Notable examples are viral proteases, such as the maturation protease VP24 in herpesvirus [12], or the adenoviral protease AVP [4]; and molecular motors involved in nucleic acid translocation, such as dsDNA packaging ATPases in bacteriophage and herpesviruses [13, 14], or the dsRNA packaging ATPase in cystoviruses (bacteriophage Φ6) [15]. The largest virus, (e.g., poxvirus) measures about 250 nm, i.e., as […] In PRD1, some of the mobile arms in its major capsid protein are embedded in the outer leaflet of the membrane, contributing to enforce the icosahedral shape [11]. You've got SUVs, minivans, pickups, motorcycles and 18-wheelers. But also proteins with a tendency to form symmetric aggregations can give rise to pleomorphic capsids. The remaining minor capsid protein, polypeptide VI, has not been unequivocally traced so far, but has been assigned to density within an internal cavity present in each hexon trimer. Podoviruses (representatives: T7, P22) have short tails, with a fiber complement that may include long, thin fibers (T7) or thick spikes (P22) [48]. Annu Rev Biochem 81:795–822, Agbandje-McKenna M, McKenna R (2011) Structural virology. 12,  17). For contractile tails, the baseplate is assembled first, and used as a seeding point for assembly of the inner tube and contractile sheath. In: Knipe DM, Howley PM, Griffin DE, Lamb RA, Martin MA (eds) Fields virology, vol II. The membrane can also be located beneath the icosahedral shell, such as in Tectiviruses (PRD1) and structurally related viruses [11, 24]. (a) Slice of a cryo-electron tomography reconstruction of herpes simplex virus-1 showing the icosahedral capsid (c), tegument (t), membrane (m) and glycoprotein spikes (s). You can click the image to magnify if you cannot see clearly. Multiple layered viruses: rotavirus capsid architecture. Lippincott Williams & Wilkins, Philadelphia, pp 1999–2069, Liu H, Jin L, Koh SB, Atanasov I, Schein S, Wu L, Zhou ZH (2010) Atomic structure of human adenovirus by cryo-EM reveals interactions among protein networks. (b) Bacteriophage PRD1 has an internal membrane (m) that adopts an icosahedral profile due to interactions with capsid proteins (arrows). J Virol 33:866–876, Giberson AN, Davidson AR, Parks RJ (2012) Chromatin structure of adenovirus DNA throughout infection. More information on the packaging motors and on the organization of nucleic acids within viral capsids can be found in  Chap. Fibers with different lengths protrude from the baseplate; fibers may also be present at the portal region and the icosahedral head. 1a and b) and the trimeric fiber (knob and distal shaft from PDB ID 1QIU, rest of the shaft modeled). Different conformations of the hexon N-terminal (left) and C-terminal (right) regions in the capsid are shown. (c) The PBCV-1 capsid (EMDB ID EMD_1597) showing the special vertex with a spike, and (d), a central slab where the asymmetry in internal contents can be appreciated. Putting together the many different elements in the virion requires an extra effort to achieve correct assembly, and thus complex viruses require sophisticated mechanisms to regulate morphogenesis. Color key indicates color changes with map radius. Capsid (Head) Protein Shell of a Virus. Others, like T4, have elongated, prolate icosahedral heads; two different proteins form the hexameric and pentameric capsomers, and several minor proteins are located at specific positions on the head surface [. J Mol Biol 406:764–774, Chappell JD, Prota AE, Dermody TS, Stehle T (2002) Crystal structure of reovirus attachment protein σ1 reveals evolutionary relationship to adenovirus fiber. To form the T4 baseplate, proteins gp6, gp7, gp8, gp10, gp11, gp25 and gp53 assemble in the form of hetero-oligomeric wedges. A similar assembly pathway has been described for non-contractile long tails. (a) Structure of adenovirus penton base (beige) and hexon (cyan) monomers, with the β-barrel jelly roll motifs highlighted in red. Finally, IIIa and VIII cooperate to bind each GOS to its five surrounding GONs. In podoviruses however, the short tail is not assembled as a separate entity, but it grows outward from the portal vertex on the virion capsid. Panels (a), (b), (c), (e) and (f) made from atomic coordinates deposited at the Protein Data Bank (PDB; Protein Data Bank is at http://www.pdb.org) with entry ID 3IYN. Curr Opin Microbiol 14:504–510, Palese P, Shaw ML (2007) Orthomyxoviridae: the viruses and their replication. Accommodation of the different components often requires deviations from high order symmetry, from mismatches to pleomorphism; and involves complex regulation of the assembly dynamics. For example, the flu virus has a membrane-like envelope around its capsid. The proximal and distal parts assemble separately, and then join before attaching to the baseplate. Structurally complex viruses present a larger variety of components in their capsids than simple viruses. (b) 3D map of the bacteriophage T7 procapsid, showing the internal core complex. Capsids function to protect the viral genetic material from damage. For example, different proteins may occupy the sixfold and fivefold coordinated positions in the icosahedral net (see  Chap. Rather, a gap exists between the DNA core and the side of the capsid containing the special vertex. (e) 3D map of the Mimivirus capsid (EMDB ID EMD_5039) showing the starfish feature (red). Cryo-EM imaging reveals high-resolution side and top views of the viral RNA replication "crown" complex structure. The biological significance of these symmetry mismatches has long intrigued virologists. Proc Natl Acad Sci U S A 102:4729–4734, Butcher SJ, Manole V, Karhu NJ (2012) Lipid-containing viruses: bacteriophage PRD1 assembly. This difference with respect to the bacteriophage case is likely related to the mode of infection of these eukaryotic viruses. With permission). However, the condensing proteins have not been identified yet. The external contractile envelope of the tail of some viruses. Inside of the virus, its ribonucleoproteins (RNPs) are shown with their coiled structures and three-bulbed polymerase complex on the ends. Enveloped. The capsid does the job of attaching to host cells. In summary, complex viruses incorporate a wide range of molecules into their capsids, including specialized host interaction, genome packaging and cementing proteins; and in some cases membranes, either internal or external. Schematic depiction of the complex bacteriophage T4 morphogenesis pathway (Reproduced from [42]. Nature 457:694–698, Pina M, Bize A, Forterre P, Prangishvili D (2011) The archeoviruses. composed of double protein shell with a complex structure. In other cases, virion components with different symmetry may interact with each other, forming a symmetry mismatch at the interface. During infection in bacteria, Qβ recruits the host-cell ribosomal S1 protein, which is composed of six oligonucleotide-oligosaccharide-binding (OB) domains, to initiate viral replication ( Wahba et al., 1974 ). (f) Mobile arms in adenovirus hexon. Accordingly, complex virus morphogenesis requires sophisticated mechanisms, tightly regulated in space and time. It is believed that scaffold redundancy confers an evolutionary advantage to ΦX174 by facilitating extremely rapid replication cycles. This figure shows three relatively-complex virions: the bacteriophage T4, with its DNA-containing head group and tail fibers that attach to host cells; adenovirus, which uses spikes from its capsid to bind to host cells; and HIV, which uses glycoproteins embedded in its envelope to bind to host cells. Sizes and Shapes of Viruses. (a) Only one type of VP2 monomer in the core shell is represented. J Virol 86:5562–5573, Aalto AP, Bitto D, Ravantti JJ, Bamford DH, Huiskonen JT, Oksanen HM (2012) Snapshot of virus evolution in hypersaline environments from the characterization of a membrane-containing salisaeta icosahedral phage 1. Two major types of the AIDS virus, HIV- 1 and HIV-2, have been identified. 11.5b). Other examples of complex, lipid-containing viruses will be described in Sect. Credit: Paul Ahlquist, Morgridge Institute for Research Molecular composition of complex viruses: capsid proteins, cementing proteins, and mobile arms. When we think about all the things that people are afraid of or should be afraid of, we can come up with a lot of different examples. Proc Natl Acad Sci U S A 101:6003–6008, Agirrezabala X, Martin-Benito J, Caston JR, Miranda R, Valpuesta JM, Carrascosa JL (2005) Maturation of phage T7 involves structural modification of both shell and inner core components. For example, flexible filamentous viruses can reach lengths in the order of microns, but most of their capsid mass is built by a single capsid protein arranged in a helical pattern [1]. Proc Natl Acad Sci U S A 109:7079–7084, Haring M, Vestergaard G, Rachel R, Chen L, Garrett RA, Prangishvili D (2005) Virology: independent virus development outside a host. Lipothrixviruses are enveloped, and the ends of their filamentous capsids are capped with structures of varied shapes (spider legs, pincers, bottle brushes), probably involved in attachment to the host. A characteristic feature of complex viruses is the presence of multiple proteins in the virion, playing specific architectural or functional roles during the viral cycle. Adenovirus and PRD1 are not the only cases indicating an evolutionary relationship between animal and bacterial viruses. In the Cystoviridae representative bacteriophage Φ6, a hexameric ssRNA packaging ATPase occupies multiple fivefold vertices of the empty procapsid [15]. (e) Cementing proteins in the adenovirus capsid. This study showed that the two fibers interact with each other at the icosahedral capsid level, and that P2 can move relatively to P5 (Reproduced from [34]. That is, the interaction is mediated by a β-strand from one of the proteins binding to the edge of a β-sheet in the other. Examples of virus shapes: Viruses can be either complex in shape or relatively simple. Other additional proteins may be incorporated to the viral particles and play fundamental roles for viability. Science 329:1038–1043, San Martín C (2012) Latest insights on adenovirus structure and assembly. The parallels between adenovirus and PRD1 extend from their DNA replication mechanisms, to their capsid architecture and the folding of their major capsid proteins [3, 11]. (b) A model for the double fiber complex in bacteriophage PRD1 vertex. In the case of myoviruses however, a further mismatch may exist, since the contractile sheath presents helical symmetry, and it is not yet clear if the inner tube follows the arrangement of the sheath or the sixfold symmetry observed in non-contractile tails [46]. Symmetry mismatches and singular vertices. However, an equilibrium between different oligomeric forms seems to be required to achieve correct capsid assembly. It has been proposed that this tube has a function in injecting the viral genome into the host cell, similar to that played by tail structures in other bacteriophages [28] (see  Chap. Its contraction ensures ejection of the virus DNA into the host cytoplasm. COMPLEX This virus structure is combined with both icosahedral and helical shapes and may have a complex outer wall or head-tall morphology. Animal Viruses: These viruses cause disease in men and other animals. Scale bar, 1,000 Å (Modified from [25]). Some archaeal viruses with tailed bacteriophage morphologies have also been reported [56]. However, discovery of the unique architectures of archaeal viruses indicates that other structural solutions exist. RSV is an RNA virus in which the genomic RNA forms part of a nuclease-resistant helical ribonucleoprotein complex. complex virus A virus with a complicated structure, such as a bacteriophage. However, the biggest danger to our species lies not with large teeth, bullets or toxins; it has to do with things you cannot even see! These viruses are among the more complex viruses. 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