The importance of chloroplast interactions for local and systemic movement of some members of the Alphaflexiviridae
J. HAMMOND (1), A. M. Vaira (2), H. S. Lim (3). (1) USDA ARS, USNA, FNPRU, Beltsville, MD, U.S.A.; (2) CNR, IVV, Torino, Italy; (3) Chungnam National University, Daejeon, Korea
<i>Alternanthera mosaic virus</i> (AltMV; genus <i>Potexvirus</i>) and <i>Lolium latent virus</i> (LoLV; <i>Lolavirus</i>) are distinct members of the family <i>Alphaflexiviridae</i>. Each requires interactions with the chloroplast in order to move efficiently cell-to-cell or systemically throughout the plant host. In AltMV, deletion of any one of the Triple Gene Block (TGB) proteins eliminates systemic movement; however, while deletion of either TGB1 or TGB2 restricts replication to the initially infected cell, deletion of TGB3 allows spread to a few adjacent epidermal cells but not movement into the mesophyll. TGB3 is targeted to the chloroplast, where it may serve as an anchor for the viral replicase complex at the chloroplast outer membrane. Alteration of two residues in the N-terminal domain of TGB3 prevents localization to the chloroplast, and disrupts interactions with nuclear-encoded Photosystem II oxygen-evolving complex protein PsbO. The LoLV genome is encapsidated in two carboxy-coterminal coat protein (CP) variants in equimolar proportion, of which the larger contains an additional 48 residues including a 42 residue chloroplast transit peptide (cTP), leading to proteolytic cleavage and an alternative means of production of the smaller CP. Mutational analysis of each of the in-frame CP start codons, and of the proteolytic cleavage site, showed that the N-terminal cTP domain is critical for efficient cell-to-cell and functional systemic movement, but is not required for virus replication.
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