Cell-to-cell movement of plant viruses. Insights from amino acid sequence comparisons of movement proteins and from analogies with cellular transport systems.

Cell-to-cell movement is a crucial step in plant virus infection. In many viruses, the movement function is secured by specific virus-encoded proteins. Amino acid sequence comparisons of these proteins revealed a vast superfamily containing a conserved sequence motif that may comprise a hydrophobic interaction domain. This superfamily combines proteins of viruses belonging to all principal groups of positive-strand RNA viruses, as well as single-stranded DNA containing geminiviruses, double-stranded DNA-containing pararetroviruses (caulimoviruses and badnaviruses), and tospoviruses that have negative-strand RNA genomes with two ambisense segments. In several groups of positive-strand RNA viruses, the movement function is provided by the proteins encoded by the so-called triple gene block including two putative small membrane-associated proteins and a putative RNA helicase. A distinct type of movement proteins with very high content of proline is found in tymoviruses. It is concluded that classification of movement proteins based on comparison of their amino acid sequences does not correlate with the type of genome nucleic acid or with grouping of viruses based on phylogenetic analysis of replicative proteins or with the virus host range. Recombination between unrelated or distantly related viruses could have played a major role in the evolution of the movement function. Limited sequence similarities were observed between i) movement proteins of dianthoviruses and the MIP family of cellular integral membrane proteins, and ii) between movement proteins of bromoviruses and cucumoviruses and M1 protein of influenza viruses which is involved in nuclear export of viral ribonucleoproteins. It is hypothesized that all movement proteins of plant viruses may mediate hydrophobic interactions between viral and cellular macromolecules.