External scaffold of spherical immature poxvirus particles is made of protein trimers, forming a honeycomb lattice

During morphogenesis, poxviruses undergo a remarkable transition from spherical immature forms to brick-shaped infectious particles lacking helical or icosahedral symmetry. In this study, we show that the transitory honeycomb lattice coating the lipoprotein membrane of immature vaccinia virus particles is formed from trimers of a 62-kD protein encoded by the viral D13L gene. Deep-etch electron microscopy demonstrated that anti-D13 antibodies bound to the external protein coat and that lattice fragments were in affinity-purified D13 preparations. Soluble D13 appeared mostly trimeric by gel electrophoresis and ultracentrifugation, which is consistent with structural requirements for a honeycomb. In the presence or absence of other virion proteins, a mutated D13 with one amino acid substitution formed stacks of membrane-unassociated flat sheets that closely resembled the curved honeycombs of immature virions except for the absence of pentagonal facets. A homologous domain that is present in D13 and capsid proteins of certain other lipid-containing viruses support the idea that the developmental stages of poxviruses reflect their evolution from an icosahedral ancestor.     

Insights into the evolution of a complex virus from the crystal structure of vaccinia virus D13

The morphogenesis of poxviruses such as vaccinia virus (VACV) sees the virion shape mature from spherical to brick-shaped. Trimeric capsomers of the VACV D13 protein form a transitory, stabilizing lattice on the surface of the initial spherical immature virus particle. The crystal structure of D13 reveals that this major scaffolding protein comprises?a double β barrel "jelly-roll" subunit arranged as pseudo-hexagonal trimers. These structural features are characteristic of the major capsid proteins of?a lineage of large icosahedral double-stranded DNA viruses including human adenovirus and the bacteriophages PRD1 and PM2. Structure-based phylogenetic analysis confirms that VACV belongs to this lineage, suggesting that (analogously to higher organism embryogenesis) early poxvirus morphogenesis reflects their evolution from a lineage of viruses sharing a common icosahedral ancestor.     

Assembly and Disassembly of the Capsid-Like External Scaffold of Immature Virions during Vaccinia Virus Morphogenesis

Infectious poxvirus particles are unusual in that they are brick shaped and lack symmetry. Nevertheless, an external honeycomb lattice comprised of a capsid-like protein dictates the spherical shape and size of immature poxvirus particles. In the case of vaccinia virus, trimers of 63-kDa D13 polypeptides form the building blocks of the lattice. In the present study, we addressed two questions: how D13, which has no transmembrane domain, associates with the immature virion (IV) membrane to form the lattice structure and how this scaffold is removed during the subsequent stage of morphogenesis. Interaction of D13 with the A17 membrane protein was demonstrated by immunoaffinity purification and Western blot analysis. In addition, the results of immunogold electron microscopy indicated a close association of A17 and D13 in crescents, as well as in vesicular structures when crescent formation was prevented. Further studies indicated that binding of A17 to D13 was abrogated by truncation of the N-terminal segment of A17. The N-terminal region of A17 was also required for the formation of crescent and IV structures. Disassembly of the D13 scaffold correlated with the processing of A17 by the I7 protease. When I7 expression was repressed, D13 was retained on aberrant virus particles. Furthermore, the morphogenesis of IVs to mature virions was blocked by mutation of the N-terminal but not the C-terminal cleavage site on A17. Taken together, these data indicate that A17 and D13 interactions regulate the assembly and disassembly of the IV scaffold.The assembly and morphogenesis of vaccinia virus (VACV) and other poxviruses occurs in specialized regions of the cytoplasm called factories. The first distinctive viral forms discerned by transmission electron microscopy are spherical immature virions (IVs) and their membrane crescent precursors, which appear to be covered by a layer of spicules (14). More-recent studies employing three-dimensional deep-etch electron microscopy revealed that the “spicule coat” of IVs is actually a continuous honeycomb lattice (20). The IVs enclose dense granular material comprising the core precursors and a DNA nucleoid. The “spicule coat” is lost as the IVs undergo a remarkable transition into dense, brick-shaped infectious mature virions (MVs).Several studies led to the identification of D13 protein trimers as the building blocks of the scaffold: (i) single amino acid changes in D13 are responsible for VACV mutants that are resistant to the drug rifampin (rifampicin) (4, 11, 42), which causes reversible formation of irregular membranes lacking the “spicule coat” (18, 29, 30); (ii) repression of D13 expression results in a phenotype identical to that caused by the drug rifampin (50); (iii) antibody to D13 labels IVs (40) on the outer surface (28, 41); (iv) in the presence of rifampin, D13 antibodies label cytoplasmic inclusions that are distinct from aberrant viral membranes (40); and (v) the results of physical and microscopic studies indicate that D13 exists as trimers of 63-kDa subunits arranged mostly in hexagons on the surface of IVs (41).Poxviruses are thought to share a common origin with members of the asfarvirus, iridovirus, phycodnavirus, and mimivirus families (23). These large DNA viruses, except for the poxviruses, have an icosahedral capsid surrounding an internal membrane (31, 47-49). Interestingly, a domain of VACV D13 has homology with the capsid proteins of these related large DNA viruses (24). Moreover, a parapoxvirus ortholog of D13 was shown to self-assemble in vitro and to have structural similarities with the capsid proteins (22). These findings, together with the honeycomb lattice structure of the IV scaffold, suggest that the infectious form of the ancestor of poxviruses may have had an icosahedral capsid and that the stages of morphogenesis recapitulate evolution (41).In the present study, we addressed two questions: how D13, which has no transmembrane domain, associates with the IV membrane to form the lattice structure and how the scaffold is removed during morphogenesis.     

Assembly of vaccinia virus: effects of rifampin on the intracellular distribution of viral protein p65.

The cytoplasmic assembly of vaccinia virus is reversibly blocked by the antibiotic rifampin, leading to the accumulation of partially membrane-delineated rifampin bodies in infected cells. Rifampin-resistant vaccinia virus mutants have point mutations in the D13L gene, which is controlled by a late promoter and expresses a 65-kDa protein, designated p65. To further characterize the mechanism of rifampin inhibition and the function of p65 in virus assembly, we raised antibodies to this protein. Immunoreactive p65 was expressed at late times of infection, and neither its expression nor its turnover was affected by rifampin. Virus-associated p65 could be extracted only with denaturing detergents from purified virions, suggesting that it is an integral viral component. Immunofluorescence studies showed that p65 is localized to the sites of virus assembly. Also, immunoelectron microscopy showed p65 to be associated with viral crescents as well as spherical, immature virions, in both cases predominantly on the inner or concave surface. In the presence of rifampin, p65 was found in large, cytoplasmic inclusion bodies that were distinct from rifampin bodies. The rifampin bodies themselves were labeled with p65 antibodies only after reversal of the rifampin block, predominantly on the viral crescents which rapidly formed following removal of the drug. We propose that p65 functions as an internal scaffold in the formation of viral crescents and immature virions, analogously to the matrix proteins of other viruses.     

The structure of a putative scaffolding protein of immature poxvirus particles as determined by electron microscopy suggests similarity with capsid proteins of large icosahedral DNA viruses

Orf virus, the prototype parapoxvirus, is responsible for contagious ecthyma in sheep and goats. The central region of the viral genome codes for proteins highly conserved among vertebrate poxviruses and which are frequently essential for viral proliferation. Analysis of the recently published genome sequence of orf virus revealed that among such essential proteins, the protein orfv075 is an orthologue of D13, the rifampin resistance gene product critical for vaccinia virus morphogenesis. Previous studies showed that D13, arranged as "spicules," is necessary for the formation of vaccinia virus immature virions, a mandatory intermediate in viral maturation. We have determined the three-dimensional structure of recombinant orfv075 at approximately 25-A resolution by electron microscopy of two-dimensional crystals. orfv075 organizes as trimers with a tripod-like main body and a propeller-like smaller domain. The molecular envelope of orfv075 shows unexpectedly good agreement to that of a distant homologue, VP54, the major capsid protein of Paramecium bursaria Chlorella virus type 1. Our structural analysis suggests that orfv075 belongs in the double-barreled capsid protein family found in many double-stranded DNA icosahedral viruses and supports the hypothesis that the nonicosahedral poxviruses and the large icosahedral DNA viruses are evolutionarily related.     

Vaccinia virus nonstructural protein encoded by the A11R gene is required for formation of the virion membrane

The vaccinia virus A11R gene has orthologs in all known poxvirus genomes, and the A11 protein has been previously reported to interact with the putative DNA packaging protein A32 in a yeast two-hybrid screen. Using antisera raised against A11 peptides, we show that the A11 protein was (i) expressed at late times with an apparent mass of 40 kDa, (ii) not incorporated into virus particles, (iii) phosphorylated independently of the viral F10 kinase, (iv) coimmunoprecipitated with A32, and (v) localized to the viral factory. To determine the role of the A11 protein and test whether it is indeed involved in DNA packaging, we constructed a recombinant vaccinia virus with an inducible A11R gene. This recombinant was dependent on inducer for single-cycle growth and plaque formation. In the absence of inducer, viral late proteins were produced at normal levels, but proteolytic processing and other posttranslational modifications of some proteins were inhibited, suggesting a block in virus particle assembly. Consistent with this observation, electron microscopy of cells infected in the absence of inducer showed virus factories with abnormal electron-dense viroplasms and intermediate density regions associated with membranes and containing the D13 protein. However, no viral membrane crescents, immature virions, or mature virions were produced. The requirement for nonvirion protein A11 in order to make normal viral membranes was an unexpected and exciting finding, since neither the origin of these membranes nor their mechanism of formation in the cytoplasm of infected cells is understood.     

Cryo electron tomography of native HIV-1 budding sites

The structure of immature and mature HIV-1 particles has been analyzed in detail by cryo electron microscopy, while no such studies have been reported for cellular HIV-1 budding sites. Here, we established a system for studying HIV-1 virus-like particle assembly and release by cryo electron tomography of intact human cells. The lattice of the structural Gag protein in budding sites was indistinguishable from that of the released immature virion, suggesting that its organization is determined at the assembly site without major subsequent rearrangements. Besides the immature lattice, a previously not described Gag lattice was detected in some budding sites and released particles; this lattice was found at high frequencies in a subset of infected T-cells. It displays the same hexagonal symmetry and spacing in the MA-CA layer as the immature lattice, but lacks density corresponding to NC-RNA-p6. Buds and released particles carrying this lattice consistently lacked the viral ribonucleoprotein complex, suggesting that they correspond to aberrant products due to premature proteolytic activation. We hypothesize that cellular and/or viral factors normally control the onset of proteolytic maturation during assembly and release, and that this control has been lost in a subset of infected T-cells leading to formation of aberrant particles.     

Deep-etch EM reveals that the early poxvirus envelope is a single membrane bilayer stabilized by a geodetic "honeycomb" surface coat

Three-dimensional "deep-etch" electron microscopy (DEEM) resolves a longstanding controversy concerning poxvirus morphogenesis. By avoiding fixative-induced membrane distortions that confounded earlier studies, DEEM shows that the primary poxvirus envelope is a single membrane bilayer coated on its external surface by a continuous honeycomb lattice. Freeze fracture of quick-frozen poxvirus-infected cells further shows that there is only one fracture plane through this primary envelope, confirming that it consists of a single lipid bilayer. DEEM also illustrates that the honeycomb coating on this envelope is completely replaced by a different paracrystalline coat as the poxvirus matures. Correlative thin section images of infected cells freeze substituted after quick-freezing, plus DEEM imaging of Tokuyasu-type cryo-thin sections of infected cells (a new application introduced here) all indicate that the honeycomb network on immature poxvirus virions is sufficiently continuous and organized, and tightly associated with the envelope throughout development, to explain how its single lipid bilayer could remain stable in the cytoplasm even before it closes into a complete sphere.     

HIV-1 maturation inhibitor bevirimat stabilizes the immature Gag lattice

Maturation of nascent virions, a key step in retroviral replication, involves cleavage of the Gag polyprotein by the viral protease into its matrix (MA), capsid (CA), and nucleocapsid (NC) components and their subsequent reorganization. Bevirimat (BVM) defines a new class of antiviral drugs termed maturation inhibitors. BVM acts by blocking the final cleavage event in Gag processing, the separation of CA from its C-terminal spacer peptide 1 (SP1). Prior evidence suggests that BVM binds to Gag assembled in immature virions, preventing the protease from accessing the CA-SP1 cleavage site. To investigate this hypothesis, we used cryo-electron tomography to examine the structures of (noninfectious) HIV-1 viral particles isolated from BVM-treated cells. We find that these particles contain an incomplete shell of density underlying the viral envelope, with a hexagonal honeycomb structure similar to the Gag lattice of immature HIV but lacking the innermost, NC-related, layer. We conclude that the shell represents a remnant of the immature Gag lattice that has been processed, except at the CA-SP1 sites, but has remained largely intact. We also compared BVM-treated particles with virions formed by the mutant CA5, in which cleavage between CA and SP1 is also blocked. Here, we find a thinner CA-related shell with no visible evidence of honeycomb organization, indicative of an altered conformation and further suggesting that binding of BVM stabilizes the immature lattice. In both cases, the observed failure to assemble mature capsids correlates with the loss of infectivity.     

Vaccinia virus L2 protein associates with the endoplasmic reticulum near the growing edge of crescent precursors of immature virions and stabilizes a subset of viral membrane proteins

The initial step in poxvirus morphogenesis, the formation of crescent membranes, occurs within cytoplasmic factories. L2 is one of several vaccinia virus proteins known to be necessary for formation of crescents and the only one synthesized early in infection. Virus replication was unaffected when the L2R open reading frame was replaced by L2R containing an N-terminal epitope tag while retaining the original promoter. L2 colocalized with the endoplasmic reticulum (ER) protein calnexin throughout the cytoplasm of infected and transfected cells. Topological studies indicated that the N terminus of L2 is exposed to the cytoplasm with the hydrophobic C terminus anchored in the ER. Using immunogold labeling and electron microscopy, L2 was detected in tubular membranes outside factories and inside factories near crescents and close to the edge or rim of crescents; a similar labeling pattern was found for the ER luminal protein disulfide isomerase (PDI). The phenotype of L2 conditional lethal mutants and the localization of L2 suggest that it participates in elongation of crescents by the addition of ER membrane to the growing edge. Small amounts of L2 and PDI were detected within immature and mature virions, perhaps trapped during assembly. The repression of L2, as well as A11 and A17, two other proteins that are required for viral crescent formation, profoundly decreased the stability of a subset of viral membrane proteins including those comprising the entry-fusion complex. To avoid degradation, these unstable membrane proteins may need to directly insert into the viral membrane or be rapidly shunted there from the ER.     

Maturation of flaviviruses starts from one or more icosahedrally independent nucleation centres

Flaviviruses assemble as fusion-incompetent immature particles and subsequently undergo conformational change leading to release of infectious virions. Flavivirus infections also produce combined 'mosaic' particles. Here, using cryo-electron tomography, we report that mosaic particles of dengue virus type 2 had glycoproteins organized into two regions of mature and immature structure. Furthermore, particles of a maturation-deficient mutant had their glycoproteins organized into two regions of immature structure with mismatching icosahedral symmetries. It is therefore apparent that the maturation-related reorganization of the flavivirus glycoproteins is not synchronized across the whole virion, but is initiated from one or more nucleation centres. Similar deviation from icosahedral symmetry might be relevant to the asymmetrical mode of genome packaging and cell entry of other viruses.     

Crystal structure of the potato leafroll virus coat protein and implications for viral assembly

Luteoviruses, poleroviruses, and enamoviruses are insect-transmitted, agricultural pathogens that infect a wide array of plants, including staple food crops. Previous cryo-electron microscopy studies of virus-like particles show that luteovirid viral capsids are built from a structural coat protein that organizes with T = 3 icosahedral symmetry. Here, we present the crystal structure of a truncated version of the coat protein monomer from potato leafroll virus at 1.80-Å resolution. In the crystal lattice, monomers pack into flat sheets that preserve the two-fold and three-fold axes of icosahedral symmetry and show minimal structural deviations when compared to the full-length subunits of the assembled virus-like particle. These observations have important implications in viral assembly and maturation and suggest that the CP N-terminus and its interactions with RNA play an important role in generating capsid curvature.     

Native hepatitis B virions and capsids visualized by electron cryomicroscopy

Hepatitis B virus (HBV) infects more than 350 million people, of which one million will die every year. The infectious virion is an enveloped capsid containing the viral polymerase and double-stranded DNA genome. The structure of the capsid assembled in vitro from expressed core protein has been studied intensively. However, little is known about the structure and assembly of native capsids present in infected cells, and even less is known about the structure of mature virions. We used electron cryomicroscopy (cryo-EM) and image analysis to examine HBV virions (Dane particles) isolated from patient serum and capsids positive and negative for HBV DNA isolated from the livers of transgenic mice. Both types of capsids assembled as icosahedral particles indistinguishable from previous image reconstructions of capsids. Likewise, the virions contained capsids with either T = 3 or T = 4 icosahedral symmetry. Projections extending from the lipid envelope were attributed to surface glycoproteins. Their packing was unexpectedly nonicosahedral but conformed to an ordered lattice. These structural features distinguish HBV from other enveloped viruses.     

Insights into virus evolution and membrane biogenesis from the structure of the marine lipid-containing bacteriophage PM2

Recent, primarily structural observations indicate that related viruses, harboring no sequence similarity, infect hosts of different domains of life. One such clade of viruses, defined by common capsid architecture and coat protein fold, is the so-called PRD1-adenovirus lineage. Here we report the structure of the marine lipid-containing bacteriophage PM2 determined by crystallographic analyses of the entire approximately 45 MDa virion and of the outer coat proteins P1 and P2, revealing PM2 to be a primeval member of the PRD1-adenovirus lineage with an icosahedral shell and canonical double beta barrel major coat protein. The view of the lipid bilayer, richly decorated with membrane proteins, constitutes a rare visualization of an in vivo membrane. The viral membrane proteins P3 and P6 are organized into a lattice, suggesting a possible assembly pathway to produce the mature virus.     

Assembly of vaccinia virus: incorporation of p14 and p32 into the membrane of the intracellular mature virus.

The cytoplasmic assembly of vaccinia virus begins with the transformation of a two-membraned cisterna derived from the intermediate compartment between the endoplasmic reticulum and the Golgi complex. This cisterna develops into a viral crescent which eventually forms a spherical immature virus (IV) that matures into the intracellular mature virus (IMV). Using immunoelectron microscopy, we determined the subcellular localization of p32 and p14, two membrane-associated proteins of vaccinia virus. p32 was associated with vaccinia virus membranes at all stages of virion assembly, starting with the viral crescents, as well as with the membranes which accumulated during the inhibition of assembly by rifampin. There was also low but significant labelling of membranes of some cellular compartments, especially those in the vicinity of the Golgi complex. In contrast, anti-p14 labelled neither the crescents nor the IV but gave strong labelling of an intermediate form between IV and IMV and was then associated with all later viral forms. This protein was also not significantly detected on identifiable cellular membranes. Both p32 and p14 were abundantly expressed on the surface of intact IMV. Our data are consistent with a model whereby p32 would become inserted into cellular membranes before being incorporated into the crescents whereas p14 would be posttranslationally associated with the viral outer membrane at a specific later stage of the viral life cycle.     

Relationships between RNA topology and nucleocapsid structure in a model icosahedral virus

The process of genome packaging in most of viruses is poorly understood, notably the role of the genome itself in the nucleocapsid structure. For simple icosahedral single-stranded RNA viruses, the branched topology due to the RNA secondary structure is thought to lower the free energy required to complete a virion. We investigate the structure of nucleocapsids packaging RNA segments with various degrees of compactness by small-angle x-ray scattering and cryotransmission electron microscopy. The structural differences are mild even though compact RNA segments lead on average to better-ordered and more uniform particles across the sample. Numerical calculations confirm that the free energy is lowered for the RNA segments displaying the larger number of branch points. The effect is, however, opposite with synthetic polyelectrolytes, in which a star topology gives rise to more disorder in the capsids than a linear topology. If RNA compactness and size account in part for the proper assembly of the nucleocapsid and the genome selectivity, other factors most likely related to the host cell environment during viral assembly must come into play as well.     

Immature and mature human astrovirus: structure, conformational changes, and similarities to hepatitis e virus

Human astroviruses (HAstVs) are a major cause of gastroenteritis. HAstV assembles from the structural protein VP90 and undergoes a cascade of proteolytic cleavages. Cleavage to VP70 is required for release of immature particles from cells, and subsequent cleavage by trypsin confers infectivity. We used electron cryomicroscopy and icosahedral image analysis to determine the first experimentally derived, three-dimensional structures of an immature VP70 virion and a fully proteolyzed, infectious virion. Both particles display T = 3 icosahedral symmetry and nearly identical solid capsid shells with diameters of ~ 350 Å. Globular spikes emanate from the capsid surface, yielding an overall diameter of ~ 440 Å. While the immature particles display 90 dimeric spikes, the mature capsid only displays 30 spikes, located on the icosahedral 2-fold axes. Loss of the 60 peripentonal spikes likely plays an important role in viral infectivity. In addition, immature HAstV bears a striking resemblance to the structure of hepatitis E virus (HEV)-like particles, as previously predicted from structural similarity of the crystal structure of the astrovirus spike domain with the HEV P-domain [Dong, J., Dong, L., Méndez, E. &; Tao, Y. (2011). Crystal structure of the human astrovirus capsid spike. Proc. Natl. Acad. Sci. USA 108, 12681–12686]. Similarities between their capsid shells and dimeric spikes and between the sequences of their capsid proteins suggest that these viral families are phylogenetically related and may share common assembly and activation mechanisms.     

A Structural Model of the Genome Packaging Process in a Membrane-Containing Double Stranded DNA Virus

Two crucial steps in the virus life cycle are genome encapsidation to form an infective virion and genome exit to infect the next host cell. In most icosahedral double-stranded (ds) DNA viruses, the viral genome enters and exits the capsid through a unique vertex. Internal membrane-containing viruses possess additional complexity as the genome must be translocated through the viral membrane bilayer. Here, we report the structure of the genome packaging complex with a membrane conduit essential for viral genome encapsidation in the tailless icosahedral membrane-containing bacteriophage PRD1. We utilize single particle electron cryo-microscopy (cryo-EM) and symmetry-free image reconstruction to determine structures of PRD1 virion, procapsid, and packaging deficient mutant particles. At the unique vertex of PRD1, the packaging complex replaces the regular 5-fold structure and crosses the lipid bilayer. These structures reveal that the packaging ATPase P9 and the packaging efficiency factor P6 form a dodecameric portal complex external to the membrane moiety, surrounded by ten major capsid protein P3 trimers. The viral transmembrane density at the special vertex is assigned to be a hexamer of heterodimer of proteins P20 and P22. The hexamer functions as a membrane conduit for the DNA and as a nucleating site for the unique vertex assembly. Our structures show a conformational alteration in the lipid membrane after the P9 and P6 are recruited to the virion. The P8-genome complex is then packaged into the procapsid through the unique vertex while the genome terminal protein P8 functions as a valve that closes the channel once the genome is inside. Comparing mature virion, procapsid, and mutant particle structures led us to propose an assembly pathway for the genome packaging apparatus in the PRD1 virion.     

Generation of filamentous instead of icosahedral particles by repression of African swine fever virus structural protein pB438L

The mechanisms involved in the construction of the icosahedral capsid of the African swine fever virus (ASFV) particle are not well understood at present. Capsid formation requires protein p72, the major capsid component, but other viral proteins are likely to play also a role in this process. We have examined the function of the ASFV structural protein pB438L, encoded by gene B438L, in virus morphogenesis. We show that protein pB438L associates with membranes during the infection, behaving as an integral membrane protein. Using a recombinant ASFV that inducibly expresses protein pB438L, we have determined that this structural protein is essential for the formation of infectious virus particles. In the absence of the protein, the virus assembly sites contain, instead of icosahedral particles, large aberrant tubular structures of viral origin as well as bilobulate forms that present morphological similarities with the tubules. The filamentous particles, which possess an aberrant core shell domain and an inner envelope, are covered by a capsid-like layer that, although containing the major capsid protein p72, does not acquire icosahedral morphology. This capsid, however, is to some extent functional, as the filamentous particles can move from the virus assembly sites to the plasma membrane and exit the cell by budding. The finding that, in the absence of protein pB438L, the viral particles formed have a tubular structure in which the icosahedral symmetry is lost supports a role for this protein in the construction or stabilization of the icosahedral vertices of the virus particle.     

New type platforms for in vitro vaccine assembly

Studying the structure of plant viruses and viral coat proteins along with the possibilities of their modification and structural remodeling is very important for the development of novel approaches to the design of biotechnological products, including those of medical use. Being nonpathogenic for mammals, in particular for humans, undoubtedly, make plant viruses advantageous for the development of new functional and biologically active materials, particularly, candidate vaccines. The present review focuses attention on characteristics and applying in biotechnology of spherical particles—the new type platforms generated by structural remodeling of plant viruses. Spherical particles do not have structural analogues among the icosahedral viruses and represent a new type of biogenic platforms. The assembly of the spherical particle–antigen immunogenic complexes in vitro bring in the possibilities of inexpensive and easy production of different vaccines.