![]() ![]() Indeed, if there were no conformational switching and two D molecules were to bind with a 45° rotation, then an infinite 8-fold screw helix would result ( The same is true for the procapsid, except the oligomerization extends only over four D molecules ( Figures 2B and 2C). Therefore, the crystal structure can be described approximately as having an 8-fold screw helix with an average rotation of 45° (alternate rotations of 51° and 39°) and translation of 15.9 Å between individual D molecules. Specifically, the interface within an asymmetric dimer involves a kinked helix α3 in D A, D 1, or D 3, whereas the interface between asymmetric dimers involves a straight helix α3 in D B or D 2, respectively. The difference between the inter- and intradimer interfaces is determined by whether helix α3 is straight or kinked. In the procapsid the D 2D 3 interface is similar to the D 1D 2 interface ( Figure 3A), and in the D AD B crystal structure the D BD A′ interface is similar to the D AD B interface ( Figure 3B). In both the D AD B crystal structure and in the procapsid structure, the interdimer interface resembles the intradimer interface. Hence, comparison with the procapsid structure provides a rare opportunity to follow the conformational switching events necessary for the construction of complex macromolecular assemblies. The crystal structure of the D scaffolding protein thus represents the state of the polypeptide prior to procapsid assembly. 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. Furthermore, application of the crystallographic 4 1 symmetry operation to one of these dimers generates a tetramer similar to the tetramer in the icosahedral asymmetric unit of the procapsid. The crystals belong to space group P4 12 12 with a dimer in the asymmetric unit that closely resembles asymmetric dimers observed in the φX174 procapsid structure. ![]() 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.
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