title: Adenosine kinase inhibition and suppression of RNA silencing by geminivirus AL2 and L2 proteins.
abstract: Most plant viruses are initiators and targets of RNA silencing and encode proteins that suppress this adaptive host defense. The DNA-containing geminiviruses are no exception, and the AL2 protein (also known as AC2, C2, and transcriptional activator protein) encoded by members of the genus Begomovirus has been shown to act as a silencing suppressor. Here, a three-component, Agrobacterium-mediated transient assay is used to further examine the silencing suppression activity of AL2 from Tomato golden mosaic virus (TGMV, a begomovirus) and to determine if the related L2 protein of Beet curly top virus (BCTV, genus Curtovirus) also has suppression activity. We show that TGMV AL2, AL2(1-100) (lacking the transcriptional activation domain), and BCTV L2 can all suppress RNA silencing directed against a green fluorescent protein (GFP) reporter gene when silencing is induced by a construct expressing an inverted repeat GFP RNA (dsGFP). We previously found that these viral proteins interact with and inactivate adenosine kinase (ADK), a cellular enzyme important for adenosine salvage and methyl cycle maintenance. Using the GFP-dsGFP system, we demonstrate here that codelivery of a construct expressing an inverted repeat ADK RNA (dsADK), or addition of an ADK inhibitor (the adenosine analogue A-134974), suppresses GFP-directed silencing in a manner similar to the geminivirus proteins. In addition, AL2/L2 suppression phenotypes and nucleic acid binding properties are shown to be different from those of the RNA virus suppressors HC-Pro and p19. These findings provide strong evidence that ADK activity is required to support RNA silencing, and indicate that the geminivirus proteins suppress silencing by a novel mechanism that involves ADK inhibition. Further, since AL2(1-100) is as effective a suppressor as the full-length AL2 protein, activation and silencing suppression appear to be independent activities.
authors.affiliation: Biotechnology Center, Ohio State University, 201 Rightmire Hall, 1060 Carmack Road, Columbus, OH 43210, USA.
authors.forename: Hui
authors.initials: H
authors.lastname: Wang
authors.forename: Kenneth J
authors.initials: KJ
authors.lastname: Buckley
authors.forename: Xiaojuan
authors.initials: X
authors.lastname: Yang
authors.forename: R Cody
authors.initials: RC
authors.lastname: Buchmann
authors.forename: David M
authors.initials: DM
authors.lastname: Bisaro
mesh_terms: D000248:Adenosine Kinase / Q000037:antagonists & inhibitors*; D036022:Capsid Proteins / Q000502:physiology*; D017791:Geminiviridae / Q000472:pathogenicity*; D020868:Gene Silencing; D017930:Genes, Reporter; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D018551:Solanum lycopersicum / Q000821:virology; D009856:Oncogene Proteins, Viral / Q000502:physiology*; D018515:Plant Leaves / Q000821:virology; D034622:RNA Interference; D012091:Repetitive Sequences, Nucleic Acid; D014026:Nicotiana / Q000821:virology; D014764:Viral Proteins / Q000502:physiology*
chemical_list: C483731:AL2 protein, Begomovirus; D036022:Capsid Proteins; C063316:L2 protein, Human papillomavirus type 16; D009856:Oncogene Proteins, Viral; D014764:Viral Proteins; D049452:Green Fluorescent Proteins; D000248:Adenosine Kinase