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Role of the zinc-finger and basic motifs of chrysanthemum virus B p12 protein in nucleic acid binding, protein localization and induction of a hypersensitive response upon expression from a viral vector

¹ A. N. Belozersky Institute of Physico-Chemical Biology, Moscow State University, 119992 Moscow, Russia
² Department of Plant Biology and Forest Genetics, Swedish University of Agricultural Sciences (SLU), Box 7080, SE-750 07 Uppsala, Sweden
³ Julius Kühn Institute (JKI), Federal Research Centre for Cultivated Plants, Institute for Biosafety of Genetically Modified Plants, Messeweg 11/12, D-38104 Braunschweig, Germany
⁴ Institute of Agricultural Biotechnology, Russian Academy of Agricultural Sciences, Timiryazevskaya 42, 127550 Moscow, Russia

Correspondence
A. G. Solovyev
solovyev{at}genebee.msu.su

The genomes of carlaviruses encode cysteine-rich proteins (CRPs) of unknown function. The 12 kDa CRP of chrysanthemum virus B (CVB), p12, has been shown previously to induce a hypersensitive response (HR) when expressed from potato virus X (PVX). This study demonstrated that a p12-induced HR was preceded by induction of a number of genes related to pathogenesis, stress and systemic acquired resistance. p12 localized predominantly to the nucleus. Interestingly, it was found that p12 bound both RNA and DNA in vitro, but notably exhibited a preference for DNA in the presence of Zn²⁺ ions. Mutational analysis of the p12 conserved sequence motifs demonstrated that the basic motif is required for p12 translocation to the nucleus, thus representing part of the protein nuclear localization signal, whereas the predicted zinc finger motif is needed for both Zn²⁺-dependent DNA binding and eliciting an HR in PVX-infected leaves. Collectively, these results link, for the first time, nuclear localization of the protein encoded by a cytoplasmically replicating virus and its DNA-binding capacity with HR induction. Furthermore, these data suggest that p12 may mediate induction of the host genes by binding to the plant genomic DNA, and emphasize that CVB p12 is functionally distinct from other known nuclear-localized proteins encoded by the plant positive-stranded RNA viruses.