Conformational transitions in Pf3 and their implications for the structure and assembly of filamentous bacterial viruses.

Laser Raman and circular dichroism spectra of filamentous bacteriophage Pf3 show that its coat protein is predominantly alpha-helical, similar to the subunits of bacteriophages Pf1 and fd. Unlike Pf1 and fd, however, the subunits of Pf3 are converted to beta-sheet structures by raising the temperature, the transition temperature depending upon phage and NaCl concentrations. On cooling, the beta structure reverts to an alpha structure the same as or similar to the native structure. On further heating it converts irreversibly to a second alpha-helical form different from the original one. The spectra also show that aromatic amino acid residues of Pf3 undergo dramatic changes in molecular environment during the alpha leads to beta transition. Similar transitions are observed to take place in the filamentous bacteriophage Xf.