Identification of the genome-linked protein in virions of Potato virus A, with comparison to other members in genus Potyvirus

Viruses of the genus Potyvirus, the largest genus of plant-infecting viruses, have a messenger-polarity ssRNA genome encapsidated by approximately 2000 units of the viral coat protein (CP), resulting in filamentous virions. Only few studies have examined potyvirus virions for the presence of other structural proteins. A protein linked covalently to the 5′-end of the genome has been identified in Tobacco vein mottling virus (TVMV) and Tobacco etch virus (TEV). In TEV, it is either the viral NIa protein or only its N-terminal domain (VPg) separated autocatalytically from the C-terminal proteinase domain (NIa-Pro). Virions of TVMV carry only the VPg. We examined virions of Potato virus A (PVA) for the genome-linked protein using immunoblotting or iodination and immunoprecipitation. The VPg ( 25 kDa) only, and not the unprocessed NIa, was detected. Another signal corresponding to 49 kDa was detected in disrupted, RNase-treated virions with anti-VPg antibodies but not with antibodies to NIa-Pro. Since it possibly represented a dimeric form of the VPg, self-interaction of the VPg was tested using the yeast two-hybrid system, which showed that the VPg self-interacts in the absence of viral RNA.