Crystallization and preliminary X-ray diffraction analysis of post-fusion six-helix bundle core structure from Newcastle disease virus F protein

Fusion of virus members from the Paramyxoviridae family involves two glycoproteins. They are termed attachment glycoprotein (HN, H or G) and fusion protein (F). The F protein contains two highly conserved heptad‐repeat (HR) regions, HR1 and HR2. Through conformational changes in the F protein, HR1 and HR2 are believed to form a stable six‐helix coiled‐coil bundle during the membrane‐fusion process. However, no crystal structure has yet been documented for this state in the Newcastle disease virus (NDV, a member of the Paramyxoviridae family) F protein, despite the recent success on its F0 crystal structure (Chen et al., 2001), which was thought to represent the pre‐fusion conformation of F glycoprotein. In this study, a single‐chain polypeptide constructed by linking two truncated HR regions of the NDV F protein has been expressed, purified and crystallized by means of the hanging‐ or sitting‐drop vapour‐diffusion method. Crystals in hexagonal and trapezoid forms with a resolution limit of 2.6 Å were obtained. These crystals belonged to space group C2, with unit‐cell parameters a = 66.4, b = 38.2, c = 102.0 Å, β = 100.2°. Crystals in the rhombic form with a resolution limit of 2.5 Å were also obtained. These crystals belonged to space group P21, with unit‐cell parameters a = 59.0, b = 31.9, c = 62.3 Å, β = 117.0°. This will add to the repertoire of viral fusion protein post‐fusion state structures and help further the understanding of the molecular mechanism of enveloped virus fusion.