The Gag proteins of Rous sarcoma virus and human immunodeficiency virus (HIV) each contain a function involved in a late step in budding, defects in which result in the accumulation of these molecules at the plasma membrane. In the Rous sarcoma virus Gag protein (Pr76 gag), this assembly domain is associated with a PPPY motif, which is located at an internal position between the MA and CA sequences. This motif is not contained anywhere within the HIV Gag protein (Pr55 gag), and the MA sequence is linked directly to CA. Instead, a late assembly function of HIV has been associated with the p6 sequence situated at the C terminus of Gag. Here we demonstrate the remarkable finding that the late assembly domains from these two unrelated Gag proteins are exchangeable between retroviruses and can function in a positionally independent manner.

No abstract

No abstract

Flock house virus (FHV) is a bipartite, positive-strand RNA insect virus that encapsidates its two genomic RNAs in a single virion. It provides a convenient model system for studying the principles underlying the copackaging of multipartite viral RNA genomes. In this study, we used a baculovirus expression system to determine if the uncoupling of viral protein synthesis from RNA replication affected the packaging of FHV RNAs. We found that neither RNA1 (which encodes the viral replicase) nor RNA2 (which encodes the capsid protein) were packaged efficiently when capsid protein was supplied in trans from nonreplicating RNA. However, capsid protein synthesized in cis from replicating RNA2 packaged RNA2 efficiently in the presence and absence of RNA1. These results demonstrated that capsid protein translation from replicating RNA2 is required for specific packaging of the FHV genome. This type of coupling between genome replication and translation and RNA packaging has not been observed previously. We hypothesize that RNA2 replication and translation must be spatially coordinated in FHV-infected cells to facilitate retrieval of the viral RNAs for encapsidation by newly synthesized capsid protein. Spatial coordination of RNA and capsid protein synthesis may be key to specific genome packaging and assembly in other RNA viruses.Flock house virus (FHV), a member of the Nodaviridae family, is a nonenveloped, icosahedral insect virus whose genome consists of two positive-strand RNA molecules (3, 28). The bipartite nature of the FHV genome organizes its nonstructural and structural genes onto RNA1 (3.1 kb) and RNA2 (1.4 kb),respectively. An open reading frame (ORF) that nearly spans the length of RNA1 encodes protein A (112 kDa), the RNAdependent RNA polymerase which directs the replication of RNA1, RNA2, and a 387-nucleotide (nt) subgenomic RNA3 (7). RNA3 corresponds to the 3Ј end of RNA1 and encodes two small proteins, B1 and B2 (10). No function has been ascribed to B1, while B2 functions as an inhibitor of RNA silencing (15). Interestingly, RNA3 is also required as a transactivator of RNA2 replication, but the mechanism by which this is achieved is not completely understood (6). RNA2 encodes the 43-kDa capsid precursor protein ␣, which is the only structural protein required for the assembly of FHV provirions (7, 9). Each provirion consists of 180 subunits of protein ␣, arranged with Tϭ3 quasiequivalent symmetry, and the two genomic RNAs. Provirions are not infectious and undergo an autocatalytic maturation process in which protein ␣ cleaves into proteins ␤ (38 kDa) and ␥ (5 kDa). Both cleavage products remain associated with the matured, infectious virion (9, 26).FHV RNA1 and RNA2 are packaged into a single virion but the mechanism by which FHV copackages its multipartite genome is still unknown (14). One hypothesis is that an interaction between RNA1 and RNA2 occurs prior to packaging, allowing the genome to be encapsidated as a complex. The absence of significant complementarity between RNA1 and RNA2, however, does ...