Fusion of paramyxovirus to the cell involves receptor binding of the HN glycoprotein and a number of conformational changes of F glycoprotein. The F protein is expressed as a homotrimer on the virus surface. In the present model, there are at least three conformations of F protein, i.e. native form, pre-hairpin intermediate and the post-fusion state. In the post-fusion state, the two highly conserved heptad repeat (HR) regions of F protein form a stable 6-helix coiled-coil bundle. However, no crystal structure is known for this state for the Newcastle disease virus, although the crystal structure of the F protein native form has been solved recently. Here we deployed an Escherichia coli in vitro expression system to engineer this 6-helix bundle by fusion of either the two HR regions (HR1, linker and HR2) or linking the 6-helix [3 x (HR1, linker and HR2)] together as a single chain. Subsequently, both of them form a stable 6-helix bundle in vitro judging by gel filtration and chemical cross-linking and the proteins show salient features of an alpha-helix structure. Crystals diffracting X-rays have been obtained from both protein preparations and the structure determination is under way. This method could be used for crystallization of the post-fusion state HR structures of other viruses.