EFFICIENT IN-VIVO AND IN-VITRO ASSEMBLY OF RETROVIRAL CAPSIDS FROM GAG PRECURSOR PROTEINS EXPRESSED IN BACTERIA

The capsid precursor protein (Gag) of Mason-Pfizer monkey virus, the p rototype type D retrovirus, has been expressed to high levels in bacte ria under the control of the phage T7 promoter. Electron microscopic s tudies of induced cells revealed the assembly of capsid-like structure s within inclusion bodies that formed at the poles of the cells 6 h af ter induction with isopropyl-beta-D-thiogalactopyranoside (IPTG). The inclusion bodies and enclosed capsid-like structures were solubilized completely in 8 M urea, but following renaturation, we observed assemb ly in vitro of capsid-like structures that demonstrated apparent icosa hedral symmetry. These results demonstrate for the first time that ret roviral capsid precursors have the propensity to self-assemble in vitr o and point to new approaches for the analysis of retroviral assembly and structure.