Molecular Description of Potato Virus Y
Potyvirus virions consist of non-enveloped filamentous structures that are 680 – 900 nm in length and 11 to 15 nm in width. Morphologically the potyvirus consists of approximately 2 000 copies of coat protein (CP) which forms a cylindrical inclusion body (CIb). The CIb is considered to be the single most important phenotypic criterion for distinguishing a potyvirus from other virus groups.
The CIb encapsulates a single strand of positive sense RNA which is in the order of 10 kb in length and has a nontranslated 5’-terminal region (5’-NTR) as well as a 3’-poly-A tail. The positive sense genome contains a single extended open reading frame and acts directly as mRNA. The 144 nucleotide 5’-NTR is particularly rich in adenine residues and has very few guanine residues. Rather than a conventional cap structure, the 5’NTR is associated with a Viral genome linked protein (VPg) which is said to act as an enhancer of transcription.
The 5’-leader sequence has an internal ribosome entry site (IRES) and cap-independent translation regulatory elements (CIREs). The IRES directs capindependent translation through a mechanism similar to that used by eukaryotes. The extended open reading frame encodes for a 350 kDa polyprotein. This polyprotein is proteolytically processed by viral proteases (NIa, HC-Pro and P1) and undergoes co- and posttranslational cleavage to yield several multi-functional proteins. These include the following: P1 (P1 Protein), HC-Pro (Helper Component Proteinase), P3 (P3 Protein), 6K1 (6-kDa Protein 1), CIb (Cylindrical Inclusion body), 6K2 (6-kDa Protein 2), VPg (Viral Genome-linked Protein), NIa-Pro (Nuclear Inclusion Protein a, Proteinase domain), NIb (Nuclear Inclusion Protein b) and the CP (Coat Protein).
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