Cowpea mosaic virus (CPMV) spreads from cell-to-cell as virus particles through tubular structures in modified plasmodesmata which are composed of viral movement protein (MP). Mutational analysis of the MP has revealed that the N-terminal and central regions of the MP are involved in tubule formation and that the C-terminal domain probably has a role in the interactions with virus particles. By constructing C-terminal deletion mutants and comoviral hybrid MPs, it was possible to delineate the C-terminal border of the tubule-forming domain to a small region between amino acids 292 and 298. Experiments with tripartite viruses in protoplasts indicated that the C-terminus of the MP is involved in the incorporation of virus particles in the tubule and that for efficient incorporation of virus particles all MP molecules incorporated in a tubule need to contain a functional C-terminus. A mutant virus coding for a MP in which the last 10 C-terminal amino acids were replaced by the green fluorescent protein (GFP) was able to form tubules in protoplasts. These tubules did not contain virus particles, probably because the GFP interferes with the incorporation of virions into the tubule. These results suggest a model for the structure of the tubule in which the C-terminus of the MP is located inside the tubular structure, where it is able to interact with virus particles.