INVOLVEMENT OF RESIDUES WITHIN PUTATIVE ALPHA-HELIX MOTIFS IN THE BEHAVIOR OF THE ALFALFA AND TOBACCO MOSAIC-VIRUS MOVEMENT PROTEINS

The movement proteins (MPs) of both alfalfa mosaic virus (AMV) and tobacco mosaic virus (TMV) have a 15-amino acid putative a helix motif that is part of a larger domain involved in the cell wall (CW) localization of these proteins. These a helices have common features since they are amphipathic and contain two clusters of acidic amino acids, EE and DE. Mutations were introduced into these helices to investigate their role in CW localization and in the activity of the MPs. Results showed that both these motifs are involved in the CW localization of the MPs, although through different mechanisms: whereas the helical structure and the EE duster of the AMV-MP were required for optimum CW localization, the DE cluster of the TMV-MP, but not the helical structure, was involved in this process. Chimeric proteins resulting from an exchange of the a helices between MPs showed that these sequences did not function in the complementary background. Moreover, the region around aa 61 of the TMV-MP is necessary for protein stability. Transgenic tobacco plants expressing a mutated AMV-MP in which the a helix was deleted or had its structure destroyed exhibited an abnormal development and a modified morphology. In parallel, a similar mutation of the TMV-MP yielded a nonfunctional protein that still accumulated in the CW but that did not compete with the viral MP during infection.