Conformational switching by the scaffolding protein D directs the assembly of bacteriophage phi X174

摘要

The three-dimensional structure of bacteriophage phiX174 external scaffolding protein D, prior to its interaction with other structural proteins, has been determined to 3.3 Angstrom by X-ray crystallography. The crystals belong to space group P4(1)2(1)2 with a dimer in the asymmetric unit that closely resembles asymmetric dimers; observed in the phi5X174 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.