Structure of a viral procapsid with molecular scaffolding

The assembly of a macromolecular structure proceeds along an ordered morphogenetic pathway, and is accomplished by the switching of proteins between discrete conformations as they are added to the nascent assembly1,2,3. Scaffolding proteins often play a catalytic role in the assembly process1,2,4, rather like molecular chaperones5. Although macromolecular assembly processes are fundamental to all biological systems, they have been characterized most thoroughly in viral systems, such as the icosahedral Escherichia coli bacteriophage φX174 (refs 6, 7). The φX174 virion contains the proteins F, G, H and J7,8. During assembly, two scaffolding proteins B and D are required for the formation of a 108S, 360-Å-diameter procapsid from pentameric precursors containing the F, G and H proteins6,9. The procapsid contains 240 copies of protein D, forming an external scaffold, and 60 copies each of the internal scaffolding protein B, the capsid protein F, and the spike protein G9,10. Maturation involves packaging of DNA and J proteins and loss of protein B, producing a 132S intermediate6,7. Subsequent removal of the external scaffold yields the mature virion. Both the F and G proteins have the eight-stranded antiparallel β-sandwich motif8,11 common to many plant and animal viruses12,13. Here we describe the structure of a procapsid-like particle at 3.5-Å resolution, showing how the scaffolding proteins coordinate assembly of the virus by interactions with the F and G proteins, and showing that the F protein undergoes conformational changes during capsid maturation.