IDENTIFICATION OF STRUCTURAL DOMAINS WITHIN THE CAULIFLOWER MOSAIC-VIRUS MOVEMENT PROTEIN BY SCANNING DELETION MUTAGENESIS AND EPITOPE TAGGING

Plant viruses encode proteins that mediate their movement from cell to cell through plasmodesmata. Currently, the mechanisms of action of th ese movement proteins (MPs) can be divided broadly into two types, req uiring or not requiring the presence of viral capsid protein. Cauliflo wer mosaic virus encodes a multifunctional MP (P1) that modifies plasm odesmata through the formation of tubules that contain virus particles . To investigate the structure of P1, 26 small deletions (scanning del etions) were used to characterize regions of P1 essential for full bio logical activity. These deletions identified an N-terminal region and a region close to but not at the C terminus as domains that could tole rate manipulation, although gross deletions of either domain abolished infection. In sequence comparisons with other caulimovirus MPs, these regions coincided with the areas of least amino acid homology. Epitop e tags inserted into either of these regions were stably maintained in systemic infections, and in extracts from infected plants, tagged P1 was detected on immunoblots. We predicted that, from the hypervariabil ity of these regions, they would be located on the surface of the nati ve P1 structure. Immunofluorescence of P1-specific tubules formed on t he surface of infected protoplasts confirmed that the N-terminal and C terminus-proximal regions were exposed-on the surface of the P1 tubul e subunit. These findings establish a structure for P1 that is likely to be applicable to other tubule-forming MPs.