Amino acid sequences of plant virus proteins mediating cell-to-cell movement were compared to each other and to protein sequences in databases. Two families of movement proteins have been identified, the members of which show statistically significant sequence similarity. The first, larger family (I) encompasses the movement proteins of tobamo-, tobra-, caulimo- and comoviruses, apple chlorotic leaf spot virus (ACLSV) and geminiviruses with bipartite genomes. Thus this family includes viruses which move by two methods, those requiring the coat protein for the cell-to-cell spread (comoviruses) and those not having this requirement (tobamoviruses). The previously unsuspected relationship between the movement proteins of RNA and DNA viruses having no RNA stage in their life cycle (geminiviruses) suggested that their movement mechanisms might be similar. The second, smaller family (II) consists of the movement proteins of tricornaviruses (bromoviruses, cucumoviruses, alfalfa mosaic virus and tobacco streak virus) and dianthoviruses. Alignment of the sequences of family I movement proteins highlighted two motifs, centred at conserved Gly and Asp residues, respectively, which are assumed to be crucial for the movement protein function(s). Screening the amino acid sequence database revealed another conserved motif that is shared by a large subset of family I movement proteins (those of caulimo- and comoviruses, and ACLSV) and the family of cellular 90K heat shock proteins (HSP90). Based on the analogy to HSP90, it is speculated that many plant virus movement proteins may mediate virus transport in a chaperone-like manner.


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