Conformational switching by the scaffolding protein D directs the assembly of bacteriophage φX174

The three-dimensional structure of bacteriophage φX174 external scaffolding protein D, prior to its interaction with other structural proteins, has been determined to 3.3 Å by X-ray crystallography. The crystals belong to space group P4₁2₁2 with a dimer in the asymmetric unit that closely resembles asymmetric dimers observed in the φX174 procapsid structure. Furthermore, application of the crystallographic 4₁ symmetry operation to one of these dimers generates a tetramer similar to the tetramer in the icosahedral asymmetric unit of the procapsid. These data suggest that both dimers and tetramers of the D protein are true morphogenetic intermediates and can form independently of other proteins involved in procapsid morphogenesis. The crystal structure of the D scaffolding protein thus represents the state of the polypeptide prior to procapsid assembly. Hence, comparison with the procapsid structure provides a rare opportunity to follow the conformational switching events necessary for the construction of complex macromolecular assemblies.