Budding of retroviruses from polarized epithelial Madin–Darby canine kidney cells (MDCK) takes place specifically at the basolateral membrane surface. This sorting event is suspected to require a specific signal harbored by the viral envelope glycoprotein and it was previously shown that, as for most basolateral proteins, the intracytoplasmic domain plays a crucial role in this targeting phenomenon. It is well known that tyrosine‐based motifs are a central element in basolateral targeting signals. In the present study, site‐directed mutagenesis was used to generate conservative or non‐conservative substitutions of each four intracytoplasmic tyrosines of the human immunodeficiency virus (HIV‐1) envelope glycoprotein. This approach revealed that the membrane‐proximal tyrosine is essential to ensure both the basolateral localization of envelope glycoprotein and the basolateral targeting of HIV‐1 virions. Substitutions of the membrane‐proximal tyrosine did not appear to affect incorporation of envelope glycoprotein into the virions, as assayed by virion infectivity and protein content, nor its capability to ensure its role in viral infection, as determined by viral multiplication kinetics. Altogether, these results indicate that the intracytoplasmic domain of the HIV‐1 envelope glycoprotein harbors a unique, tyrosine‐based, basolateral targeting signal. Such a tyrosine‐based targeting signal may play a fundamental role in HIV transmission and pathogenesis.

The synthesis of the Gag-Pol polyprotein, the precursor of the enzymes of the human immunodeficiency virus type 1 (HIV-1), requires a programmed −1 ribosomal frameshift. This frameshift has been investigated so far only for subtype B of HIV-1 group M. In this subtype, the frameshift stimulatory signal was found to be a two-stem helix, in which a three-purine bulge interrupts the two stems. In this study, using a luciferase reporter system, we compare, for the first time, the frameshift efficiency of all the subtypes of group M. Mutants of subtype B, including a natural variant were also investigated. Our results with mutants of subtype B confirm that the bulge and the lower stem of the frameshift stimulatory signal contribute to the frameshift in addition to the upper stem-loop considered previously as the sole participant. Our results also show that the frameshift stimulatory signal of all of the other subtypes of group M can be folded into the same structure as in subtype B, despite sequence variations. Moreover, the frameshift efficiency of these subtypes, when assessed in cultured cells, falls within a narrow window (the maximal deviation from the mean value calculated from the experimental values of all the subtypes being ∼35%), although the predicted thermodynamic stability of the frameshift stimulatory signal differs between the subtypes (from −17.2 kcal/mole to −26.2 kcal/mole). The fact that the frameshift efficiencies fall within a narrow range for all of the subtypes of HIV-1 group M stresses the potential of the frameshift event as an antiviral target.

Human immunodeficiency virus type 1 (HIV-1) has been shown to exhibit a specific basolateral release in polarized epithelial cells. Previous investigators have used vaccinia virus recombinants expressing HIV proteins to demonstrate that virus release is nonpolarized in the absence of viral envelope glycoproteins. In this study, we developed a transient expression system which allows the use of Madin-Darby canine kidney polarized epithelial cells directly grown on semipermeable membranes. This procedure allowed us to investigate polarized HIV viral budding following introduction of proviral DNA constructs. Expression of env gene products in trans demonstrated the ability to polarize env-negative viruses in a dose-dependent manner. The targeting signal for polarized virus release was shown to be present in the envelope gp4l transmembrane protein and absent from the gpl20 portion ofenv. At least part of this signal is within the gp41 intracytoplasmic domain. Mutants of the pl79a9 matrix protein were shown to be nonpolarized only when unable to interact with the envelope glycoproteins. Together, these data are consistent with a model of polarized virus budding in which capsid proteins, lacking a targeting signal, are targeted for specific basolateral release via an interaction of p17 with the envelope glycoprotein containing the polarization signal in its intracytoplasmic domain.

We have studied the integrated proviruses in Moloney murine leukemia virus-induced rat thymomas. By Southern blot analysis, we found several complete integrated proviruses in each tumor. In most tumors, we could not detect defective or recombinant proviruses. Several of these integrated proviruses (with their flanking cellular sequences) from a single tumor were cloned in Charon 4A. Their flanking cellular fragments were subcloned into pBR322 and used as a probe to screen other thymoma DNAs. With one clone (pMo-

Upon viral infection, a tug of war is triggered between host cells and viruses to maintain/gain control of vital cellular functions, the result of which will ultimately dictate the fate of the host cell. Among these essential cellular functions, alternative splicing (AS) is an important RNA maturation step that allows exons, or parts of exons, and introns to be retained in mature transcripts, thereby expanding proteome diversity and function. AS is widespread in higher eukaryotes, as it is estimated that nearly all genes in humans are alternatively spliced. Recent evidence has shown that upon infection by numerous viruses, the AS landscape of host‐cells is affected. In this review, we summarize recent advances in our understanding of how virus infection impacts the AS of cellular transcripts. We also present various molecular mechanisms allowing viruses to modulate cellular AS. Finally, the functional consequences of these changes in the RNA splicing signatures during virus–host interactions are discussed. This article is categorized under: RNA in Disease and Development > RNA in Disease RNA Processing > Splicing Regulation/Alternative Splicing

Previous studies have shown that the reovirus 1 core protein harbors a putative nucleotide-binding motif and exhibits an affinity for nucleic acids. In addition, a nucleoside triphosphate phosphohydrolase activity present in reovirus cores has been recently assigned to 1 using gene reassortment analysis. In this study, it was demonstrated that the recombinant 1 protein, expressed in the yeast Pichia pastoris, is able to hydrolyze nucleoside 5-triphosphates or deoxynucleoside 5-triphosphates. This activity was absolutely dependent on the presence of a divalent cation, Mg 2؉ or Mn 2؉. The protein can also unwind double-stranded nucleic acid molecules in the presence of a nucleoside 5-triphosphate or deoxynucleoside 5-triphosphate. These results provide the first biochemical evidence that the reovirus 1 protein is a nucleoside triphosphate phosphohydrolase/helicase and strongly support the idea that 1 participates in transcription of the viral genome.Mammalian reoviruses are members of the Reoviridae family, and since their genome is made up of 10 segments of double-stranded RNA (dsRNA) 1 and replicates in the cytoplasm, they must encode their own transcriptional and replicative enzymes (1). During reovirus infection, the viral genome remains in the inner capsid (core) of the virus, composed of two major (1 and 2) and two minor (3 and 2) proteins. Gene reassortment experiments have resulted in the assignment of transcriptase activity to 3 (2). Although functions of other core proteins have not been firmly established, it is suspected that additional enzymatic functions are needed to achieve transcription and replication of the viral genome. For example, it has been postulated, by analogy with other viruses, that a helicase function could be present in the viral core (1).Nucleic acid helicases unwind double-stranded DNA and/or RNA, a process energetically coupled to the hydrolysis of nucleoside 5Ј-triphosphates (NTPs) or deoxynucleoside 5Ј-triphosphates (dNTPs) (3, 4). Helicases play a key role in nucleic acid replication, transcription, splicing, translocation, recombination, and repair (5-8). Helicases of prokaryotic, eukaryotic, and viral origins have been isolated and classified into defined superfamilies (9 -14). These proteins are characterized by seven conserved motifs designated I, Ia, and II-VI (15). Motifs I and II are very well conserved and correspond to the A and B consensus sequences of a nucleotide-binding domain (16). Superfamily II includes an expanding group of DNA and RNA helicases that harbor a DEA(D/H) sequence in motif II (17). The sequences present in motifs III-V are less strictly conserved, and their roles are not clearly defined, whereas motif VI is supposed to be involved in nucleic acid binding given its high content of positively charged amino acids (13).The 1 protein, a major component of the reovirus core, exhibits an affinity for dsRNA and dsDNA in filter binding assays and can also bind single-stranded RNA in gel retardation assays (18).2 Furthermore, analysis of gene reassortment ha...

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Alternative splicing (AS) is a central mechanism of genetic regulation which modifies the sequence of RNA transcripts in higher eukaryotes. AS has been shown to increase both the variability and diversity of the cellular proteome by changing the composition of resulting proteins through differential choice of exons to be included in mature mRNAs. In the present study, alterations to the global RNA splicing landscape of cellular genes upon viral infection were investigated using mammalian reovirus as a model. Our study provides the first comprehensive portrait of global changes in the RNA splicing signatures that occur in eukaryotic cells following infection with a human virus. We identify 240 modified alternative splicing events upon infection which belong to transcripts frequently involved in the regulation of gene expression and RNA metabolism. Using mass spectrometry, we also confirm modifications to transcript-specific peptides resulting from AS in virus-infected cells. These findings provide additional insights into the complexity of virus-host interactions as these splice variants expand proteome diversity and function during viral infection.