SUMMARYFrog virus 3 (FV3) assembles in morphologically distinct assembly sites in the cytoplasm of infected cells. As the assembly sites form, the intermediate filaments (IF) aggregate, delimit the assembly sites, and remain so throughout infection. To determine the molecular basis of reorganization of IF, we analysed the vimentin of uninfected and FV3-infected cells by two-dimensional gel electrophoresis. The results showed that (i) the vimentin was more acidic in FV3-infected cells than in uninfected cells, (ii) the acidification of vimentin in FV3-infected cells was possibly due to a fourfold increase in phosphorylation, and (iii) the phosphorylation of vimentin preceded the reorganization of IF around virus assembly sites. A temperature-sensitive mutant of FV3 (ts9467), which at the non-permissive temperature neither reorganized IF nor formed assembly sites, failed to increase the phosphorylation of vimentin. Together, the above results suggest that changes in phosphorylation may modulate IF organization and that changes in IF organization are required for FV3 assembly site formation.

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