STRUCTURE OF A VIRAL PROCAPSID WITH MOLECULAR SCAFFOLDING

The assembly of a macromolecular structure proceeds along an ordered m orphogenetic pathway, and is accomplished by the switching of proteins between discrete conformations as they are added to the nascent assem bly(1-3). Scaffolding proteins often play a catalytic role in the asse mbly process(1,2,4), rather like molecular chaperones(5). Although mac romolecular assembly processes are fundamental to all biological syste ms, they have been characterized most thoroughly in viral systems, suc h as the icosahedral Escherichia coli bacteriophage Phi X174 (refs 6, 7). The Phi X174 virion contains the proteins F, G, H and J(7,8). asse mbly, two scaffolding proteins B and D are required for formation of a 108S, 360-Angstrom-diameter procapsid from pentameric precursors cont aining the F, G and H protein(6,9). 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 pr otein G(9,10). Maturation involves packaging of DNA and J proteins and loss of protein B, producing a 132S intermediate(6,7). Subsequent rem oval of the external scaffold yields the mature virion. Both the F and G proteins have the eight- stranded antiparallel beta-sandwich motifs (8,11) common to many plant and animal viruses(12,13). Here we describ e the structure of a procapsid-like particle at 3.5-Angstrom resolutio n, showing how the scaffolding proteins coordinate assembly of the vir us by interactions with the F and G proteins, and showing that the F p rotein undergoes conformational changes during capsid maturation.