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J. Virol. doi:10.1128/JVI.02246-07
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Class VIII Myosins Are Required for Plasmodesmatal Localization of a Closteroviral Hsp70 Homolog

Department of Botany and Plant Pathology and Center for Genome Research and Biocomputing, Oregon State University, Corvallis, Oregon 97331

^* To whom correspondence should be addressed. Email: doljav{at}science.oregonstate.edu.

	Abstract

The Hsp70 homolog (Hsp70h) of Beet yellows virus (BYV) functions in virion assembly and cell-to-cell movement and is autonomously targeted to plasmodesmata in association with the actomyosin motility system (Prokhnevsky et al., 2005, J. Virol. 79, 14421-14428). Myosins are a diverse category of molecular motors that possess a motor domain and a tail domain involved in cargo binding. Plants have two classes of myosins, VIII and XI, whose specific functions are poorly understood. We used dominant negative inhibition to identify myosins required for Hsp70h localization to plasmodesmata. Six full-length myosin cDNAs from BYV host plant Nicotiana benthamiana were sequenced and shown to encode apparent orthologs of the Arabidopsis myosins VIII-1, VIII-2, VIII-B, XI-2, XI-F, and XI-K. We found that the ectopic expression of the tail domains of each of the class VIII, but not class XI myosins inhibited plasmodesmatal localization of Hsp70h. In contrast, overexpression of the motor domains or the entire molecules of the class VIII myosins did not affect Hsp70h targeting. Further mapping revealed that the minimal cargo-binding part of the myosin VIII tails was both essential and sufficient for the inhibition of the proper Hsp70h localization. Interestingly, plasmodesmatal localization of the Tobacco mosaic virus movement protein and Arabidopsis protein RGP2 was not affected by myosin VIII tail overexpression. Collectively, our data implicate class VIII myosins in the protein delivery to plasmodesmata and suggest that more than one mechanism of such delivery exist in plants.