Specificity of in vitro binding of primate Type C viral RNA and the homologous viral p12 core protein

Analysis of deletions and thermosensitive mutations in Rous sarcoma virus gag protein p10

Rous sarcoma virus protein p10 is a gag component of the virion present in stoichiometric amount but of unknown function. To characterize this protein, a series of mutants of p10 with linker insertions or deletions was generated by site-directed mutagenesis of a cloned proviral DNA. The deletions and two of the linkers insertions, which disrupted proline pairs, reduced the yield of virus particles upon transfection. These two linker insertion mutants were moreover thermosensitive for this phenotype, producing fewer virus particles at 41 degrees C than at 36 degrees C. Examination of the intracellular viral proteins demonstrated that for all mutants, the amount of gag precursor was similar to the wild-type level. Moreover, the amount of mature gag CA that could be detected by this analysis was similar between each of the mutants and the wild type. This finding suggests that the transport of gag to the membrane and the initial stages of maturation were not affected by the mutations. The virus particles contained normal amounts of active reverse transcriptase, showing that the gag-pol polyprotein was incorporated and cleaved properly. Viral RNA was quantitatively and qualitatively similar in mutant and wild-type virions. However, the infectivity of the mutants virions differed; one of the thermosensitive linker insertions that had no effect on particle production at 36 degrees C was nevertheless noninfectious at that temperature. Together, these data suggest that the p10 protein is involved in a late steps of virus maturation, possibly budding, and perhaps also in an early event of viral infection.

Bovine leukemia virus RNA sequences involved in dimerization and specific gag protein binding: close relation to the packaging sites of avian, murine, and human retroviruses

In vitro detection of a specific complex of the bovine leukemia virus (BLV) MA(p15) protein and the 5'-terminal RNA dimer led to the hypothesis that the NH2-terminal domain of retrovirus gag protein precursor is involved in the selective viral RNA packaging mechanism. Here we describe mapping of the BLV RNA for dimer-forming and MA(p15)-binding abilities by a simple cDNA probing method followed by mutation analyses with the reactive U5-5' gag RNA. The RNA dimerization is mediated by the region harboring U5, the primer binding site (PBS), and the 30 bases immediately downstream of PBS. This conclusion is supported by computer-assisted RNA secondary-structure analysis which predicted a multibranched stem-loop folding throughout the dimer region determined. Another region from PBS to the 5'-terminal 60 residues of the gag gene, partially overlapping the dimer region, likely provides essential elements for the MA(p15) binding reaction, although the presence of either the 3' or 5' neighboring sequences increases the complex-forming efficiency significantly, and each of the substructures predicted within the core region has, if any, only very weak affinity to MA(p15). These in vitro characterizations of the BLV RNA may reflect general features of the specific protein-RNA interaction in the packaging events of various retroviruses. 5'-terminal folded structures of retroviral RNA molecules and their biological activities are discussed.

Bovine leukemia virus matrix-associated protein MA(p15): further processing and formation of a specific complex with the dimer of the 5'-terminal genomic RNA fragment

The retrovirus precursor protein has an arrangement of several characteristic domains with which it achieves selective and efficient packaging of the genome RNA during particle assembly. In this study, we analyzed the composition of the bovine leukemia virus (BLV) gag proteins and examined their RNA-binding properties in gel mobility shift assays, using various genomic RNA probes synthesized in vitro. Results obtained in amino acid sequence and composition analyses indicate that the matrix-associated protein MA(p15) is further processed by the BLV protease (PR) to generate MA(p10), a short peptide of seven amino acid residues, and p4. The gag precursor is now mapped as NH2-MA(p10)-p4-CA(p24)-NC(p12)-COOH. MA(p15) formed a specific complex with the dimer RNA of the U5-5' gag region presumed to contain the BLV packaging signal but not with other RNAs. The NH2-terminal cleavage product, MA(p10), bound all RNA fragments tested, while the COOH-terminal peptides with a sequence common to mammalian type C retroviruses had little affinity for RNA. The nucleocapsid protein NC(p12) bound to RNAs nonspecifically and randomly in the presence or absence of zinc ions. These results suggest a possible interaction of the NH2 terminus of the gag precursor with the 5' terminus of the genomic RNA in an early phase of particle assembly, when the conserved structure between the MA and CA domains might be involved.

Murine leukemia virus maturation: protease region required for conversion from "immature" to "mature" core form and for virus infectivity

Murine leukemia virus (MuLV) genome encodes a protease (Y. Yoshinaka, I. Katoh, T.D. Copeland, and S. Oroszlan (1985), Proc. Natl. Acad. Sci. USA 82, 1618-1622), which has been shown to cause maturation, specified as morphological conversion from "immature" to "mature" form of virus cores. To examine whether "immature" particles have infectivity or not, we constructed mutant DNAs with deletions in the protease region. The NIH/3T3 cells transfected with mutant DNAs produced "immature" particles, having immature morphology and containing Pr65gag, a polyprotein precursor of core proteins. The specific infectivity of the extracellularly released and purified particles was shown to be greatly reduced based on reverse transcriptase activity and protein content as compared with the "mature" particles obtained from wild-type DNA-transfected cells. The mutant genomes encoded functionally normal surface glycoprotein, gp70. These results strongly suggest that maturation of MuLV from "immature" to "mature" form of virus particles is indispensable to virus infectivity. The importance of processing of gag and pol, as well as transmembrane protein precursors by the viral protease is discussed.

Encapsidation sequences for spleen necrosis virus, an avian retrovirus, are between the 5' long terminal repeat and the start of the gag gene

The minimal cis-acting sequences outside the long terminal repeat (LTR) required for formation of an infectious retrovirus cloning vector were determined with recombinants of spleen necrosis virus (SNV) DNA and herpes simplex virus type 1 thymidine kinase gene. The 3' end of SNV DNA was removed to within 40 base pairs (bp) from the 3' LTR with only a 2-fold effect on the recovery of infectious recombinant virus. However, when the 5' end of SNV DNA was removed to within 100 bp from the 5' LTR, infectious recombinant virus was not recovered. Deletion mutants constructed around this latter region showed that nucleotides between 100 and 285 bp from the 5' LTR are necessary for encapsidation of genomic viral RNA. We call this region required for encapsidation E.

BALB/c myeloma retroviruses: peptide mapping and immunological analysis of the pp12 structural protein

We have compared the pp12 structural protein of the MO-21 and FL-1 BALB/c myeloma retroviruses with the pp12 of several prototype retroviruses. Chymotryptic peptide maps of 125I-labeled, immune-precipitated pp12 proteins revealed that the MO-21 and FL-1 proteins can be distinguished from one another. The MO-21 pp12 most closely resembled the NIH-xenotrophic virus pp12, and the FL-1 pp12 most closely resembled the pp12 of BV-2 and WN 1802 B. Competition radioimmunoassay studies showed that the MO-21 and FL-1 pp12 proteins are also antigenically distinct from one another and that both contain pp12 antigenic determinants of a xenotropic virus. These data support our proposal that these two BALB/c viruses contain a gag gene that was generated by recombination between endogenous eco- and xenotropic viral sequences.

Amino acid sequence homology between histone H5 and murine leukemia virus phosphoprotein p12

The amino terminal acid sequences of several mouse leukemia virus phosphoproteins (p12) show definite homology with the amino terminal conserved region of H5 histones, the phosphorylated nuclear proteins of nucleated erythrocytes. Differences in the amino acid compositions of the two groups of proteins seem to rule out the possibility that they evolved from a single common ancestral gene. The finding of sequence homology between viral p12's and cellular histones, however, is consistent with evolution of retrovirus structural proteins by a process of differentiation from preexisting cellular genes. The conserved primary and secondary structure at the amino terminal region, common to both groups of proteins, may be related to their common function of nucleic acid binding modulated by phosphorylation.

The in vitro of Rous sarcoma virus RNA and function of the viral protein during the viral replication

The gag gene and pol gene of the Rous sarcoma virus are translated in vitro from the 35S viral RNA. The env gene cannot be translated in vitro from the 35S RNA. For the in vitro translation of the src gene. 3' end fragments of the viral RNA are used. The gag protein p15 has a proteolytic activity and specifically processes its own protein precursor pr76. The gag protein p19 suppresses the in vitro translation of the pol gene.

A method for classification of 5' termini of retroviruses

A new method for the classification of retroviruses is presented. The scheme is based on the length and sequence of a DNA transcript of the 5' end of the genome. The method can be used to detect similarities between distantly related viruses as well as to discriminate between very closely related viruses. The method is applied to viruses isolated from mice, baboons, gibbons, a woolly monkey and chickens.

Phosphorylation of murine type C viral p12 proteins regulates their extent of binding to the homologous viral RNA

The purified p12 phosphoprotein of Rauscher murine leukemia virus was fractionated by ion exchange chromatography into subpopulations of molecules containing different amounts of covalently linked phosphate. Of the various phosphorylated forms of p12 protein purified from virions, only a species containing relatively little phosphate can bind in vitro to purified homologous 70S viral RNA. Using ultraviolet irradiation to stabilize ribonucleoprotein complexes in intact virions, the same molecular species of p12 phosphoprotein can be isolated in close association with the 70S viral genome. The results show that phosphorylation of type C viral p12 proteins influences the extent, but not the specificity, of their interaction with homologous viral RNA.

The genome-associated, specific RNA binding proteins of avian and mammalian type C viruses

A structural protein purified from the Rous sarcoma virus (RSV) can specificially bind in vitro to purified avian, but not mammalian, type C viral RNA. Following ultraviolet irradiation of viral particles under conditions which stabilize the polyploid 70S viral RNA, the same polypeptide can be directly purified from the RSV genome. Based on its electrophoretic mobility in polyacrylamide gels containing sodium dodecylsulfate, the RNA binding protein has been identified as the major phosphoprotein (p19) of avian type C viruses. Similar experiments show that the major phosphoproteins of mammalian type C viruses (p12 for murine viruses and p16 for endogenous primate viruses) are also the specific RNA binding proteins and, similarly, are found closely associated with the 70S RNA genomes in the intact viral particles.

Poly rC-oligo dG specific template activity of mouse viral DNA polymerase detected by isoelectric focusing

An acid fraction from polyacrylamide gel isoelectric focusing contained only DNA polymerase template activity for (rC)n-(dG)12-18 with little or no activity with (rA)n-(dT)12-18. A factor which stimulated (rC)n-(dG) 12-18 activity was also detected in low molecular weight fractions from G-100 Sephadex chromatography of the purified enzyme. Since (rC)n-(dG)12-18 acitivty is thought to be descriptive of viral DNA polymerase, this stimulation is proposed to be significant for the regulation and specificity of viral DNA synthesis.