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Leslie J Parent – 1st expert on this subject based on the ideXlab platform
TNPO3-mediated nuclear entry of the Rous sarcoma virus Gag protein is independent of the cargo-binding domainbioRxiv, 2020Co-Authors: Breanna L. Rice, Matthew S. Stake, Leslie J ParentAbstract:
Retroviral Gag polyproteins orchestrate the assembly and release of nascent virus particles from the plasma membranes of infected cells. Although it was traditionally thought that Gag proteins trafficked directly from the cytosol to the plasma membrane, we discovered that the oncogenic avian Alpharetrovirus Rous sarcoma virus (RSV) Gag protein undergoes transient nucleocytoplasmic transport as an intrinsic step in virus assembly. Using a genetic approach in yeast, we identified three karyopherins that engage the two independent nuclear localization signals (NLSs) in Gag. The primary NLS is in the nucleocapsid (NC) domain of Gag and binds directly to importin-α, which recruits importin-β to mediate nuclear entry. The second NLS, which resides in the matrix (MA) domain, is dependent on importin-11 and transportin-3 (TNPO3), known as MTR10p and Kap120p in yeast, although it is not clear whether these import factors are independent or additive. The functionality of importin α/β and importin-11 has been verified in avian cells, whereas the role of TNPO3 has not been studied. In this report, we demonstrate that TNPO3 mediates nuclear entry of Gag and directly binds to Gag. To our surprise, this interaction did not require the cargo-binding domain of TNPO3, which typically mediates nuclear entry for other binding partners of TNPO3 including SR-domain containing splicing factors and tRNAs that re-enter the nucleus. These results suggest that RSV hijacks the host nuclear import pathway using a unique mechanism, potentially allowing other cargo to bind TNPO3 simultaneously.
Orchestrating the Selection and Packaging of Genomic RNA by Retroviruses: An Ensemble of Viral and Host FactorsViruses, 2016Co-Authors: Rebecca J. Kaddis Maldonado, Leslie J ParentAbstract:
Infectious retrovirus particles contain two copies of unspliced viral RNA that serve as the viral genome. Unspliced retroviral RNA is transcribed in the nucleus by the host RNA polymerase II and has three potential fates: (1) it can be spliced into subgenomic messenger RNAs (mRNAs) for the translation of viral proteins; or it can remain unspliced to serve as either (2) the mRNA for the translation of Gag and Gag–Pol; or (3) the genomic RNA (gRNA) that is packaged into virions. The Gag structural protein recognizes and binds the unspliced viral RNA to select it as a genome, which is selected in preference to spliced viral RNAs and cellular RNAs. In this review, we summarize the current state of understanding about how retroviral packaging is orchestrated within the cell and explore potential new mechanisms based on recent discoveries in the field. We discuss the cis-acting elements in the unspliced viral RNA and the properties of the Gag protein that are required for their interaction. In addition, we discuss the role of host factors in influencing the fate of the newly transcribed viral RNA, current models for how retroviruses distinguish unspliced viral mRNA from viral genomic RNA, and the possible subcellular sites of genomic RNA dimerization and selection by Gag. Although this review centers primarily on the wealth of data available for the Alpharetrovirus Rous sarcoma virus, in which a discrete RNA packaging sequence has been identified, we have also summarized the cis- and trans-acting factors as well as the mechanisms governing gRNA packaging of other retroviruses for comparison.
Interplay between the alpharetroviral Gag protein and SR Proteins SF2 and SC35 in the nucleusFrontiers in Microbiology, 2015Co-Authors: Breanna L. Rice, Matthew S. Stake, Rebecca J. Kaddis, Timothy L. Lochmann, Leslie J ParentAbstract:
Retroviruses are positive-sense, single-stranded RNA viruses that reverse transcribe their RNA genomes into double-stranded DNA for integration into the host cell chromosome. The integrated provirus is used as a template for the transcription of viral RNA. The full-length viral RNA can be used for the translation of the Gag and Gag-Pol structural proteins or as the genomic RNA (gRNA) for encapsidation into new virions by the Gag protein. The mechanism by which Gag selectively incorporates unspliced gRNA into virus particles is poorly understood. Although Gag was previously thought to localize exclusively to the cytoplasm and plasma membrane where particles are released, we found that the Gag protein of Rous sarcoma virus, an Alpharetrovirus, undergoes transient nuclear trafficking. When the nuclear export signal of RSV Gag is mutated (Gag.L219A), the protein accumulates in discrete subnuclear foci reminiscent of nuclear bodies such as splicing speckles, paraspeckles, and PML bodies. In this report, we observed that RSV Gag.L219A foci appeared to be tethered in the nucleus, partially co-localizing with the splicing speckle components SC35 and SF2. Overexpression of SC35 increased the number of Gag.L219A nucleoplasmic foci, suggesting that SC35 may facilitate the formation of Gag foci. We previously reported that RSV Gag nuclear trafficking is required for efficient gRNA packaging. Together with the data presented herein, our findings raise the intriguing hypothesis that RSV Gag may co-opt splicing factors to localize near transcription sites. Because splicing occurs co-transcriptionally, we speculate that this mechanism could allow Gag to associate with unspliced viral RNA shortly after its transcription initiation in the nucleus, before the viral RNA can be spliced or exported from the nucleus as an mRNA template.
Maxine L Linial – 2nd expert on this subject based on the ideXlab platform
deletion of a cys his motif from the Alpharetrovirus nucleocapsid domain reveals late domain mutant like budding defectsVirology, 2006Co-Authors: Maxine L LinialAbstract:
The Rous sarcoma virus (RSV) Gag polyprotein is the only protein required for virus assembly and release. We previously found that deletion of either one of the two Cys-His (CH) motifs in the RSV nucleocapsid (NC) protein did not abrogate Gag-Gag interactions, RNA binding, or packaging but greatly reduced virus production (E-G. Lee, A. Alidina et al., J. Virol. 77: 2010-2020, 2003). In this report, we have further investigated the effects of mutations in the CH motifs on virus assembly and release. Precise deletion of either CH motif, without affecting surrounding basic residues, reduced virus production by approximately 10-fold, similar to levels seen for late (L) domain mutants. Strikingly, transmission electron microscopy revealed that virions of both DeltaCH1 and DeltaCH2 mutants were assembled normally at the plasma membrane but were arrested in budding. Virus particles remained tethered to the membrane or to each other, reminiscent of L domain mutants, although the release defect appears to be independent of the L domain functions. Therefore, two CH motifs are likely to be required for budding independent of a requirement for either Gag-Gag interactions or RNA packaging.
Deletion of a Cys-His motif from the Alpharetrovirus nucleocapsid domain reveals late domain mutant-like budding defects.Virology, 2006Co-Authors: Maxine L LinialAbstract:
Abstract The Rous sarcoma virus (RSV) Gag polyprotein is the only protein required for virus assembly and release. We previously found that deletion of either one of the two Cys–His (CH) motifs in the RSV nucleocapsid (NC) protein did not abrogate Gag–Gag interactions, RNA binding, or packaging but greatly reduced virus production (E-G. Lee, A. Alidina et al., J. Virol. 77: 2010–2020, 2003). In this report, we have further investigated the effects of mutations in the CH motifs on virus assembly and release. Precise deletion of either CH motif, without affecting surrounding basic residues, reduced virus production by approximately 10-fold, similar to levels seen for late (L) domain mutants. Strikingly, transmission electron microscopy revealed that virions of both ΔCH1 and ΔCH2 mutants were assembled normally at the plasma membrane but were arrested in budding. Virus particles remained tethered to the membrane or to each other, reminiscent of L domain mutants, although the release defect appears to be independent of the L domain functions. Therefore, two CH motifs are likely to be required for budding independent of a requirement for either Gag–Gag interactions or RNA packaging.
Basic Residues of the Retroviral Nucleocapsid Play Different Roles in Gag-Gag and Gag-Ψ RNA InteractionsJournal of Virology, 2004Co-Authors: Maxine L LinialAbstract:
The Orthoretrovirus Gag interaction (I) domain maps to the nucleocapsid (NC) domain in the Gag polyprotein. We used the yeast two-hybrid system to analyze the role of Alpharetrovirus NC in Gag-Gag interactions and also examined the efficiency of viral assembly and release in vivo. We could delete either or both of the two Cys-His (CH) boxes without abrogating Gag-Gag interactions. We found that as few as eight clustered basic residues, attached to the C terminus of the spacer peptide separating the capsid (CA) and NC domains in the absence of NC, was sufficient for Gag-Gag interactions. Our results support the idea that a sufficient number of basic residues, rather than the CH boxes, play the important role in Gag multimerization. We also examined the requirement for basic residues in Gag for packaging of specific packaging signal (Ψ)-containing RNA. Using a yeast three-hybrid RNA-protein interaction assay, second-site suppressors of a packaging-defective Gag mutant were isolated, which restored Ψ RNA binding. These suppressors mapped to the p10 or CA domains in Gag and resulted in either introduction of a positively charged residue or elimination of a negatively charged one. These results imply that the structural interactions of NC with other domains of Gag are necessary for Ψ RNA binding. Taken together, our results show that while Gag assembly only requires a certain number of positively charged amino acids, Gag binding to genomic RNA for packaging requires more complex interactions inherent in the protein tertiary structure.
Lisa Z Scheifele – 3rd expert on this subject based on the ideXlab platform
importin β family members mediate Alpharetrovirus gag nuclear entry via interactions with matrix and nucleocapsidJournal of Virology, 2006Co-Authors: Kristin L Butterfieldgerson, Lisa Z Scheifele, Eileen P Ryan, Anita K Hopper, Leslie J ParentAbstract:
The retroviral Gag polyprotein orchestrates the assembly and release of virus particles from infected cells. We previously reported that nuclear transport of the Rous sarcoma virus (RSV) Gag protein is intrinsic to the virus assembly pathway. To identify cis- and trans-acting factors governing nucleocytoplasmic trafficking, we developed novel vectors to express regions of Gag in Saccharomyces cerevisiae. The localization of Gag proteins was examined in the wild type and in mutant strains deficient in members of the importin-β family. We confirmed the Crm1p dependence of the previously identified Gag p10 nuclear export signal. The known nuclear localization signal (NLS) in MA (matrix) was also functional in S. cerevisiae, and additionally we discovered a novel NLS within the NC (nucleocapsid) domain of Gag. MA utilizes Kap120p and Mtr10p import receptors while nuclear entry of NC involves the classical importin-α/β (Kap60p/95p) pathway. NC also possesses nuclear targeting activity in avian cells and contains the primary signal for the import of the Gag polyprotein. Thus, the nucleocytoplasmic dynamics of RSV Gag depend upon the counterbalance of Crm1p-mediated export with two independent NLSs, each interacting with distinct nuclear import factors.