Phosphorylation and proteolytic cleavage of gag proteins in budded simian immunodeficiency virus

Identification and characterization of vp7 gene in Bombyx mori cytoplasmic polyhedrosis virus

The genome of Bombyx mori cytoplasmic polyhedrosis virus (BmCPV) contains 10 double stranded RNA segments (S1-S10). The segment 7 (S7) encodes 50kDa protein which is considered as a structural protein. The expression pattern and function of p50 in the virus life cycle are still unclear. In this study, the viral structural protein 7 (VP7) polyclonal antibody was prepared with immunized mouse to explore the presence of small VP7 gene-encoded proteins in Bombyx mori cytoplasmic polyhedrosis virus. The expression pattern of vp7 gene was investigated by its overexpression in BmN cells. In addition to VP7, supplementary band was identified with western blotting technique. The virion, BmCPV infected cells and midguts were also examined using western blotting technique. 4, 2 and 5 bands were detected in the corresponding samples, respectively. The replication of BmCPV genome in the cultured cells and midgut of silkworm was decreased by reducing the expression level of vp7 gene using RNA interference. In immunoprecipitation experiments, using a polyclonal antiserum directed against the VP7, one additional shorter band in BmCPV infected midguts was detected, and then the band was analyzed with mass spectrum (MS), the MS results showed thatone candidate interacted protein (VP7 voltage-dependent anion-selective channel-like isoform, VDAC) was identified from silkworm. We concluded that the novel viral product was generated with a leaky scanning mechanism and the VDAC may be an interacted protein with VP7.

Phosphorylation of human immunodeficiency virus type 1 capsid protein at serine 16, required for peptidyl-prolyl isomerase-dependent uncoating, is mediated by virion-incorporated extracellular signal-regulated kinase 2

We reported previously that Pin1 facilitates human immunodeficiency virus type 1 (HIV-1) uncoating by interacting with the capsid core through the phosphorylated Ser(16)-Pro(17) motif. However, the specific kinase responsible for Ser(16) phosphorylation has remained unknown. Here, we showed that virion-associated extracellular signal-regulated kinase 2 (ERK2) phosphorylates Ser(16). The characterization of immature virions produced by exposing chronically HIV-1LAV-1-infected CEM/LAV-1 cells to 10 µM saquinavir indicated that Ser(16) is phosphorylated after the initiation of Pr55(Gag) processing. Furthermore, a mass spectrometry-based in vitro kinase assay demonstrated that ERK2 specifically phosphorylated the Ser(16) residue in the Ser(16)-Pro(17) motif-containing substrate. The treatment of CEM/LAV-1 cells with the ERK2 inhibitor sc-222229 decreased the Ser(16) phosphorylation level inside virions, and virus partially defective in Ser(16) phosphorylation showed impaired reverse transcription and attenuated replication owing to attenuated Pin1-dependent uncoating. Furthermore, the suppression of ERK2 expression by RNA interference in CEM/LAV-1 cells resulted in suppressed ERK2 packaging inside virions and decreased the Ser(16) phosphorylation level inside virions. Interestingly, the ERK2-packaging-defective virus showed impaired reverse transcription and attenuated HIV-1 replication. Taken together, these findings provide insights into the as-yet-obscure processes in Pin1-dependent HIV-1 uncoating.

Regulation of SIV mac 239 basal long terminal repeat activity and viral replication in macrophages: functional roles of two CCAAT/enhancer-binding protein beta sites in activation and interferon beta-mediated suppression

CCAAT/enhancer-binding protein (C/EBP) beta and C/EBP sites in the HIV-1 long terminal repeat (LTR) are crucial for HIV-1 replication in monocyte/macrophages and for the ability of interferon beta (IFN beta) to inhibit ongoing active HIV replication in these cells. This IFN beta-mediated down-regulation involves induction of the truncated, dominant-negative isoform of C/EBP beta referred to as liver-enriched transcriptional inhibitory protein (LIP). Although binding of the C/EBP beta isoform to C/EBP sites in the simian immunodeficiency virus (SIV) LTR has previously been examined, the importance of these sites in core promoter-mediated transcription, virus replication, IFN beta-mediated regulation, and the relative binding of the two isoforms (C/EBP beta and LIP) has not been investigated. Here, we specifically examine two C/EBP sites, JC1 (-100 bp) and DS1 (+134 bp), located within the minimal region of the SIV LTR, required for core promoter-mediated transcription and virus replication in macrophages. Our studies revealed that the JC1 but not DS1 C/EBP site is important for basal level transcription, whereas the DS1 C/EBP site is imperative for productive virus replication in primary macrophages. In contrast, either JC1 or DS1 C/EBP site is sufficient to mediate IFN beta-induced down-regulation of SIV LTR activity and virus replication in these cells. We also characterized the differential binding properties of C/EBP beta and LIP to the JC1 and DS1 sites. In conjunction with previous studies from our laboratory, we demonstrate the importance of these sites in virus gene expression, and we propose a model for their role in establishing latency and persistence in macrophages in the brain.

VSV-G pseudotyping rescues HIV-1 CA mutations that impair core assembly or stability

BACKGROUND

The machinery of early HIV-1 replication still remains to be elucidated. Recently the viral core was reported to persist in the infected cell cytoplasm as an assembled particle, giving rise to the reverse transcription complex responsible for the synthesis of proviral DNA and its transport to the nucleus. Numerous studies have demonstrated that reverse transcription of the HIV-1 genome into proviral DNA is tightly dependent upon proper assembly of the capsid (CA) protein into mature cores that display appropriate stability. The functional impact of structural properties of the core in early replicative steps has yet to be determined.

RESULTS

Here, we show that infectivity of HIV-1 mutants bearing S149A and S178A mutations in CA can be efficiently restored when pseudotyped with vesicular stomatitis virus envelope glycoprotein, that addresses the mutant cores through the endocytic pathway rather than by fusion at the plasma membrane. The mechanisms by which these mutations disrupt virus infectivity were investigated. S149A and S178A mutants were unable to complete reverse transcription and/or produce 2-LTR DNA. Morphological analysis of viral particles and in vitro uncoating assays of isolated cores demonstrated that infectivity defects resulted from disruption of the viral core assembly and stability for S149A and S178A mutants, respectively. Consistent with these results, both mutants failed to saturate TRIM-antiviral restriction activity.

CONCLUSION

Defects generated at the level of core assembly and stability by S149A and S178A mutations are sensitive to the way of delivery of viral nucleoprotein complexes into the target cell. Addressing CA mutants through the endocytic pathway may compensate for defects generated at the reverse transcription/nuclear import level subsequent to impairment of core assembly or stability.

Phosphorylation of the termini of Cauliflower mosaic virus precapsid protein is important for productive infection

Cauliflower mosaic virus (CaMV) coat protein precursor (pre-CP) has 489 amino acids (p57) and is processed by the viral proteinase into three major forms: p44, p39, and p37. The N- and C-terminal extensions of pre-CP are released during maturation by the virus-encoded proteinase. We showed that these extensions are phosphorylated at several sites by host casein kinase II (CKII). We have identified the phosphorylated amino acids using an in vitro phosphorylation assay and tested the effect of mutation of these sites on viral infectivity. Mutation of serines S66, S68, and S72 to alanine in the N-terminal extension abolished phosphorylation of the protein in vitro. Also, mutation of all S and T residues in the C-terminus (450 to 489) made this region insensitive to CKII. Amino acid substitutions also were introduced into a full-length infectious clone of CaMV. Mutated forms of the virus with S66, S68, and S72 substituted with A or D showed a delay in symptom development and affected the infectivity of the virus. However, a mutant with an A substitution of all the S and T residues of the C-terminal extension of CP was not infectious. These results suggest that phosphorylation of the N- and C-termini of CaMV pre-CP plays an important role in the initiation of viral infection.

The SIVmac239 Pr55Gag isoform, SIV p43, suppresses proteolytic cleavage of Pr55Gag

In human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) the gag gene encodes the precursor polyprotein Pr55Gag, which is cleaved by the viral protease to produce the major structural proteins. Recently, it has been shown that HIV and SIV gag RNAs contain internal ribosome entry sites (IRESs) that mediate translation of Pr55Gag [Pr57Gag in HIV type 2 (HIV-2)] isoforms. Previously, we demonstrated that SIVmac239 p43(-), a mutant that does not express the Pr55Gag isoform, SIV p43, replicates more efficiently than wild-type (WT) SIVmac239 in cell culture. In this study, we characterize SIVmac239 p43(-) virion production and demonstrate that, in the absence of SIV p43, cleavage of Pr55Gag is increased in budded virions, resulting in a higher percentage of mature particles. Additionally, intracellular cleavage of Pr55Gag is increased in SIVmac239 p43(-), suggesting that SIV p43 suppresses premature cleavage of Pr55Gag by the viral protease.

An internal ribosome entry site promotes translation of a novel SIV Pr55(Gag) isoform

In complex retroviruses including simian immunodeficiency virus (SIV) and human immunodeficiency virus type 1 (HIV-1), the major structural proteins are encoded by the gag gene and translated as a precursor polyprotein, Pr55(Gag). An internal ribosome entry site (IRES) within the coding region of HIV-1 and HIV type 2 (HIV-2) gag RNA mediates expression of N-terminally truncated isoforms of the precursor polyprotein. In this study, we identify an N-terminally truncated SIV Pr55(Gag) isoform expressed from the SIV gag gene SIV p43. We demonstrate that translation of p43 occurs independently of Pr55(Gag) translation and initiates at an in-frame AUG within the gag transcript. We test several mechanisms that could mediate translation of p43 and report that translation of SIV p43 is driven by an IRES located entirely within the coding region of gag mRNA. Additionally, we present data that suggest SIV p43 affects viral replication in cell culture.

Conserved serines in simian immunodeficiency virus capsid are required for virus budding

The simian immunodeficiency virus (SIV) capsid protein (CA), a constituent of the Pr55Gag polyprotein, is phosphorylated in virions but not in virus-producing cells (Rue, S.M., Roos, J.W., Tarwater, P.M., Clements, J.E., Barber, S.A., 2005. Phosphorylation and proteolytic cleavage of gag proteins in budded simian immunodeficiency virus. J. Virol. 79 (4), 2484-2492.). Using phosphoamino acid analysis of CA, we show that serine is the primary phosphate acceptor. A series of substitution mutants of serines in the CA domain of Pr55Gag were constructed in the infectious viral clone SIVmac239. These virus mutants were examined for defects in virus replication and virion infectivity, release, and morphology, as well as alterations in phosphorylation of CA-containing proteins. Although the virus mutants exhibited a number of replication defects, none of these defects could be directly attributed to aberrant CA phosphorylation. A novel defect was a block in early budding, which was common among several virus mutants with substitutions in the CA N terminus. Together, these results indicate that certain residues in the CA N terminus are crucial for early budding events.