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The Corticoviridae is a family of icosahedral internal membrane-containing viruses with double-stranded circular DNA genomes of ~10 000 bases in length. Only one virus species, Pseudoalteromonas virus PM2, has been recognized. Bacteriophage PM2 infects Gram-negative bacteria. Currently, PM2 does not share any significant sequence similarity to any other virus. PM2 was isolated from seawater in 1968 and was already reported in the 1st Report of ICTV. PM2 is the first bacteriophage in which the presence of lipids in the virion was demonstrated. Since only one genus Corticovirus is recognized, the family description corresponds to the genus description.
Table 1.Corticoviridae. Characteristics of the family Corticoviridae
Pseudoalteromonas phage PM2, species Pseudoalteromonas phage PM2 (AF155037), genus Corticovirus
Icosahedral internal membrane-containing virions of ~57 nm with a single capsid protein P2, single spike protein P1 and 8 membrane associated proteins P3-P10
10.1 kb of highly supercoiled circular double-stranded DNA
Rolling circle replication initiated by virus encoded protein P12
Prokaryotic translation using viral mRNA and host ribosomes
Bacteria, Gram-negative Pseudoalteromonas strains
One genus containing one species
PM2 is a virulent virus infecting Gram-negative Pseudoalteromonas species (Espejo and Canelo 1968, Kivelä et al., 1999). The characteristic feature of the corticovirus is the highly supercoiled circular double-stranded DNA genome. The virions are composed of an icosahedral protein capsid and an internal protein-rich membrane enclosing the genome (Abrescia et al., 2008). PM2 does not share significant sequence similarity to any other virus.
Icosahedral virions consist of an internal membrane and an outer protein capsid that has a diameter of 57 nm between facets (Figure 1.Corticoviridae) (Abrescia et al., 2008). The genome is enclosed by the membrane. The capsid consists of 200 major capsid protein P2 trimers that are organized on a pseudo T=21 lattice (Abrescia et al., 2008). Protein P2 is composed of two beta-barrels disposed normal to the capsid surface. The P2 trimers have pseudo-six-fold symmetry and the structure is stabilized by calcium ions. Spikes protrude about 8 nm from the capsid surface at the five-fold vertices. The spikes are homopentamers and formed of protein P1. P1 is composed of three beta-barrel domains arranged end to end. The distal C-terminal domains of P1 are used for receptor recognition. The N-termini of P1 form pentagonal assemblies at the vertices. The inner membrane (47 nm in diameter) contains host plasma membrane-derived phospholipids and virus-encoded proteins P3 to P10 (Kivelä et al., 2002). Transmembrane proteins P3 and P6 are organized into a lattice on the membrane vesicle surface, on which the outer protein capsid is assembled (Abrescia et al., 2008).
Figure 1.Corticoviridae. (A) Negative stain electron micrograph of Pseudoalteromonas phage PM2 particles. The bar represents 50 nm. (B) X-ray crystallographic structure of a virion of Pseudoalteromonas phage PM2 at 7 Å resolution, viewed along two-fold axis of symmetry (courtesy of N.G.A. Abrescia) and (C) a schematic presentation of the virion. Capsid diameter is ~57 nm (from facet to facet).
The mass of the virion is about 4.5×107 Da and it is distributed among protein (~72%), lipid (~14%) and nucleic acid (~14%) (Camerini-Otero and Franklin 1975, Camerini-Otero et al., 1974). The buoyant density in CsCl is 1.28 g cm−3 and in sucrose 1.26 g cm−3, and the S20,w is 293S (Kivelä et al., 1999, Camerini-Otero and Franklin 1975). Virions are stable at pH 6–8, and are very sensitive to ether, chloroform and detergents (Espejo and Canelo 1968). The virion stability is strongly dependent on sodium and calcium ions (Kivelä et al., 2002). Virions are sensitive to freezing.
The genome is a highly supercoiled circular dsDNA of 10,079 bp (6.6×106 Da). DNA comprises about 14% of the virion weight and the G+C content is 42.2%. The phage PM2 genome has been sequenced (Männistö et al., 1999)(AF155037).
The genome has 21 putative genes, 17 of which have been shown to code for structural proteins (P1–P10) and nonstructural proteins (P12–P18; Table 2.Corticoviridae). Proteins form about 72% of the virus particle weight.
Table 2.Corticoviridae. Pseudoalteromonas phage PM2 proteins
Spike protein, receptor binding (S)
Major capsid protein (S)
Major membrane protein (S)
Putative packaging ATPase (S)
Replication initiation protein (N)
Transcription factor (N)
Regulative function (N)
Lysis factor (N)
Particles are about 14% lipid by weight (Camerini-Otero and Franklin 1975). The membrane contains about 34% phosphatidyl ethanolamine and about 66% phosphatidyl glycerol and trace amounts of phosphatidic acid and acyl-phosphatidyl glycerol (Braunstein and Franklin 1971, Camerini-Otero and Franklin 1972, Tsukagoshi et al., 1976). The lipids are derived from the host plasma membrane, but their composition deviates from that of the host bacterium. Lipids form an internal membrane with virus-specific membrane associated proteins.
To infect and replicate, PM2 delivers its genome across the cell envelope of two known marine host strains: Gram-negative Pseudoalteromonas species ER72M2 and BAL-31. Virions adsorb via the distal tips of the spike proteins to uncharacterized receptors (Abrescia et al., 2008). The internal membrane mediates the translocation of the supercoiled genome across the host cell envelopes most probably via fusion in a process which is not fully understood. Replication of the PM2 genome, most probably by a rolling circle mechanism, takes place in proximity to the host cytoplasmic membrane. The largest PM2 gene XII encoding protein P12 shares significant sequence similarity with the superfamily I group of replication initiation proteins (Männistö et al., 1999). The genome is organized in three operons (Figure 2.Corticoviridae). Operons OEL and OER encode early function gene products: the replication initiation protein P12 and transcription regulatory proteins P13, P14, P15 and P16. Expression of the genes for structural proteins is under the control of the late promoter (OL), which is activated by the phage-encoded transcription factors P13 and P14 (Mannistö et al., 2003). The mature virions are released from the cell by lysis. Lysis factor P17 permeabilizes the cytoplasmic membrane and acts like a holin, whereas lysis factor P18 disrupt the outer membrane, while peptidoglycan is most probably disrupted by the host lytic factors (Krupovic et al., 2007).
Figure 2.Corticoviridae. Genome organization of Pseudoalteromonas phage PM2 (PM2). The genome is a 10,079 bp highly supercoiled circular dsDNA molecule containing 21 ORFs (as arrows showing their orientations) and three operons (OER, OEL and OL). ORFs shown to code for functional proteins are classified as genes and given a Roman numeral. The different colors indicate the ORFs (yellow), a gene for replication initiation protein (orange) and the following groups of genes: transcriptional regulation (magenta), structural proteins (blue) and lysis (green).
Phages are virulent and replicate in two known strains of marine host bacteria of the genus Pseudoalteromonas. Although PM2 is virulent and the sole member of the family Corticoviridae, using a comparative genomic approach, integrated corticoviral genetic elements have been identified to commonly reside within aquatic bacterial chromosomes (Krupovic and Bamford 2007).
Cortico: from Latin cortex, “crust”, “bark”.
Corticovirus virion morphology resembles other tailless icosahedral viruses with internal membrane such as viruses in the family Tectiviridae which have a lipid bilayer underneath the isometric protein capsid. Corticoviruses and tectiviruses appear to differ by the genome organization and the infection mechanism, since no tectivirus-specific tail-like membrane tube is seen upon corticoviral infection. PM2 major capsid protein is a trimeric protein with two beta-barrels forming hexagonal capsomers (Abrescia et al., 2008). The same viral jelly-roll fold has also been described at least for bacteriophage PRD1 (family Tectiviridae) (Benson et al., 1999), the archaeal Sulfolobus turreted icosahedral virus (STIV; family Turriviridae) (Maaty et al., 2006), human adenovirus (family Adenoviridae)(Rux et al., 2003), and for large eukaryotic viruses Paramecium bursaria Chlorella virus 1 (family Phycodnaviridae) (Nandhagopal et al., 2002) and Chilo iridescent virus (family Iridoviridae) as well as in virophage Sputnik (family Lavidaviridae) (Zhang et al., 2012). In addition, bacterial and archaeal viruses in the family Sphaerolipoviridae most probably share the same major capsid fold (Gil-Carton et al., 2015, Demina et al., 2016).
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