Structure and assembly of turnip crinkle virus. II. Mechanism of reassembly in vitro.

Dissociation of turnip crinkle virus (TCV) at elevated pH and ionic strength produces free dimers of the coat protein and a ribonucleoprotein complex that contains the viral RNA, six coat-protein subunits, and the minor protein species, p80 (a covalently linked coat-protein dimer). This "rp-complex" is stable for several days in high salt at pH 8.5. Reassembly of TCV can be accomplished under physiological conditions, using isolated coat protein and either rp-complex or protein-free RNA. If rp-complex is used in reassembly, the same subunits remain bound to RNA on subsequent dissociation; if free RNA is used, rp-complex is regenerated. In both cases, the assembly is selective for viral RNA in competition experiments with heterologous RNA. Electron microscopy shows that assembly proceeds by continuous growth of a shell from an initiating structure, rather than by formation of distinct intermediates. We suggest that rp-complex is the initiating structure, suggest a model based on the organization of the TCV particle, and propose a mechanism for TCV assembly.