SUMMARYInsertions and deletions have been introduced into an infectious cDNA clone of M RNA of cowpea mosaic virus (CPMV), in the coding regions of the 58K/48K and capsid proteins. Transcripts derived from these mutant clones appeared to be replicated in cowpea protoplasts as detected by immunofluorescent staining and Northern blotting. However in cowpea plants, mutations in either region restricted the replication of the viral RNAs to the inoculated cells and thus prevented a successful systemic infection of the plant. These results indicate that the M RNA-encoded 58K/48K proteins are involved in cell-to-cell transport of CPMV, and that the virus can spread only if the RNA is encapsidated in particles.

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Tubular structures extending from plasmodesmata in cowpea mosaic virus (CPMV)-infected tissue have been implicated to play an important role in cell-to-cell movement of this virus. Using a cauliflower mosaic virus 35S promoter-based transient expression vector, we show that expression of only the CPMV M RNA-encoded 48-kDa protein (48K protein) in cowpea protoplasts is sufficient to induce these structures. Strikingly,

Cell-to-cell movement of cowpea mosaic virus particles in plants takes place with the help of tubules that penetrate presumably modified plasmodesmata. These tubules, which are built up by the virus-encoded 48-kDa movement protein (MP), are also formed on single protoplast cells. To determine whether the MP contains different functional domains, the effect of mutations in its coding region was studied. Mutations between amino acids 1 and 313 led to complete abolishment of the tubule-forming capacity, while a deletion in the C-terminal region resulted in tubules that could not take up virus particles. From these observations, it is concluded that the MP contains at least two distinct domains, one that is involved in tubule formation and that spans amino acids 1 and 313 and a second that is probably involved in the incorporation of virus particles in the tubule and that is located in the C terminus between amino acids 314 and 331.