The host protein Staufen1 interacts with the Pr55Gag zinc fingers and regulates HIV-1 assembly via its N-terminus

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Characterization of the interaction between the HIV-1 Gag structural polyprotein and the cellular ribosomal protein L7 and its implication in viral nucleic acid remodeling

Background: In HIV-1 infected cells, the integrated viral DNA is transcribed by the host cell machinery to generate the full length HIV-1 RNA (FL RNA) that serves as mRNA encoding for the Gag and GagPol precursors. Virion formation is orchestrated by Gag, and the current view is that a specific interaction between newly made Gag molecules and FL RNA initiates the process. This in turn would cause FL RNA dimerization by the NC domain of Gag (GagNC). However the RNA chaperoning activity of unprocessed Gag is low as compared to the mature NC protein. This prompted us to search for GagNC co-factors. Results: Here we report that RPL7, a major ribosomal protein involved in translation regulation, is a partner of Gag via its interaction with the NC domain. This interaction is mediated by the NC zinc fingers and the N- and C-termini of RPL7, respectively, but seems independent of RNA binding, Gag oligomerization and its interaction with the plasma membrane. Interestingly, RPL7 is shown for the first time to exhibit a potent DNA/RNA chaperone activity higher than that of Gag. In addition, Gag and RPL7 can function in concert to drive rapid nucleic acid hybridization. Conclusions: Our results show that GagNC interacts with the ribosomal protein RPL7 endowed with nucleic acid chaperone activity, favoring the notion that RPL7 could be a Gag helper chaperoning factor possibly contributing to the start of Gag assembly.Instituto de Estudios Inmunológicos y Fisiopatológico

Properties and Functions of Feline Immunodeficiency Virus Gag Domains in Virion Assembly and Budding

Feline immunodeficiency virus (FIV) is an important cat pathogen worldwide whose biological and pathophysiological properties resemble those of human immunodeficiency virus type 1 (HIV-1). Therefore, the study of FIV not only benefits its natural host but is also useful for the development of antiviral strategies directed against HIV-1 infections in humans. FIV assembly results from the multimerization of a single but complex viral polypeptide, the Gag precursor. In this review, we will first give an overview of the current knowledge of the proteins encoded by the FIV pol, env, rev, vif, and orf-A genes, and then we will describe and discuss in detail the critical roles that each of the FIV Gag domains plays in virion morphogenesis. Since retroviral assembly is an attractive target for therapeutic interventions, gaining a better understanding of this process is highly desirable.Fil: González, Ana Silvia. Universidad de Belgrano. Facultad de Ciencias Exactas y Naturales. Laboratorio de Virología; ArgentinaFil: Affranchino, Jose Luis. Universidad de Belgrano. Facultad de Ciencias Exactas y Naturales. Laboratorio de Virología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

REGULATION OF GAG TRAFFICKING DURING RETROVIRUS ASSEMBLY AND BUDDING

Retroviral Gag polyproteins are necessary and sufficient for virus budding, but little is known about how thousands of Gag polyproteins are transported to the budding sites. The actin cytoskeleton has long been speculated to take a role in retrovirus assembly and recent studies suggest that HIV-1 assembly is regulated as early as viral RNA nuclear export, however specific mechanisms for these regulations are unknown. In contrast to numerous studies of HIV-1 Gag assembly and budding, relatively little is reported for these fundamental pathways among animal lentiviruses. In this project, we used bimolecular fluorescence complementation (BiFC) (1) to reveal intimate (<15nm) and specific associations between EIAV Gag and actin, but not tubulin; (2) to characterize and compare assembly sites and budding efficiencies of EIAV and HIV-1 Gag in both human and rodent cells when the mRNA nuclear export context is altered to be Rev-dependent or Rev-independent; (3) to reveal co-assembly of Rev-dependent and Rev-independent HIV-1 Gag and rescued assembly of Rev-independent HIV-1 Gag in human cells by in cis provided membrane targeting signals. The results of these studies showed that (1) multimerization of EIAV Gag was required for association with filamentous actin and this association correlated with Gag budding efficiency, suggesting that association of Gag multimers with filamentous actin is important for efficient virion production; (2) HIV-1 and EIAV Gag assembled in different cellular at sites, and HIV-1 but not EIAV Gag assembly was affected by mRNA nuclear export pathways, suggesting that alternative cellular pathways can be adapted for lentiviral Gag assembly and budding; (3) Rev-independent HIV-1 Gag was deficient in lipid raft targeting and its assembly and budding could be restored by membrane targeting signals provided in trans or in cis, suggesting that raft association is critical for HIV-1 assembly and budding and is regulated as early as nuclear export of Gag-encoding mRNA. The findings presented in these studies are significant for public health because a better understanding of the mechanism of retrovirus assembly and budding increase the potential to develop novel antiviral therapies targeting this critical step in the viral life cycle

Investigating the Role of Stem-Loop 1 in the Assembly Process of HIV-1

An important step in the production of infectious HIV-1 particles is maturation of the virus core. This process is completed by cleavage of the capsid (CA) domain of Gag, from its precursor, CA-SP1, by the viral protease. Large deletions in stem-loop 1 (SL1) in the 5’ untranslated region (UTR) of HIV-1 genomic RNA (gRNA) delay CA-SP1 processing. SL1 harbours the dimerisation initiation site (DIS) palindrome suggesting that efficient Gag processing may be linked to gRNA dimerization as shown in HIV-2. However, a dimerisation mutant with normal Gag processing was identified. Gag processing defects are hallmarks of late domain mutants, and SL1 mutation was found to result in reduced virus release. HIV-1 hijacks the host’s endosomal complexes required for transport (ESCRT) pathway to enable budding. An ESCRT-associated protein, ALIX, is known to be capable of binding to the nucleocapsid (NC) domain of Gag using lipids or RNA as a ‘bridge’ in vitro. It was hypothesised that SL1 mutation disrupts an RNA-dependent interaction that occurs during virus assembly. Consistent with this, an intact SL1 was found to be required for efficient ALIX function. Increasing the abundance of gRNA in the cell by expressing it in trans accelerated CA-SP1 processing in a manner that required ALIX’s binding motif in p6. Gag processing could also be accelerated by introducing previously identified compensatory mutations into the SP1 and NC domains of Gag, in a manner reminiscent of the actions of maturation inhibitor resistance mutations. The effects of the compensatory mutations were also dependent on intact late domain motifs. These data suggest that gRNA is involved in regulating virus budding and maturation through interaction with ALIX. A model is proposed whereby the packaging signal (psi) region of gRNA acts as a bridge between Gag and ALIX, acting as a checkpoint mechanism to promote Gag processing and optimise release of virions that have successfully packaged gRNA

Investigation of the sequences and structural elements required for HIV-2 infectivity and genome dimerisation

A unique feature of retroviruses is that two copies of the genomic RNA are packaged in each particle. The selective encapsidation of viral genomes is ensured by the binding of the Nucleocapsid to a specific motif on the RNA genome, the packaging signal (Psi). The Psi regions of many retroviruses overlap with sequences that promote the dimerisation of the genome, the dimerisation initiation site (DIS), and it has been suggested that the two mechanisms are closely linked. The aim of the research presented herein was to identify the sequences and structural elements required for the dimerisation of HIV-2 genomic RNA and to investigate the relationship between HIV-2 genome dimerisation and encapsidation, infectivity and particle morphogenesis. Mutations of two palindromic sequences, introduced in an infectious molecular clone of the HIV-2rod isolate, revealed that a palindrome within HIV-2 Psi was important for genome dimerisation. In contrast with previous studies, the palindrome termed DIS is not required for genome dimerisation and viral replication. Viruses bearing mutations within the Psi region failed to dimerise and to replicate in T-cells, a defect that could not be rescued by targeting more genomes to the cells. Psi-deleted viruses also displayed a defect in particle morphogenesis. A reduced packaging efficiency, combined with the presence of RNA monomers or unstable dimers in these virions, resulted in the production of fewer mature particles. However an increase in the number of particles containing two cores was observed. Further characterisation of the sequences and structural elements required for RNA dimerisation, packaging and viral replication showed that the formation of stem B is not critical for viral replication. However, a GGAG purine-rich motif at position 392-395 of the HIV-2rod genome is absolutely essential for genome dimerisation and viral infectivity, and a correlation was observed between dimer formation and viral replication

Cytoplasmic Localization of HIV-1 Vif Is Necessary for Apobec3G Neutralization and Viral Replication: A Dissertation

The binding of HIV-1 Vif to the cellular cytidine deaminase Apobec3G and subsequent prevention of Apobec3G virion incorporation have recently been identified as critical steps for the successful completion of the HIV-1 viral life cycle. This interaction occurs in the cytoplasm where Vif complexes with Apobec3G and directs its degradation via the proteasome pathway or sequesters it away from the assembling virion, thereby preventing viral packaging of Apobec3G. While many recent studies have focused on several aspects of Vif interaction with Apobec3G, the subcellular localization of Vif and Apobec3G during the viral life cycle have not been fully considered. Inhibition of Apobec3G requires direct interaction of Vif with Apobec3G, which can only be achieved when both proteins are present in the same subcellular compartment. In this thesis, a unique approach was utilized to study the impact of Vif subcellular localization on Vif function. The question of whether localization could influence function was brought about during the course of studying a severely attenuated viral isolate from a long-term non-progressor who displayed a remarkable disease course. Initial observations indicated that this highly attenuated virus contained a mutant Vif protein that inhibited growth and replication. Upon further investigation, it was found that the Vif defect was atypical in that the mutant was fully functional in in vitro assays, but that it was aberrantly localized to the nucleus in the cell. This provided the basis for the study of Vif localization and its contribution to Vif function. In addition to the unique Vif mutant that was employed, while determining the localization and replication phenotypes of the differentially localized Vif proteins, a novel pathway for Vif function was defined. Copious publications have recently defined the mechanism for Vif inhibition of Apobec3G. Vif is able to recruit Apobec3G into a complex that is targeted for degradation by the proteasome. However, this directed degradation model did not fully explain the complete neutralization of Apobec3G observed in cell culture. Other recent works have proposed the existence of a second, complementary pathway for Vif function. This pathway is defined here as formation of an aggresome that prevents Apobec3G packaging by binding and sequestering Apobec3G in a perinuclear aggregate. This second mechanism is believed to work in parallel with the already defined directed degradation pathway to promote complete exclusion of Apobec3G from the virion. The data presented here provide insight into two areas of HIV research. First, the work on the naturally occurring Vif mutant isolated from a long-term non-progress or confirms the importance of Vif in in vivo pathogenesis and points to Vif as a potentially useful gene for manipulation in vaccine or therapy design due to its critical contributions to in vivo virus replication. Additionally, the work done to address the subcellular localization of Vif led to the proposal of a second pathway for Vif function. This could have implications in the field of basic Vif research in terms of completely understanding and defining the functions of Vif. Again, a more complete knowledge about Vif can help in the development of novel therapies aimed at disrupting Vif function and abrogating HIV-1 replication

Regulation of HIV Gag conformation and complex formation

The results of this research will help in a better understanding of HIV morphogenesis, which will hopefully lead to a structure based drug design towards the virus