proteins. However, the N-terminal half of MVP binds to a specific domain in the C-terminus of VPARP. Furthermore, VPARP contains amino acid stretches mediating intramolecular binding.     

Characterisation of major vault protein during the life cycle of the human parasite Schistosoma mansoni

Vaults are ribonucleoproteins (13 MDa) highly conserved among lower and higher eukaryotes. Their association produces a complex composed of three proteins named Major Vault Protein (MVP), vault (PolyADP-ribose) polymerase (VPARP) and Telomerase-associated protein (TEP1), plus a small untranslated RNA. The exact function of this complex is unknown, although the biological role of vaults has been associated with multidrug resistance phenotypes and signal transduction pathways. Genomic analysis showed that model organisms, such as Caenorhabditis elegans and Drosophila melanogaster, do not possess genes encoding vaults. However, we have found that vault-related genes are present in the Schistosoma mansoni genome. These observations raised questions on the involvement of vaults in mechanisms of adaptation of the parasite in its mammalian host. Therefore, molecular characterisation of the putative Major Vault Protein performed using bioinformatics tools showed that this vault component is highly conserved in S. mansoni. The MVP expression level was quantified by qRT-PCR using total RNA from susceptible (LE) and resistant (LE-PZQ) adult worm lineages, cercariae and mechanically transformed schistosomula (MTS) cultured for 3.5, 24, 48 and 72 h in vitro. Our results suggest a stage-specific expression in all developmental stages analysed. Western blotting has shown up-regulation of SmMVP in the MTS-3.5, 72 h and resistant adult worms, and similar levels in all other stages. Furthermore, SmMVP was found differentially expressed in adult males and females from the susceptible lineage. Further studies should clarify whether SmMVP is somehow linked to drug resistance in S. mansoni.     

RNA location and modeling of a WD40 repeat domain within the vault

The vault complex is a ubiquitous 13-MDa ribonucleoprotein assembly, composed of three proteins (TEP1, 240 kDa; VPARP, 193 kDa; and MVP, 100 kDa) that are highly conserved in eukaryotes and an untranslated RNA (vRNA). The vault has been shown to affect multidrug resistance in cancer cells, and one particular component, MVP, is thought to play a role in the transport of drug from the nucleus. To locate the position of the vRNA, vaults were treated with RNases, and cryo-electron microscopy (cryo-EM) was performed on the resulting complexes. Using single-particle reconstruction techniques, 3,476 particle images were combined to generate a 22-A-resolution structure. Difference mapping between the RNase-treated vault and the previously calculated intact vault reconstructions reveals the vRNA to be at the ends of the vault caps. In this position, the vRNA may interact with both the interior and exterior environments of the vault. The finding of a 16-fold density ring at the top of the cap has allowed modeling of the WD40 repeat domain of the vault TEP1 protein within the cryo-EM vault density. Both stoichiometric considerations and the finding of higher resolution for the computationally selected and refined "barrel only" images indicate a possible symmetry mismatch between the barrel and the caps. The molecular architecture of the complex is emerging, with 96 copies of MVP composing the eightfold symmetric barrel, and the vRNA together with one copy of TEP1 and four predicted copies of VPARP comprising each cap.     

The La RNA-binding protein interacts with the vault RNA and is a vault-associated protein

Vaults are highly conserved ubiquitous ribonucleoprotein particles with an undefined function. Three protein species (p240/TEP1, p193/VPARP, and p100/MVP) and a small RNA comprise the 13-MDa vault particle. The expression of the unique 100-kDa major vault protein is sufficient to form the basic vault structure. Previously, we have shown that stable association of the vault RNA with the vault particle is dependent on its interaction with the p240/TEP1 protein. To identify other proteins that interact with the vault RNA, we used a UV-cross-linking assay. We find that a portion of the vault RNA is complexed with the La autoantigen in a separate smaller ribonucleoprotein particle. La interacts with the vault RNA (both in vivo and in vitro) presumably through binding to 3'-uridylates. Moreover, we also demonstrate that the La autoantigen is the 50-kDa protein that we have previously reported as a protein that co-purifies with vaults.     

Detection of the Mr 110,000 lung resistance-related protein LRP/MVP with monoclonal antibodies.

The Mr 110,000 lung resistance-related protein (LRP), also termed the major vault protein (MVP), constitutes >70% of subcellular ribonucleoprotein particles called vaults. Overexpression of LRP/MVP and vaults has been linked directly to MDR in cancer cells. Clinically, LRP/MVP expression can be of value to predict response to chemotherapy and prognosis. Monoclonal antibodies (MAbs) against LRP/MVP have played a critical role in determining the relevance of this protein in clinical drug resistance. We compared the applicability of the previously described MAbs LRP-56, LMR-5, LRP, 1027, 1032, and newly isolated MAbs MVP-9, MVP-16, MVP-18, and MVP-37 for the immunodetection of LRP/MVP by immunoblotting analysis and by immunocyto- and histochemistry. The availability of a broader panel of reagents for the specific and sensitive immunodetection of LRP/MVP should greatly facilitate biological and clinical studies of vault-related MDR.     

Major vault protein is a substrate of endogenous protein kinases in CHO and PC12 cells.

Major vault protein (MVP) is the predominant member of a large cytosolic ribonucleoprotein particle, termed vault. We have previously shown that MVP derived from electric ray electric organ becomes phosphorylated by protein kinase C in vitro and by tyrosine kinase in vivo. Here we show that MVP from two mammalian cell lines (CHO and PC12 cell) becomes highly phosphorylated by endogenous protein kinases in cell-free systems. The susceptibility to protein kinases differs substantially from those observed in MVP derived from electric organ. Phosphorylation of MVP depends on the presence of Mg2+ and can be inhibited by the chelating agent EDTA. Inhibitors of casein kinase II attenuate the phosphorylation of MVP. In contrast to CHO cells, addition of recombinant casein kinase II enhances the phosphorylation of MVP in PC12 cells. Endogenous kinase activity is of particulate nature and copurifies with vault particles. Immuno-affinity purified vaults containing recombinant tagged MVP expressed in CHO cells reveal no autophosphorylation, suggesting that protein kinase activity is not an intrinsic property of vaults. Our results suggest that cell-specific phosphorylation of MVP may play a critical role in vault function.     

Solution structure of a two-repeat fragment of major vault protein

Major vault protein (MVP) is the main constituent of vaults, large ribonucleoprotein particles implicated in resistance to cancer therapy and correlated with poor survival prognosis. Here, we report the structure of the main repeat element in human MVP. The approximately 55 amino acid residue MVP domain has a unique, novel fold that consists of a three-stranded antiparallel beta-sheet. The solution NMR structure of a two-domain fragment reveals the interdomain contacts and relative orientations of the two MVP domains. We use these results to model the assembly of 672 MVP domains from 96 MVP molecules into the ribs of the 13MDa vault structure. The unique features include a thin, skin-like structure with polar residues on both the cytoplasmic and internal surface, and a pole-to-pole arrangement of MVP molecules. These studies provide a starting point for understanding the self-assembly of MVP into vaults and their interactions with other proteins. Chemical shift perturbation studies identified the binding site of vault poly(ADP-ribose) polymerase, another component of vault particles, indicating that MVP domains form a new class of interaction-mediating modules.     

Major vault protein does not play a role in chemoresistance or drug localization in a non-small cell lung cancer cell line

The human major vault protein (MVP) is the primary component of the 13 MDa vault complex. MVP has been implicated in the development of non-P-glycoprotein-mediated drug resistance in cancer cells. Here we present several lines of evidence that dispute this assertion. siRNAs capable of specifically and efficiently knocking down expression of MVP do not alter the ability of resistant cells to remove doxorubicin from the nucleus and do not increase sensitivity to the drug. Conversely, upregulation of MVP in chemosensitive cells does not confer increased drug resistance. In multi-drug resistant (MDR) lung carcinoma cells, fluorescence microscopy reveals that doxorubicin enters the nucleus and is then removed, inconsistent with suggestions that vaults either act to prevent the drug from entering the nucleus or are involved as a nuclear efflux pump. These data suggest that vaults play no direct role in the MDR phenotype in non-small cell lung carcinoma cells and that their cellular function remains unknown. These results also have important implications concerning the value of MVP as a drug target and as a prognostic marker for chemotherapy failure. Our results suggest the need for further investigation into the link between upregulation of vaults and malignancy, the mechanism behind non-P-gp-mediated drug resistance, and the role of vaults in human cells.     

The genomic sequence of the murine major vault protein and its promoter

Vaults are ribonucleoproteins of unknown function, consisting of three different proteins and multiple copies of small untranslated RNA molecules. One of the protein subunits has been identified as TEP1, a protein that is also associated with the telomerase complex. Another protein appears to contain a functional PARP domain and is hence called VPARP. The third protein, major vault protein (MVP), is believed to make up 70% of the total mass of the vault complex and to be responsible for the typical barrel-shaped structure of vaults. We have isolated the murine MVP cDNA and compared the amino acid sequence with MVP from other species. Over 90% of sequence identity was found between mouse, human and rat, and a considerable degree of identity between mouse and MVPs from lower eukaryotes. We also found that the genomic structure of the murine MVP gene closely resembles the organization of the human MVP gene, both consisting of 15 exons of which most have exactly the same size. Finally we have isolated a genomic region upstream (and partially overlapping) the first untranslated exon, that displayed promoter activity in a luciferase reporter assay. Furthermore, we showed that the sequences from the first exon together with the 5'-end of the first intron enhance the promoter activity, implying the presence of essential promoter elements in this region. Alignment of the murine promoter region with the homologous sequences of the human gene revealed an identity of 58%. The apparent presence of conserved promoter elements suggests a similar regulation of human and murine MVP expression.     

Vault mobility depends in part on microtubules and vaults can be recruited to the nuclear envelope

Vaults are ribonucleoproteins that may function in intracellular transport processes. We investigated the intracellular distribution and dynamics of vaults in non-small cell lung cancer cells in which vaults are labeled with the green fluorescent protein. Immunofluorescence experiments showed that vaults are dispersed throughout the cytoplasm; a small fraction is found in close proximity to microtubules. Immunoprecipitation experiments corroborated these results showing co-precipitation of MVP and beta-tubulin. Using quantitative fluorescence-recovery after photobleaching (FRAP), we demonstrated that vault mobility over longer distances in part depends on intact microtubules; vaults moving slower when microtubules are depolymerized by nocodazole. Biochemical fractionation indicated a small fraction of MVP associated with the nucleus, however, no GFP-tagged vaults could be observed inside the nucleus. We observed an accumulation of vaults at the nuclear envelope upon treatment of cells with the protein synthesis inhibitor cycloheximide. Analysis of nucleo-cytoplasmic transport using a fluorescent substrate containing a classical NLS and NES expressed in MVP+/+ and MVP-/- mouse embryonic fibroblasts indicated no differences in nuclear import/export kinetics, suggesting no role for vaults in these processes. We hypothesize that a subset of vaults moves directionally via microtubules, possibly towards the nucleus.     

Crosstalk between Src and major vault protein in epidermal growth factor-dependent cell signalling

Vaults are highly conserved, ubiquitous ribonucleoprotein (RNP) particles with an unidentified function. For the three protein species (TEP1, VPARP, and MVP) and a small RNA that comprises vault, expression of the unique 100-kDa major vault protein (MVP) is sufficient to form the basic vault structure. To identify and characterize proteins that interact with the Src homology 2 (SH2) domain of Src and potentially regulate Src activity, we used a pull-down assay using GST-Src-SH2 fusion proteins. We found MVP as a Src-SH2 binding protein in human stomach tissue. Interaction of Src and MVP was also observed in 253J stomach cancer cells. A subcellular localization study using immunofluorescence microscopy shows that epidermal growth factor (EGF) stimulation triggers MVP translocation from the nucleus to the cytosol and perinuclear region where it colocalizes with Src. We found that the interaction between Src and MVP is critically dependent on Src activity and protein (MVP) tyrosyl phosphorylation, which are induced by EGF stimulation. Our results also indicate MVP to be a novel substrate of Src and phosphorylated in an EGF-dependent manner. Interestingly, purified MVP inhibited the in vitro tyrosine kinase activity of Src in a concentration-dependent manner. MVP overexpression downregulates EGF-dependent ERK activation in Src overexpressing cells. To our knowledge, this is the first report of MVP interacting with a protein tyrosine kinase involved in a distinct cell signalling pathway. It appears that MVP is a novel regulator of Src-mediated signalling cascades.     

Vault nanocapsule dissociation into halves triggered at low pH

Vaults are self-assembled ribonucleoprotein nanocapsules that consist of multiple copies of three proteins (major vault protein, VPARP, and TEP1) and an untranslated RNA. Although their function has not been determined, vaults are found in nearly all eukaryotic cells. This study describes the use of fluorescence spectroscopy, multiangle laser light scattering (MALLS), and the quartz crystal microbalance (QCM) as tools in investigating recombinant vault conformational change in response to a varied solution pH. Identification of conditions for reversible vault disassembly and reassembly could enable application of these nanocapsules in drug delivery and in nanomaterials synthesis. Initial monitoring of changes in the intrinsic fluorescence intensity of vaults showed a 60% increase at pH 3.4 compared to that at pH 6.5, suggesting vaults exhibit a more open conformation at low pH. Fluorescence quenching studies provided further evidence of a vault structural change at low pH. MALLS data suggested a decrease in molecular mass accompanied by a clear increase in the radius of gyration as the solution pH was shifted from 6.5 to 3.4. This result prompted the hypothesis that vaults dissociate at least partially at low pH. Using the QCM to study adsorption of the vault onto self-assembled monolayers, data that suggest vault dissociation at low pH, even when the vault is in an adsorbed state, were also obtained. Finally, transmission electron microscopy (TEM) of negatively stained vaults at pH 6.5 and 3.4 confirmed the fluorescence spectroscopy, MALLS, and QCM findings by providing visual evidence that vaults disassemble into halves as the solution pH is lowered from 6.5 to 3.4.     

On the role of major vault protein in the resistance of senescent human diploid fibroblasts to apoptosis

Major vault protein (MVP), the main component of vault complex, is overexpressed in many multidrug-resistant cancer cell lines, suggesting a possible role for MVP in cell signaling and survival. In this study, we have found that MVP is markedly increased in senescent human diploid fibroblasts (HDFs) as well as in aged organs. We examined whether MVP expression might be affected by apoptotic stress in an aging-dependent manner. We treated young and senescent HDFs with apoptosis-inducing agents such as H(2)O(2), staurosporine and thapsigargin, and monitored MVP expression. We found that MVP expression is markedly reduced in young HDFs but not in senescent HDFs, in response to apoptotic stresses. Downregulation of MVP increased the sensitivity of senescent HDFs to apoptosis. Also, the level of antiapoptotic B-cell lymphoma protein-2 (Bcl-2) was significantly reduced and the accumulation of c-Jun increased in MVP knocked-down senescent HDFs. Moreover, treatment of MVP knocked-down senescent HDFs with SP600125, a specific c-Jun NH(2)-terminal kinase (JNK) inhibitor, restored the level of Bcl-2 protein. Taken together, these results suggest that MVP is important in the resistance of senescent HDFs to apoptosis by modulation of Bcl-2 expression by JNK pathway.     

Assembly of vault-like particles in insect cells expressing only the major vault protein

Vaults are the largest (13 megadalton) cytoplasmic ribonucleoprotein particles known to exist in eukaryotic cells. They have a unique barrel-shaped structure with 8-fold symmetry. Although the precise function of vaults is unknown, their wide distribution and highly conserved morphology in eukaryotes suggests that their function is essential and that their structure must be important for their function. The 100-kDa major vault protein (MVP) constitutes approximately 75% of the particle mass and is predicted to form the central barrel portion of the vault. To gain insight into the mechanisms for vault assembly, we have expressed rat MVP in the Sf9 insect cell line using a baculovirus vector. Our results show that the expression of the rat MVP alone can direct the formation of particles that have biochemical characteristics similar to endogenous rat vaults and display the distinct vault-like morphology when negatively stained and examined by electron microscopy. These particles are the first example of a single protein polymerizing into a non-spherically, non-cylindrically symmetrical structure. Understanding vault assembly will enable us to design agents that disrupt vault formation and hence aid in elucidating vault function in vivo.