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 m any viruses, the movement function is secured by specific virus-encode d proteins. Amino acid sequence comparisons of these proteins revealed a vast superfamily containing a conserved sequence motif that may com prise a hydrophobic interaction domain. This superfamily combines prot eins of viruses belonging to all principal groups of positive-strand R NA viruses, as well as single-stranded DNA containing geminiviruses, d ouble-stranded DNA-containing pararetroviruses (caulimoviruses and bad naviruses), and tospoviruses that have negative-strand RNA genomes wit h two ambisense segments. In several groups of positive-strand RNA vir uses, the movement function is provided by the proteins encoded by the so-called triple gene block including two putative small membrane-ass ociated proteins and a putative RNA helicase. A distinct type of movem ent proteins with very high content of proline is found in tymoviruses . It is concluded that classification of movement proteins based on co mparison of their amino acid sequences does not correlate with the typ e of genome nucleic acid or with grouping of viruses based on phylogen etic analysis of replicative proteins or with the virus host range. Re combination 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 d ianthoviruses and the MIP family of cellular integral membrane protein s, and ii) between movement proteins of bromoviruses and cucumoviruses and M1 protein of influenza viruses which is involved in nuclear expo rt of viral ribonucleoproteins. It is hypothesized that all movement p roteins of plant viruses may mediate hydrophobic interactions between viral and cellular macromolecules.