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J. Virol. doi:10.1128/JVI.01031-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

The conserved FRNK box in HC-Pro, a plant viral suppressor of gene silencing, is required for small RNA binding and mediates symptom development

Departments of Plant Pathology and Ornamental Horticulture, Agricultural Research Organization, the Volcani Center, P.O.Box 6 Bet Dagan 50250, Israel, AgroScience GmbH, AlPlanta-Institute for Plant Research, Breitenweg 71, 67435 Neustadt, Germany and Department of Plant Pathology, Iowa State University, Ames, IA 50011-1020, USA

^* To whom correspondence should be addressed. Email: amitg{at}agri.gov.il.

	Abstract

The helper component-proteinase (HC-Pro) protein of potyviruses is a suppressor of gene silencing and has been shown to elicit plant developmental defect-like symptoms. In Zucchini yellow mosaic virus (ZYMV) a mutation in the highly conserved FR₁₈₀NK box of HC-Pro to FI₁₈₀NK causes attenuation of these symptoms. At 5 days post inoculation and before symptoms appear, virus accumulation, HC-Pro protein levels and viral siRNA levels are similar for the severe (FRNK) and the attenuated (FINK) strains. At this stage, ZYMV^FRNK caused greater accumulation of most miRNAs, and especially of their complementary miRNA*s, in systemically infected leaves as compared to attenuated ZYMV^FINK. HC-Pro^FRNK specifically bound artificial siRNA and miRNA/miRNA* duplexes with a much higher affinity than the mutated HC-Pro^FINK. Further analysis of the mutant and wild-type HC-Pro proteins revealed that suppressor activity of the ZYMV-HC^FINK mutant was not diminished. However, the FINK mutation caused loss of HC-Pro suppressor function in other potyviruses. Replacement of the second positively-charged amino-acid in the ZYMV FRNK box to FRNA also caused symptom attenuation and reduced small RNA duplex binding affinity without loss of suppressor activity. Our data suggest that the highly conserved FRNK box in the HC-Pro of potyviruses is a probable point of contact with siRNA and miRNA duplexes. The interaction of the FRNK box with populations of miRNAs directly influences their accumulation levels and regulatory functions resulting in symptom development.

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