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Characterization of VPg and the polyprotein processing of Cocksfoot mottle virus (genus Sobemovirus)

Institute of Biotechnology, Program for Plant Molecular Biology, Viikki Biocenter, PO Box 56, FIN-00014 University of Helsinki, Finland¹
Institute of Agricultural Biotechnology, Timiryazevskaya st. 42, Moscow 127550, Russia²
National Institute of Chemical Physics and Biophysics³ and Gene Technology Center, Tallinn Technical University⁴, Akadeemia tee 23, EE12618 Tallinn, Estonia

Author for correspondence: Kristiina Mäkinen. Fax +358 9 19159571. e-mail kristiina.makinen{at}helsinki.fi

The polyprotein of Cocksfoot mottle virus (CfMV; genus Sobemovirus) is translated from two overlapping open reading frames (ORFs) 2a and 2b by a -1 ribosomal frameshifting mechanism. In this study, a 12 kDa protein was purified from viral RNA-derived samples that appears to correspond to the CfMV genome-linked protein (VPg). According to the determined N-terminal amino acid sequence, the VPg domain is located between the serine proteinase and replicase motifs and the N terminus of VPg is cleaved from the polyprotein between glutamic acid and asparagine residues. Western blot analysis of infected plant material showed that the polyprotein is processed at several additional sites. An antiserum against the ORF 2a product recognized six distinct proteins, whereas, of these, the VPg antiserum clearly recognized only a 24 kDa protein. This indicates that the fully processed 12 kDa VPg detected in viral RNA-derived samples is a minor product in infected plants. An antiserum against the ORF 2b product recognized a 58 kDa protein, which indicates that the fully processed replicase is entirely or almost entirely encoded by ORF 2b. The origin of the detected cleavage products and a proposed polyprotein processing model are discussed.

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