Virus-like particles (VLPs) are molecules that closely resemble viruses , but are non-infectious because they contain no viral genetic material. They can be naturally occurring or synthesized through the individual expression of viral structural proteins, which can then self assemble into the virus-like structure. Combinations of structural capsid proteins from different viruses can be used to create recombinant VLPs. Both in-vivo assembly (i.e., assembly inside E. coli bacteria via recombinant co-expression of multiple proteins) and in-vitro assembly (i.e., protein self-assembly in a reaction vessel using stoichiometric quantities of previously purified proteins) have been successfully shown to form virus-like particles. VLPs derived from the Hepatitis B virus (HBV) and composed of the small HBV derived surface antigen ( HBsAg ) were described in 1968 from patient sera. VLPs have been produced from components of a wide variety of virus families including Parvoviridae (e.g. adeno-associated virus ), Retroviridae (e.g. HIV ), Flaviviridae (e.g. Hepatitis C virus ), Paramyxoviridae (e.g. Nipah ) and bacteriophages (e.g. Qβ, AP205). VLPs can be produced in multiple cell culture systems including bacteria, mammalian cell lines, insect cell lines, yeast and plant cells.
35-426: VLPs can also refer to structures produced by some LTR retrotransposons (under Ortervirales ) in nature. These are defective, immature virions , sometimes containing genetic material, that are generally non-infective due to the lack of a functional viral envelope . In addition, wasps produce polydnavirus vectors with pathogenic genes (but not core viral genes) or gene-less VLPs to help control their host. VLPs are
70-685: A candidate delivery system for genes or other therapeutics. These drug delivery agents have been shown to effectively target cancer cells in vitro . It is hypothesized that VLPs may accumulate in tumor sites due to the enhanced permeability and retention effect , which could be useful for drug delivery or tumor imaging. VLPs are useful as vaccines . VLPs contain repetitive, high density displays of viral surface proteins that present conformational viral epitopes that can elicit strong T cell and B cell immune responses . The particles' small radius of roughly 20-200 nm allows sufficient draining into lymph nodes . Since VLPs cannot replicate, they provide
105-641: A few hundred base pairs to 30 kb, the largest species reported to date are members of the Burro retrotransposon family in Schmidtea mediterranea . In plant genomes, LTR retrotransposons are the major repetitive sequence class constituting more than 75% of the maize genome. LTR retrotransposons make up about 8% of the human genome and approximately 10% of the mouse genome. LTR retrotransposons have direct long terminal repeats that range from ~100 bp to over 5 kb in size. LTR retrotransposons are further sub-classified into
140-634: A membrane protein. Such proteins can only be separated from the membranes by using detergents , nonpolar solvents , or sometimes denaturing agents. Proteins that adhere only temporarily to cellular membranes are known as peripheral membrane proteins . These proteins can either associate with integral membrane proteins, or independently insert in the lipid bilayer in several ways. Three-dimensional structures of ~160 different integral membrane proteins have been determined at atomic resolution by X-ray crystallography or nuclear magnetic resonance spectroscopy . They are challenging subjects for study owing to
175-487: A minimum of two genes, gag and pol , that are sufficient for their replication. Gag encodes a polyprotein with a capsid and a nucleocapsid domain. Gag proteins form virus-like particles in the cytoplasm inside which reverse-transcription occurs. The Pol gene produces three proteins: a protease (PR), a reverse transcriptase endowed with an RT (reverse-transcriptase) and an RNAse H domains, and an integrase (IN). Typically, LTR-retrotransposon mRNAs are produced by
210-421: A new location, creating short TSDs (Target Site Duplications) and adding a new copy in the host genome Ty1- copia retrotransposons are abundant in species ranging from single-cell algae to bryophytes , gymnosperms , and angiosperms . They encode four protein domains in the following order: protease , integrase , reverse transcriptase , and ribonuclease H . At least two classification systems exist for
245-435: A safer alternative to attenuated viruses . VLPs were used to develop FDA-approved vaccines for Hepatitis B and human papillomavirus , which are commercially available. A selection of viruslike particle-based vaccines against human papilloma virus (HPV) such as Cervarix by GlaxoSmithKline along with Gardasil and Gardasil-9, are available, produced by Merck & Co . Gardasil consists of recombinant VLPs assembled from
280-730: A separate research, 3.5 nm small Cytochrome C with peroxidase-like activity was encapsulated inside a 9 nm small Dps protein cage to form organelle-inspired protein cage structure. The VLP lipoparticle was developed to aid the study of integral membrane proteins . Lipoparticles are stable, highly purified, homogeneous VLPs that are engineered to contain high concentrations of a conformationally intact membrane protein of interest. Integral Membrane proteins are involved in diverse biological functions and are targeted by nearly 50% of existing therapeutic drugs. However, because of their hydrophobic domains, membrane proteins are difficult to manipulate outside of living cells. Lipoparticles can incorporate
315-501: A specific cell type or for raising an immune response is useful. In some cases a protein of interest can be genetically fused to the viral coat protein. However, this approach sometimes leads to impaired VLP assembly and has limited utility if the targeting agent is not protein-based. An alternative is to assemble the VLP and then use chemical crosslinkers, reactive unnatural amino acids or SpyTag/SpyCatcher reaction in order to covalently attach
350-427: A wide variety of structurally intact membrane proteins, including G protein-coupled receptors (GPCR)s, ion channels and viral Envelopes. Lipoparticles provide a platform for numerous applications including antibody screening, production of immunogens and ligand binding assays. The understanding of self-assembly of VLPs was once based on viral assembly. This is rational as long as the VLP assembly takes place inside
385-412: Is a type of membrane protein that is permanently attached to the biological membrane . All transmembrane proteins can be classified as IMPs, but not all IMPs are transmembrane proteins. IMPs comprise a significant fraction of the proteins encoded in an organism's genome . Proteins that cross the membrane are surrounded by annular lipids , which are defined as lipids that are in direct contact with
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#1733202661526420-441: Is one of eleven Sushi-like neogenes identified in the human genome. The BEL/pao family is found in animals. Although retroviruses are often classified separately, they share many features with LTR retrotransposons. A major difference with Ty1- copia and Ty3- gypsy retrotransposons is that retroviruses have an envelope protein (ENV). A retrovirus can be transformed into an LTR retrotransposon through inactivation or deletion of
455-467: Is produced to form virus-like particles. Reverse transcription usually initiates at a short sequence located immediately downstream of the 5’-LTR and termed the primer binding site (PBS). Specific host tRNAs bind to the PBS and act as primers for reverse-transcription, which occurs in a complex and multi-step process, ultimately producing a double- stranded cDNA molecule. The cDNA is finally integrated into
490-424: Is the transmembrane protein , which spans the entire biological membrane . Single-pass membrane proteins cross the membrane only once, while multi-pass membrane proteins weave in and out, crossing the membrane several times. Single pass membrane proteins can be categorized as Type I, which are positioned such that their carboxyl-terminus is towards the cytosol , or Type II, which have their amino-terminus towards
525-1162: Is thought that the chromodomain directs retrotransposon integration to specific target sites. According to sequence and structure of the chromodomain, chromoviruses are subdivided into the four clades CRM, Tekay, Reina and Galadriel. Chromoviruses from each clade show distinctive integration patterns, e.g. into centromeres or into the rRNA genes. Ogre-elements are gigantic Ty3-retrotransposons reaching lengths up to 25 kb. Ogre elements have been first described in Pisum sativum . Metaviruses describe conventional Ty3- gypsy retrotransposons that do not contain additional domains or ORFs. The Sushi family of Ty3 long terminal repeat retrotransposons were first identified in teleost fish and Sushi-like neogenes were subsequently identified in mammals. Mammalian retrotransposon-derived transcripts (MARTs) cannot transpose but have retained open reading frames, demonstrate high levels of evolutionary conservation and are subject to selective pressures, which suggests some have become neofunctionalized genes with new cellular functions. Retrotransposon gag-like-3 (RTL3/ZCCHC5/MART3)
560-854: The Influenza A virus subtype H5N1 and the 1918 flu pandemic . Novavax and Medicago Inc. have run clinical trials of their VLP flu vaccines. Several VLP vaccines for COVID-19 , including Novavax , are under development. VLPs have been used to develop a pre-clinical vaccine candidate against chikungunya virus. Compartmentalization is a common theme in biology. Nature is full of examples of hierarchically compartmentalized multicomponent structures that self-assembles from individual building blocks. Taking inspiration from nature, synthetic approaches using polymers, phase-separated microdroplets, lipids and proteins have been used to mimic hierarchical compartmentalization of natural systems and to form functional bio-inspired nanomaterials. For example, protein self-assembly
595-662: The "target" integral protein from its amino acid sequence and an experimental three-dimensional structure of a related homologous protein. This procedure has been extensively used for ligand - G protein–coupled receptors (GPCR) and their complexes. IMPs include transporters , linkers, channels , receptors , enzymes , structural membrane-anchoring domains, proteins involved in accumulation and transduction of energy , and proteins responsible for cell adhesion . Classification of transporters can be found in Transporter Classification Database . As an example of
630-656: The L1 proteins of HPV types 6, 11, 16, and 18 expressed in yeast . It is adjuvanted with aluminum hydroxyphosphate sulfate. Gardasil-9 consists of L1 epitopes of 31, 33, 45, 52 and 58 in addition to the listed L1 epitopes found in Gardasil. Cervarix consists of recombinant VLPs assembled from the L1 proteins of HPV types 16 and 18, expressed in insect cells, and is adjuvanted with 3-O-Desacyl-4-monophosphoryl lipid (MPL) A and aluminum hydroxide. The first VLP vaccine that addresses malaria, Mosquirix, ( RTS,S ) has been approved by EU regulators. It
665-576: The MASIVEdb Sirevirus database. Ty3-retrotransposons are widely distributed in the plant kingdom, including both gymnosperms and angiosperms . They encode at least four protein domains in the order: protease , reverse transcriptase , ribonuclease H , and integrase . Based on structure, presence/absence of specific protein domains, and conserved protein sequence motifs, they can be subdivided into several lineages: Errantiviruses contain an additional defective envelope ORF with similarities to
700-637: The Ty1- copia -like ( Pseudoviridae ), Ty3-like ( Metaviridae , formally referred to as Gypsy-like, a name that is being considered for retirement ), and BEL-Pao-like ( Belpaoviridae ) groups based on both their degree of sequence similarity and the order of encoded gene products. Ty1- copia and Ty3-Metaviridae groups of retrotransposons are commonly found in high copy number (up to a few million copies per haploid nucleus ) in animals, fungi, protista, and plants genomes. BEL-Pao like elements have so far only been found in animals. All functional LTR-retrotransposons encode
735-661: The coding domains of autonomous Ty1- copia or Ty3- gypsy retrotransposons. Among the TRIMs, the Cassandra family plays an exceptional role, as the family is unusually wide-spread among higher plants. In contrast to all other characterized TRIMs, Cassandra elements harbor a 5S rRNA promoter in their LTR sequence. Due to their short overall length and the relatively high contribution of the flanking LTRs, TRIMs are prone to re-arrangements by recombination. Integral membrane proteins An integral , or intrinsic , membrane protein ( IMP )
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#1733202661526770-413: The cytosol. Type III proteins have multiple transmembrane domains in a single polypeptide, while type IV consists of several different polypeptides assembled together in a channel through the membrane. Type V proteins are anchored to the lipid bilayer through covalently linked lipids. Finally Type VI proteins have both transmembrane domains and lipid anchors. Integral monotopic proteins are associated with
805-554: The difficulties associated with extraction and crystallization . In addition, structures of many water - soluble protein domains of IMPs are available in the Protein Data Bank . Their membrane-anchoring α-helices have been removed to facilitate the extraction and crystallization . Search integral membrane proteins in the PDB (based on gene ontology classification) IMPs can be divided into two groups: The most common type of IMP
840-458: The domains that enable extracellular mobility. If such a retrovirus infects and subsequently inserts itself in the genome in germ line cells, it may become transmitted vertically and become an Endogenous Retrovirus . Some LTR retrotransposons lack all of their coding domains. Due to their short size, they are referred to as terminal repeat retrotransposons in miniature (TRIMs). Nevertheless, TRIMs can be able to retrotranspose, as they may rely on
875-754: The function or structure of the proteins. Several successful methods are available for performing the extraction including the uses of "detergents, low ionic salt (salting out), shearing force, and rapid pressure change". The Protein Structure Initiative (PSI), funded by the U.S. National Institute of General Medical Sciences (NIGMS), part of the National Institutes of Health (NIH), has among its aim to determine three-dimensional protein structures and to develop techniques for use in structural biology , including for membrane proteins. Homology modeling can be used to construct an atomic-resolution model of
910-469: The host RNA pol II acting on a promoter located in their 5’ LTR. The Gag and Pol genes are encoded in the same mRNA. Depending on the host species, two different strategies can be used to express the two polyproteins: a fusion into a single open reading frame (ORF) that is then cleaved or the introduction of a frameshift between the two ORFs. Occasional ribosomal frameshifting allows the production of both proteins, while ensuring that much more Gag protein
945-424: The host cell ( in vivo ), though the self-assembly event was found in vitro from the very beginning of the study about viral assembly. Study also reveals that in vitro assembly of VLPs competes with aggregation and certain mechanisms exist inside the cell to prevent the formation of aggregates while assembly is ongoing. Attaching proteins, nucleic acids, or small molecules to the VLP surface, such as for targeting
980-412: The membrane from one side but do not span the lipid bilayer completely. Many challenges facing the study of integral membrane proteins are attributed to the extraction of those proteins from the phospholipid bilayer . Since integral proteins span the width of the phospholipid bilayer, their extraction involves disrupting the phospholipids surrounding them, without causing any damage that would interrupt
1015-551: The molecule of interest. This method is effective at directing the immune response against the attached molecule, thereby inducing high levels of neutralizing antibody and even being able to break tolerance to self-proteins displayed on VLPs. LTR retrotransposon LTR retrotransposons are class I transposable elements (TEs) characterized by the presence of long terminal repeats (LTRs) directly flanking an internal coding region. As retrotransposons, they mobilize through reverse transcription of their mRNA and integration of
1050-475: The newly created cDNA into another genomic location. Their mechanism of retrotransposition is shared with retroviruses , with the difference that the rate of horizontal transfer in LTR-retrotransposons is much lower than the vertical transfer by passing active TE insertions to the progeny. LTR retrotransposons that form virus-like particles are classified under Ortervirales . Their size ranges from
1085-487: The relationship between the IMP (in this case the bacterial phototrapping pigment, bacteriorhodopsin) and the membrane formed by the phospholipid bilayer is illustrated below. In this case the integral membrane protein spans the phospholipid bilayer seven times. The part of the protein that is embedded in the hydrophobic regions of the bilayer are alpha helical and composed of predominantly hydrophobic amino acids. The C terminal end of
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1120-566: The retroviral envelope gene. First described as Athila-elements in Arabidopsis thaliana , they have been later identified in many species, such as Glycine max and Beta vulgaris . Chromoviruses contain an additional chromodomain ( chr omatin o rganization mo difier domain) at the C-terminus of their integrase protein. They are widespread in plants and fungi, probably retaining protein domains during evolution of these two kingdoms. It
1155-662: The subdivision of Ty1- copia retrotransposons into five lineages: Sireviruses /Maximus, Oryco/Ivana, Retrofit/Ale, TORK (subdivided in Angela/Sto, TAR/Fourf, GMR/Tork), and Bianca. Sireviruses /Maximus retrotransposons contain an additional putative envelope gene. This lineage is named for the founder element SIRE1 in the Glycine max genome, and was later described in many species such as Zea mays , Arabidopsis thaliana , Beta vulgaris , and Pinus pinaster . Plant Sireviruses of many sequenced plant genomes are summarized at
1190-629: Was expressed in yeast. RTS,S is a portion of the Plasmodium falciparum circumsporozoite protein fused to the Hepatitis B surface antigen (RTS), combined with Hepatitis B surface antigen (S), and adjuvanted with AS01 (consisting of (MPL)A and saponin ). Vaccine production can begin as soon as the virus strain is sequenced and can take as little as 12 weeks, compared to 9 months for traditional vaccines. In early clinical trials, VLP vaccines for influenza appeared to provide complete protection against both
1225-589: Was used to encapsulate multiple copies of ferritin protein cages as sub-compartments inside P22 virus-like particle as larger compartment essentially forming a Matryoshka-like nested cage-within-cage structure. The authors further demonstrated stoichiometric encapsulation of cellobiose-hydrolysing β-glycosidase enzyme CelB along with ferritin protein cages using in-vitro self-assembly strategy to form multi-compartment cell-inspired protein cage structure. Using similar strategy, glutathione biosynthesizing enzymes were encapsulated inside bacteriophage P22 virus-like particles. In
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