FUNCTIONAL CHIMERAS OF THE ROUS-SARCOMA VIRUS AND HUMAN-IMMUNODEFICIENCY-VIRUS GAG PROTEINS

The Gag protein encoded by Rous sarcoma virus (RSV) is the only viral product required for the process of budding whereby virus particles ar e formed at the plasma membrane. Deletion analysis of this Gag molecul e has revealed several regions (assembly domains) that are important f or budding. One of these domains is located at the amino terminus and is needed for membrane binding. Another is located within the carboxy- terminal third of the protein. Though there is little sequence homolog y among the Gag proteins of unrelated retroviruses, it seemed possible that their assembly domains might be functionally conserved, and to e xplore this idea, numerous Gag chimeras were made. The results indicat e that the first 10 amino acids of the human immunodeficiency virus (H IV) Gag protein can suppress the block to budding caused by deletions in the RSV MA sequence, much as described previously for the first 10 residues from the Src oncoprotein (J. W. Wills, R. C. Craven, R. A. We ldon, Jr., T. D. Nelle, and C. R. Erdie, J. Virol. 65:3804-3812, 1991) . In addition, the carboxy-terminal half of the HIV Gag protein was fu sed to a truncated RSV Gag molecule, mutant Bg-Bs which is unable to d irect core assembly. This chimera was able to produce particles at a r ate identical to that of RSV and of a density similar to that of authe ntic virions. Deletion analysis of the carboxy-terminal chimera reveal ed two small regions within the HIV NC protein that were sufficient fo r endowing mutant Bg-Bs with these properties. Chimeras lacking both r egions produced particles of a low density, suggesting that these sequ ences may be involved in the tight packing of Gag molecules during ass embly. In a related set of experiments, replacement of the RSV proteas e with that of HIV resulted in premature processing within the RSV seq uence and a block to budding. Particle assembly was restored when the HIV PR activity was inactivated by mutagenesis. Collectively, the data presented here illustrate the functional similarities of Gag proteins from unrelated retroviruses.