Maturation-induced conformational changes of HIV-1 capsid protein and identification of two high affinity sites for cyclophilins in the C-terminal domain.
Viral incorporation of cyclophilin A (CyPA) during the assembly of human immunodeficiency virus type-1 (HIV-1) is crucial for efficient viral replication. CyPA binds to the previously identified Gly-Pro90 site of the capsid protein p24, but its role remained unclear. Here we report two new interaction sites between cyclophilins and p24. Both are located in the C-terminal domain of p24 around Gly-Pro157 and Gly-Pro224. Peptides corresponding to these regions showed higher affinities (Kd approximately 0.3 microM) for both CyPA and cyclophilin B than the best peptide derived from the Gly-Pro90 site ( approximately 8 microM) and thus revealed new sequence motifs flanking Gly-Pro that are important for tight interaction of peptide ligands with cyclophilins. Between CyPA and an immature (unprocessed) form of p24, a Kd of approximately 8 microM was measured, which corresponded with the Kd of the best of the Gly-Pro90 peptides, indicating an association via this site. Processing of immature p24 by the viral protease, yielding mature p24, elicited a conformational change in its C-terminal domain that was signaled by the covalently attached fluorescence label acrylodan. Consequently, CyPA and cyclophilin B bound with much higher affinities ( approximately 0.6 and 0.25 microM) to the new, i.e. maturation-generated sites. Since this domain is essential for p24 oligomerization and capsid cone formation, CyPA bound to the new sites might impair the regularity of the capsid cone and thus facilitate in vivo core disassembly after host infection. (+info)
The cleavable carboxyl-terminus of the small coat protein of cowpea mosaic virus is involved in RNA encapsidation.
The site of cleavage of the small coat protein of cowpea mosaic virus has been precisely mapped and the proteolysis has been shown to result in the loss of 24 amino acids from the carboxyl-terminus of the protein. A series of premature termination and deletion mutants was constructed to investigate the role or roles of these carboxyl-terminal amino acids in the viral replication cycle. Mutants containing premature termination codons at or downstream of the cleavage site were viable but reverted to wild-type after a single passage through cowpea plants, indicating that the carboxyl-terminal amino acids are important. Mutants with the equivalent deletions were genetically stable and shown to be debilitated with respect to virus accumulation. The specific infectivity of preparations of a deletion mutant (DM4) lacking all 24 amino acids was 6-fold less than that of a wild-type preparation. This was shown to be a result of DM4 preparations containing a much increased percentage (73%) of empty (RNA-free) particles, a finding that implicates the cleavable carboxyl-terminal residues in the packaging of the virion RNAs. (+info)
Precise packaging of the three genomic segments of the double-stranded-RNA bacteriophage phi6.
Bacteriophage phi6 has a genome of three segments of double-stranded RNA. Each virus particle contains one each of the three segments. Packaging is effected by the acquisition, in a serially dependent manner, of the plus strands of the genomic segments into empty procapsids. The empty procapsids are compressed in shape and expand during packaging. The packaging program involves discrete steps that are determined by the amount of RNA inside the procapsid. The steps involve the exposure and concealment of binding sites on the outer surface of the procapsid for the plus strands of the three genomic segments. The plus strand of segment S can be packaged alone, while packaging of the plus strand of segment M depends upon prior packaging of S. Packaging of the plus strand of L depends upon the prior packaging of M. Minus-strand synthesis begins when the particle has a full complement of plus strands. Plus-strand synthesis commences upon the completion of minus-strand synthesis. All of the reactions of packaging, minus-strand synthesis, and plus-strand synthesis can be accomplished in vitro with isolated procapsids. Live-virus constructions that are in accord with the model have been prepared. Mutant virus with changes in the packaging program have been isolated and analyzed. (+info)
Interactions of heterologous DNA with polyomavirus major structural protein, VP1.
'Empty' polyomavirus pseudocapsids, self-assembled from the major structural protein VP1, bind DNA non-specifically and can deliver it into the nuclei of mammalian cells for expression [Forstova et al. (1995) Hum. Gene Ther. 6, 297-3061. Formation of suitable VP1-DNA complexes appears to be the limiting step in this route of gene delivery. Here, the character of VP1-DNA interactions has been studied in detail. Electron microscopy revealed that VP1 pseudocapsids can create in vitro at least two types of interactions with double-stranded DNA: (i) highly stable complexes, requiring free DNA ends, where the DNA is partially encapsidated; and, (ii) weaker interactions of pseudocapsids with internal parts of the DNA chain. (+info)
Noncytopathic flavivirus replicon RNA-based system for expression and delivery of heterologous genes.
Noncytopathic replicons of the flavivirus Kunjin (KUN) were employed for expression and delivery of heterologous genes. Replicon vector C20DX2Arep, containing a unique cloning site followed by the sequence of 2A autoprotease of foot-and-mouth disease virus, was constructed and used for expression of a number of heterologous genes including chloramphenicol acetyltransferase (CAT), green fluorescent protein (GFP), beta-galactosidase, glycoprotein G of vesicular stomatitis virus, and the Core and NS3 genes of hepatitis C virus. The expression and proper processing of these genes upon transfection of BHK21 cells with the recombinant replicon RNAs were demonstrated by immunofluorescence, radioimmunoprecipitation, and appropriate reporter gene assays. Most of these recombinant KUN replicon RNAs were also successfully packaged into secreted virus-like particles (VLPs) by subsequent transfection with Semliki Forest virus replicon RNA expressing KUN structural genes. Infection of BHK21 and Vero cells with these VLPs resulted in continuous replication of the recombinant replicon RNAs and prolonged expression of the cloned genes without any cytopathic effect. We also developed a replicon vector for generation of stable cell lines continuously expressing heterologous genes by inserting an encephalomyelocarditis virus internal ribosomal entry site-neomycin transferase gene cassette into the 3'-untranslated region of the C20DX2Arep vector. Using this vector (C20DX2ArepNeo), stable BHK cell lines persistently expressing GFP and CAT genes for up to 17 passages were established. Thus noncytopathic KUN replicon vectors with the ability to be packaged into VLPs should provide a useful tool for the development of noninfectious and noncytopathic vaccines as well as for gene therapy applications. (+info)
Opposing effects of human immunodeficiency virus type 1 matrix mutations support a myristyl switch model of gag membrane targeting.
Targeting of the human immunodeficiency virus type 1 (HIV-1) Gag precursor Pr55(gag) to the plasma membrane, the site of virus assembly, is primarily mediated by the N-terminal matrix (MA) domain. N-myristylation of MA is essential for the stable association of Pr55(gag) with membranes and for virus assembly. We now show that single amino acid substitutions near the N terminus of MA can dramatically impair assembly without compromising myristylation. Subcellular fractionation demonstrated that Gag membrane binding was compromised to a similar extent as in the absence of the myristyl acceptor site, indicating that the myristyl group was not available for membrane insertion. Remarkably, the effects of the N-terminal modifications could be completely suppressed by second-site mutations in the globular core of MA. The compensatory mutations enhanced Gag membrane binding and increased viral particle yields above wild-type levels, consistent with an increase in the exposure of the myristyl group. Our results support a model in which the compact globular core of MA sequesters the myristyl group to prevent aberrant binding to intracellular membranes, while the N terminus is critical to allow the controlled exposure of the myristyl group for insertion into the plasma membrane. (+info)
Foamy virus capsids require the cognate envelope protein for particle export.
Unlike other subclasses of the Retroviridae the Spumavirinae, its prototype member being the so-called human foamy virus (HFV), require the expression of the envelope (Env) glycoprotein for viral particle egress. Both the murine leukemia virus (MuLV) Env and the vesicular stomatitis virus G protein, which efficiently pseudotype other retrovirus capsids, were not able to support export of HFV particles. Analysis of deletion and point mutants of the HFV Env protein revealed that the HFV Env cytoplasmic domain (CyD) is dispensable for HFV particle envelopment, release, and infectivity, whereas deletion of the membrane-spanning-domain (MSD) led to an accumulation of naked capsids in the cytoplasm. Neither alternative membrane association of HFV Env deletion mutants lacking the MSD and CyD via phosphoglycolipid anchor nor domain swapping mutants, with the MSD or CyD of MuLV Env and VSV-G exchanged against the corresponding HFV domains, could restore particle envelopment and the release defect of pseudotypes. However, replacement of the HFV MSD with that of MuLV led to budding of HFV capsids at the intracellular membranes. These virions were of apparently wild-type morphology but were not naturally released into the supernatant and they were noninfectious. (+info)
Vif and the p55(Gag) polyprotein of human immunodeficiency virus type 1 are present in colocalizing membrane-free cytoplasmic complexes.
The Vif protein of human immunodeficiency virus type 1 (HIV-1) is a potent regulator of viral infectivity. Current data posit that Vif functions late in replication to modulate assembly, budding, and/or maturation. Consistent with this model, earlier indirect immunofluorescence analyses of HIV-1-infected cells demonstrated that Vif and Gag colocalize to a substantial degree (J. H. M. Simon, R. A. M. Fouchier, T. E. Southerling, C. B. Guerra, C. K. Grant, and M. H. Malim, J. Virol. 71:5259-5267, 1997). Here, we describe a series of subcellular fractionation studies which indicate that Vif and the p55(Gag) polyprotein are present in membrane-free cytoplasmic complexes that copurify in sucrose density gradients and are stable in nonionic detergents. Both Vif and Gag are targeted to these complexes independent of each other, and their association with them appears to be mediated by protein-protein interactions. We propose that these complexes may represent viral assembly intermediates and that Vif is appropriately localized to influence the final stages of the viral life cycle and, therefore, the infectivity of progeny virions. (+info)