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1 Department of Virology, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel
2 Department of Biochemistry and Cell Biology, State University of New York at Stony Brook, Stony Brook, NY 11794-5215, USA
3 Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK
Correspondence
Amit Gal-On
amitg{at}volcani.agri.gov.il
The 2b protein encoded by Cucumber mosaic virus (CMV) has been shown to be a silencing suppressor and pathogenicity determinant in solanaceous hosts, but a movement determinant in cucumber. In addition, synergistic interactions between CMV and Zucchini yellow mosaic virus (ZYMV) have been described in several cucurbit species. Here, it was shown that deletion of the 2b gene from CMV prevented extensive systemic movement of the virus in zucchini squash, which could not be complemented by co-infection with ZYMV. Thus, ZYMV expressing a silencing suppressor with a different target could not complement the CMV 2b-specific movement function. Expression of the 2b protein from an attenuated ZYMV vector resulted in a synergistic response, largely restoring infection symptoms of wild-type ZYMV in several cucurbit species. Deletion or alteration of either of two nuclear localization signals (NLSs) did not affect nuclear localization in two assays, but did affect pathogenicity in several cucurbit species, whilst deletion of both NLSs led to loss of nuclear localization. The 2b protein interacted with an Arabidopsis thaliana karyopherin 
protein (AtKAP) in the yeast two-hybrid system, as did each of the two single NLS-deletion mutants. However, 2b protein containing a deletion of both NLSs was unable to interact with AtKAP. These data suggest that the 2b protein localizes to the nucleus by using the karyopherin -mediated system, but demonstrate that nuclear localization was insufficient for enhancement of the 2b-mediated pathogenic response in cucurbit hosts. Thus, the sequences corresponding to the two NLSs must have another role leading to pathogenicity enhancement.
Present address: Department of Plant Pathology, Iowa State University, Ames, IA 50011-1020, USA.
These authors contributed equally to this work.
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