THE PLANT CELL, Vol 7, Issue 5 561-572, Copyright © 1995 by American Society of Plant Biologists
Identification of Structural Domains within the Cauliflower Mosaic Virus Movement Protein by Scanning Deletion Mutagenesis and Epitope Tagging
C. L. Thomas and A. J. Maule
Department of Virus Research, John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7UH, United Kingdom
Plant viruses encode proteins that mediate their movement from cell to cell
through plasmodesmata. Currently, the mechanisms of action of these
movement proteins (MPs) can be divided broadly into two types, requiring or
not requiring the presence of viral capsid protein. Cauliflower mosaic
virus encodes a multifunctional MP (P1) that modifies plasmodesmata through
the formation of tubules that contain virus particles. To investigate the
structure of P1, 26 small deletions (scanning deletions) were used to
characterize regions of P1 essential for full biological activity. These
deletions identified an N-terminal region and a region close to but not at
the C terminus as domains that could tolerate 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. Epitope 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 hypervariability of these regions, they would be located on the
surface of the native P1 structure. Immunofluorescence of P1-specific
tubules formed on the surface of infected protoplasts confirmed that the
N-terminal and C terminus-proximal regions were exposed on the surface of
the P1 tubule subunit. These findings establish a structure for P1 that is
likely to be applicable to other tubule-forming MPs.
