Потивирустар: құрамы, қасиеттері және таралуы

Genus of positive strand RNA viruses in the family Potyviridae

Potyvirus is a genus of positive strand RNA viruses (named after its type species, Potato virus Y (PVY)) in the family Potyviridae. Plants serve as natural hosts. Like begomoviruses, members of this genus may cause significant losses in agricultural, pastoral, horticultural, and ornamental crops. More than 200 species of aphids spread potyviruses, and most are from the subfamily Aphidinae (genera Macrosiphum and Myzus). The genus contains 190 species and potyviruses account for about thirty percent of all currently known plant viruses.

The virion is non enveloped with a flexuous and filamentous nucleocapsid, 680 to 900 nanometers (nm) long and is 11–20 nm in diameter. In the species with a monopartite genome, a genome linked VPg protein is covalently bound to the 5' end and the 3' end is polyadenylated. The genome encodes a single open reading frame (ORF) expressed as a 350 kDa polyprotein precursor. This polyprotein is processed into ten smaller proteins: protein 1 protease (P1 Pro), helper component protease (HC Pro), protein 3 (P3), cylindrical inclusion (CI), viral protein genome linked (Vpg), nuclear inclusion A (NIa), nuclear inclusion B (NIb), capsid protein (CP) and two small putative proteins known as 6K1 and 6K2. The P3 cistron also contains an overlapping reading frame called "Pretty interesting Potyviridae ORF" (PIPO). PIPO codes for an alternative C terminus to the P3 protein, which is generated into a subset of transcripts by a +2 frameshift caused by a ribosome slippage mechanism at a conserved GA6 repeat sequence. The resulting protein is called P3N PIPO. A similar mechanism is thought to produce an alternative reading frame within the P1 cistron, named "pretty interesting sweet potato potyvirus ORF" (PISPO), in a number of sweet potato infecting potyviruses including sweet potato feathery mottle virus.

P1 (~33 kilodaltons (kDa) in molecular weight) is a serine protease which facilitates its own cleavage from the polyprotein at the P1 HC Pro junction. P1 consists of a conserved C terminal protease domain and an N terminal region which has a high level of variation in sequence and length between potyvirus species but exhibits conserved patterns of intrinsic disorder. P1 is also promotes viral RNA replication, though it is not required for it. HC Pro (~52 KDa) is a cysteine protease which cleaves a glycine glycine dipeptide at its own C terminus. HC Pro's activity is regulated by the adjacent P1 protein: before P1 cleaves itself off the P1 HC Pro intermediate, the P1 terminus reduces HC Pro's RNA silencing suppression activity. Though the exact mechanism is unknown, HC Pro has been proposed to attach to host aphid mouth parts through its N terminal zinc finger like domain and anchor virions through its interactions with the capsid protein. P3 (~41 kDa) is a membrane protein which is required for viral replication and accumulates in viral replication vesicles. It mediates the interactions between replication vesicles and movement complex proteins which may allow replication vesicles to be recruited to the movement complex for efficient intercellular movement. P3 also interacts with large subunit of the ribulose 1,5 bisphosphate carboxylase/oxygenase. CI (~71 kDa) is an RNA helicase with ATPase activity. Its most unusual property is its ability to form large and highly symmetrical conical and cylindrical inclusions with a central hollow cylinder from which laminate sheets radiate outward and fold in on themselves in a pattern often described as "pinwheels". These inclusions are easily seen in transmission electron micrographs of infected tissues and were historically used as a diagnostic criterion for potyvirus infections. CI inclusions are a major component of the potyviral movement complex which is assembled at plasmodesmata. CI is also required for viral replication and is present on replication membranes. Its exact contributions to replication are not clear but, as an RNA helicase, CI is likely facilitating replication by dismantling the secondary structures of viral RNA. NIa (~50 kDa) forms crystalline inclusions in the host nucleus. It is cleaved into NIa Pro and VPg. NIa Pro (~27 kDa) is a cysteine protease which processes most of the cleavage sites of the polyprotein. The only exceptions are the self cleavages of P1 and HC Pro. The high degree of cleavage sequence specificity and conservation has made NIa Pro (often that of Tobacco etch virus) a valuable tool in biotechnology, especially in applications which require removing affinity tags from recombinant proteins after affinity purification. NIa Pro has also shown to exhibit sequence independent DNase activity and to interfere with host DNA methylation suggesting that NIa and/or NIa Pro are altering in host gene expression. Potyviral NIa Pro shares a high level of homology with the picornaviral 3C protease. VPg (~22 kDa) is covalently attched to the 5' end of the viral genomic RNA through uridylation and is thought to act as a primer for viral genome replication similarly to the VPg proteins of picornaviridae. It is a highly disordered protein and its flexibility has been suggested to allow it to interact with many other viral proteins. VPg also interacts with various host proteins including eukaryotic initiation factor 4E (eIF4E), eukaryotic elongation factor 1A (eEF1A), and poly(A) binding protein (PABP). NIb (~59 kDa) is a superfamily II RNA dependent RNA polymerase (RdRp) which polymerises viral RNA during replication. Like NIa, NIb forms inclusions in the host nucleus where it is transported due to its two nuclear localisation sequences. NIb has the three domain "palm, thumb, and fingers" structure typical of RdRps. 6K1 (~6 kDa) the function is not known, but because it accumulates in replication vesicles and has a transmembrane domain, 6K1 is thought to contribute to virus induced vesicle formation. 6K2 (~6 kDa) is a transmembrane protein which rearranges host membranes into virus induced membrane structures. It interacts with various ER exit site proteins to produce vesicular and tubular extensions which eventually mature into replication vesicles. 6K2 has three main domains: the N terminal domain which is required for cell to cell movement, the central hydrophobic transmembrane alpha helix, and the C terminal domain which is required for viral replication. P3N PIPO (~25 kDa) is a dedicated movement protein which anchors the movement complex to the plasmodesma. It may also modulate the plasmodesmatal size exclusion limit by interacting with host proteins which sever plasmodesmatal actin filaments and reduce callose deposition. It interacts with both the large and small subunits of the ribulose 1,5 bisphosphate carboxylase/oxygenase. CP (~30 35 kDa) is the capsid protein. It has two terminal domains which are disordered and exposed at the surface of the virion. The central core domain contains an RNA binding pocket which binds to viral RNA. The structure of the capsid protein is highly conserved in potyviruses, though there is a relatively high degree of sequence variability. In addition to encapsidating the virion, CP core domain is required for intercellular movement and contributes to seed transmission. Certain atypical potyviruses code for additional proteins or protein domains, such as P1 PISPO, Alkylation B (AlkB), and inosine triphosphate pyrophosphatase (known as ITPase or HAM1). Such anomalies are often situated in the hypervariable P1 HC Pro region. They do not circulate or multiply within the aphid and typically only persist in the aphid for a few minutes. Certain potyviruses have been shown to alter the feeding patterns of their aphid vectors, which may manifest as longer periods of time spent on infected plants, reduced non probing feeding time, and increased phloem sap ingestion. Seed and pollen transmission has been documented in certain potyvirus species, for instance in PVY and Turnip mosaic virus (TUMV). Vegetative transmission by infected tubers or grafting material are of particular concern for certain agricultural crops, such as potato and fruit trees, respectively. Transmission can also occur by physical contact with infected plants or with contaminated tools, clothes, or even water.

After entry, potyvirus particles are uncoated and genomic RNA is released into host cytoplasm. Potyviral RNA mimicks host mRNA: the 5' VPg protein shares functional similarities with the 5' cap and the 3' end is polyadenylated. VPg and its interactions with eIF4E and eIF4(iso)E allow the virus to utilise host cap dependent translation machinery for its translation. Similarly to eukaryotic translation, the VPg eIF4E interaction assembles the eIF4F complex around viral RNA. A number of weak internal ribosome entry sites (IRES) have been identified in many potyvirus species but it is not known whether cap independent translation is an important translation mechanism for potyviruses. The viral 6K2 protein coordinates the rearrangement of host membranes into various infection associated structures which, depending of the potyvirus species, can include anything from small round viral vesicles to complex globular structures with many cisternae or lobes. These structures are dotted with viral replication complexes and are often called "replication vesicles", "viroplasm" or "viral factories". Replication vesicle membranes are derived from a variety of host organelles and the sources differ between potyvirus species. Some membrane sources include the ER, chloroplasts, Golgi apparatus, and vacuoles. The exact replication mechanism is not known but it involves a negative sense RNA intermediate and requires both viral and host proteins. Viral proteins detected in replication complexes include HC Pro, P3, 6K1, 6K2, CI, VPg, NIa Pro, and NIb. Host factors present in replication vesicles include eIF4A and several heat shock proteins.

Like most plant viruses, potyviruses have evolved to move from one plant cell to another through plasmodesmata. However, unlike some well studied plant viruses, such as the Tobacco mosaic virus, potyviruses do not have a single movement protein but instead assemble a movement complex around the plasmodesma. This complex is primarily composed of three viral proteins: CI, CP, P3N PIPO. Conical CI inclusions are anchored to plasmodesmata by P3N PIPO during the early stages of potyvirus infection. This allows the inclusion to funnel either viral particles or viral RNA CP complexes through the plasmodesma. Replication vesicles are also recruited to the movement complex, suggesting that replication and movement are coupled. Replication vesicles are recruited by P3N PIPO, which interacts with both CI and P3 through the shared P3N domain. They appear to have evolved in southwest Eurasia or north Africa. The estimated mutation rate is about 1.15 nucleotide substitutions/site/year.

Agriculture was introduced into Australia in the 18th century. This introduction also included plant pathogens. Thirty eight potyvirus species have been isolated in Australia. Eighteen potyviruses have been found only in Australia and are presumed to be endemic there. The remaining twenty appear to have been introduced with agriculture.

Historically, potyvirus diagnostics relied on the detection of various proteinaceous inclusions in infected plant cells. These may appear as crystals in either the cytoplasm or in the nucleus, as amorphous X bodies, membranous bodies, viroplasms or pinwheels. The inclusions may or may not (depending on the species) contain virions. These inclusions can be seen by light microscopy in leaf strips of infected plant tissue stained with Orange Green (protein stain) but not Azure A (nucleic acid stain). Modern detection methods rely primarily on reverse transcription PCR.

Potyviruses were further divided into the PVY, SCMV, BYMV, BCMV species groups in 1992. Gibbs and Ohshima 2010 produced a more extensive molecular phylogeny with the same four, but also several new groups: the BtMV, ChVMV, DaMV, OYDV, PRSV, TEV, and TuMV.

Contains 16 species including the type species of the genus (potato virus Y). The primary hosts are: Nine Solanaceae, three Amaranthus, three Asteraceae, one Lilium, and one Amaryllis.