Role of human immunodeficiency virus type 1 matrix phosphorylation in an early postentry step of virus replication

Current Status of Biomedical Products for Gene and Cell Therapy of Recessive Dystrophic Epidermolysis Bullosa

This detailed review describes innovative strategies and current products for gene and cell therapy at different stages of research and development to treat recessive dystrophic epidermolysis bullosa (RDEB) which is associated with the functional deficiency of collagen type VII alpha 1 (C7) caused by defects in the COL7A1 gene. The use of allogenic mesenchymal stem/stromal cells, which can be injected intradermally and intravenously, appears to be the most promising approach in the field of RDEB cell therapy. Injections of genetically modified autologous dermal fibroblasts are also worth mentioning under this framework. The most common methods of RDEB gene therapy are gene replacement using viral vectors and gene editing using programmable nucleases. Ex vivo epidermal transplants (ETs) based on autologous keratinocytes (Ks) have been developed using gene therapy methods; one such ET successively passed phase III clinical trials. Products based on the use of two-layer transplants have also been developed with both types of skin cells producing C7. Gene products have also been developed for local use. To date, significant progress has been achieved in the development of efficient biomedical products to treat RDEB, one of the most severe hereditary diseases.

Evolution toward beta common chain receptor usage links the matrix proteins of HIV-1 and its ancestors to human erythropoietin

The HIV-1 matrix protein p17 (p17) is a pleiotropic molecule impacting on different cell types. Its interaction with many cellular proteins underlines the importance of the viral protein as a major determinant of human specific adaptation. We previously showed the proangiogenic capability of p17. Here, by integrating functional analysis and receptor binding, we identify a functional epitope that displays molecular mimicry with human erythropoietin (EPO) and promotes angiogenesis through common beta chain receptor (βCR) activation. The functional EPO-like epitope was found to be present in the matrix protein of HIV-1 ancestors SIV originated in chimpanzees (SIVcpz) and gorillas (SIVgor) but not in that of HIV-2 and its ancestor SIVsmm from sooty mangabeys. According to biological data, evolution of the EPO-like epitope showed a clear differentiation between HIV-1/SIVcpz-gor and HIV-2/SIVsmm branches, thus highlighting this epitope on p17 as a divergent signature discriminating HIV-1 and HIV-2 ancestors. P17 is known to enhance HIV-1 replication. Similarly to other βCR ligands, p17 is capable of attracting and activating HIV-1 target cells and promoting a proinflammatory microenvironment. Thus, it is tempting to speculate that acquisition of an epitope on the matrix proteins of HIV-1 ancestors capable of triggering βCR may have represented a critical step to enhance viral aggressiveness and early human-to-human SIVcpz/gor dissemination. The hypothesis that the p17/βCR interaction and βCR abnormal stimulation may also play a role in sustaining chronic activation and inflammation, thus marking the difference between HIV-1 and HIV-2 in term of pathogenicity, needs further investigation.

Post-Translational Modifications of Retroviral HIV-1 Gag Precursors: An Overview of Their Biological Role

Protein post-translational modifications (PTMs) play key roles in eukaryotes since they finely regulate numerous mechanisms used to diversify the protein functions and to modulate their signaling networks. Besides, these chemical modifications also take part in the viral hijacking of the host, and also contribute to the cellular response to viral infections. All domains of the human immunodeficiency virus type 1 (HIV-1) Gag precursor of 55-kDa (Pr55^Gag), which is the central actor for viral RNA specific recruitment and genome packaging, are post-translationally modified. In this review, we summarize the current knowledge about HIV-1 Pr55^Gag PTMs such as myristoylation, phosphorylation, ubiquitination, sumoylation, methylation, and ISGylation in order to figure out how these modifications affect the precursor functions and viral replication. Indeed, in HIV-1, PTMs regulate the precursor trafficking between cell compartments and its anchoring at the plasma membrane, where viral assembly occurs. Interestingly, PTMs also allow Pr55^Gag to hijack the cell machinery to achieve viral budding as they drive recognition between viral proteins or cellular components such as the ESCRT machinery. Finally, we will describe and compare PTMs of several other retroviral Gag proteins to give a global overview of their role in the retroviral life cycle.

Toward Combined Cell and Gene Therapy for Genodermatoses

To date, more than 200 monogenic, often devastating, skin diseases have been described. Because of unmet medical needs, development of long-lasting and curative therapies has been consistently attempted, with the aim of correcting the underlying molecular defect. In this review, we will specifically address the few combined cell and gene therapy strategies that made it to the clinics. Based on these studies, what can be envisioned for the future is a patient-oriented strategy, built on the specific features of the individual in need. Most likely, a combination of different strategies, approaches, and advanced therapies will be required to reach the finish line at the end of the long and winding road hampering the achievement of definitive treatments for genodermatoses.

Cellular and viral determinants of retroviral nuclear entry

Retroviruses must integrate their cDNA into the host genome to generate proviruses. Viral DNA-protein complexes interact with cellular proteins and produce pre-integration complexes, which carry the viral genome and cross the nuclear pore channel to enter the nucleus and integrate viral DNA into host chromosomal DNA. If the reverse transcripts fail to integrate, linear or circular DNA species such as 1- and 2-long terminal repeats are generated. Such complexes encounter numerous cellular proteins in the cytoplasm, which restrict viral infection and protect the nucleus. To overcome host cell defenses, the pathogens have evolved several evasion strategies. Viral proteins often contain nuclear localization signals, allowing entry into the nucleus. Among more than 1000 proteins identified as required for HIV infection by RNA interference screening, karyopherins, cleavage and polyadenylation specific factor 6, and nucleoporins have been predominantly studied. This review discusses current opinions about the synergistic relationship between the viral and cellular factors involved in nuclear import, with focus on the unveiled mysteries of the host-pathogen interaction, and highlights novel approaches to pinpoint therapeutic targets.

Deletions in the fifth alpha helix of HIV-1 matrix block virus release

The matrix (MA) protein of HIV-1 is the N-terminal component of the Gag structural protein and is critical for the early and late stages of viral replication. MA contains five α-helices (α1-α5). Deletions in the N-terminus of α5 as small as three amino acids impaired virus release. Electron microscopy of one deletion mutant (MA∆96-120) showed that its particles were tethered to the surface of cells by membranous stalks. Immunoblots indicated all mutants were processed completely, but mutants with large deletions had alternative processing intermediates. Consistent with the EM data, MA∆96-120 retained membrane association and multimerization capability. Co-expression of this mutant inhibited wild type particle release. Alanine scanning mutation in this region did not affect virus release, although the progeny virions were poorly infectious. Combined, these data demonstrate that structural ablation of the α5 of MA inhibits virus release.

Membrane interaction of retroviral Gag proteins

Assembly of an infectious retroviral particle relies on multimerization of the Gag polyprotein at the inner leaflet of the plasma membrane. The three domains of Gag common to all retroviruses - MA, CA, and NC - provide the signals for membrane binding, assembly, and viral RNA packaging, respectively. These signals do not function independently of one another. For example, Gag multimerization enhances membrane binding and is more efficient when NC is interacting with RNA. MA binding to the plasma membrane is governed by several principles, including electrostatics, recognition of specific lipid head groups, hydrophobic interactions, and membrane order. HIV-1 uses many of these principles while Rous sarcoma virus (RSV) appears to use fewer. This review describes the principles that govern Gag interactions with membranes, focusing on RSV and HIV-1 Gag. The review also defines lipid and membrane behavior, and discusses the complexities in determining how lipid and membrane behavior impact Gag membrane binding.

The Nef-infectivity enigma: mechanisms of enhanced lentiviral infection

The Nef protein is an essential factor for lentiviral pathogenesis in humans and other simians. Despite a multitude of functions attributed to this protein, the exact role of Nef in disease progression remains unclear. One of its most intriguing functions is the ability of Nef to enhance the infectivity of viral particles. In this review we will discuss current insights in the mechanism of this well-known, yet poorly understood Nef effect. We will elaborate on effects of Nef, on both virion biogenesis and the early stage of the cellular infection, that might be involved in infectivity enhancement. In addition, we provide an overview of different HIV-1 Nef domains important for optimal infectivity and briefly discuss some possible sources of the frequent discrepancies in the field. Hereby we aim to contribute to a better understanding of this highly conserved and therapeutically attractive Nef function.

A highly conserved residue in the C-terminal helix of HIV-1 matrix is required for envelope incorporation into virus particles

The incorporation of viral envelope (Env) glycoproteins into nascent particles is an essential step in the production of infectious human immunodeficiency virus type 1 (HIV-1). This process has been shown to require interactions between Env and the matrix (MA) domain of the Gag polyprotein. Previous studies indicate that several residues in the N-terminal region of MA are required for Env incorporation. However, the precise mechanism by which Env proteins are acquired during virus assembly has yet to be fully defined. Here, we examine whether a highly conserved glutamate at position 99 in the C-terminal helix is required for MA function and HIV-1 replication. We analyze a panel of mutant viruses that contain different amino acid substitutions at this position using viral infectivity studies, virus-cell fusion assays, and immunoblotting. We find that E99V mutant viruses are defective for fusion with cell membranes and thus are noninfectious. We show that E99V mutant particles of HIV-1 strains LAI and NL4.3 lack wild-type levels of Env proteins. We identify a compensatory substitution in MA residue 84 and show that it can reverse the E99V-associated defects. Taken together, these results indicate that the C-terminal hydrophobic pocket of MA, which encompasses both residues 84 and 99, has a previously unsuspected and key role in HIV-1 Env incorporation.

Cellular kinases incorporated into HIV-1 particles: passive or active passengers?

Phosphorylation is one of the major mechanisms by which the activities of protein factors can be regulated. Such regulation impacts multiple key-functions of mammalian cells, including signal transduction, nucleo-cytoplasmic shuttling, macromolecular complexes assembly, DNA binding and regulation of enzymatic activities to name a few. To ensure their capacities to replicate and propagate efficiently in their hosts, viruses may rely on the phosphorylation of viral proteins to assist diverse steps of their life cycle. It has been known for several decades that particles from diverse virus families contain some protein kinase activity. While large DNA viruses generally encode for viral kinases, RNA viruses and more precisely retroviruses have acquired the capacity to hijack the signaling machinery of the host cell and to embark cellular kinases when budding. Such property was demonstrated for HIV-1 more than a decade ago. This review summarizes the knowledge acquired in the field of HIV-1-associated kinases and discusses their possible function in the retroviral life cycle.

A novel yeast-based recombination method to clone and propagate diverse HIV-1 isolates

Replication studies on human immunodeficiency virus 1 (HIV-1) rely on a few laboratory strains that are divergent from dominant HIV-1 subtypes in the epidemic. Several phenotypic differences between diverse HIV-1 isolates and subtypes could affect vaccine development and treatment, but this research field lacks robust cloning/virus production systems to study drug sensitivity, replication kinetics, or to develop personalized vaccines. Extreme HIV-1 heterogeneity leaves few restriction enzyme sites for bacterial cloning strategies. In this study, we describe an alternative approach that involves direct introduction of any HIV-1 coding regions (e.g., any gene from a patient sample) into an HIV-1 DNA vector using yeast recombination. This technique uses positive and negative selectable markers in yeast and avoids the need for purification and screening of the DNA substrates and cloning products. Replication-competent virus is then produced from a modified mammalian 293T packaging cell line transfected with this yeast-derived HIV-1 vector. Although HIV-1 served as the prototype, this cloning strategy is now being developed for other diverse virus species such as hepatitis C virus and influenza virus.

Mutation of critical serine residues in HIV-1 matrix result in an envelope incorporation defect which can be rescued by truncation of the gp41 cytoplasmic tail

The human immunodeficiency virus type 1 (HIV-1) matrix (MA) domain is involved in both early and late events of the viral life cycle. Simultaneous mutation of critical serine residues in MA has been shown previously to dramatically reduce phosphorylation of MA. However, the role of phosphorylation in viral replication remains unclear. Viruses harboring serine to alanine substitutions at positions 9, 67, 72, and 77 are severely impaired in their ability to infect target cells. In addition, the serine mutant viruses are defective in their ability to fuse with target cell membranes. Interestingly, both the fusion defect and the infectivity defect can be rescued by truncation of the long cytoplasmic tail of gp41 envelope protein (gp41CT). Sucrose density gradient analysis also reveals that these mutant viruses have reduced levels of gp120 envelope protein incorporated into the virions as compared to wild type virus. Truncation of the gp41CT rescues the envelope incorporation defect. Here we propose a model in which mutation of specific serine residues prevents MA interaction with lipid rafts during HIV-1 assembly and thereby impairs recruitment of envelope to the sites of viral budding.

Nucleocapsid protein function in early infection processes

The role of nucleocapsid protein (NC) in the early steps of retroviral replication appears largely that of a facilitator for reverse transcription and integration. Using a wide variety of cell-free assay systems, the properties of mature NC proteins (e.g. HIV-1 p7(NC) or MLV p10(NC)) as nucleic acid chaperones have been extensively investigated. The effect of NC on tRNA annealing, reverse transcription initiation, minus-strand-transfer, processivity of reverse transcription, plus-strand-transfer, strand-displacement synthesis, 3' processing of viral DNA by integrase, and integrase-mediated strand-transfer has been determined by a large number of laboratories. Interestingly, these reactions can all be accomplished to varying degrees in the absence of NC; some are facilitated by both viral and non-viral proteins and peptides that may or may not be involved in vivo. What is one to conclude from the observation that NC is not strictly required for these necessary reactions to occur? NC likely enhances the efficiency of each of these steps, thereby vastly improving the productivity of infection. In other words, one of the major roles of NC is to enhance the effectiveness of early infection, thereby increasing the probability of productive replication and ultimately of retrovirus survival.

Mutations that mimic phosphorylation of the HIV-1 matrix protein do not perturb the myristyl switch

Recent studies indicate that the matrix domain (MA) of the HIV-1 Gag polyprotein directs Gag to the plasma membrane for virus assembly via a phosphatidylinositol-4,5-bisphosphate (PIP(2))-dependent myristyl switch mechanism. MA also has been reported to direct nuclear trafficking via nuclear import and export functions, and some studies suggest that nuclear targeting may be regulated by MA phosphorylation (although this proposal remains controversial). We have prepared and studied a series of HIV-1 MA mutants containing Ser-to-Asp substitutions designed to mimic phosphorylation, including substitutions in regions of the protein involved in protein-protein interactions and known to influence the myristyl switch (S6D, S9D, S67D, S72D, S6D/S9D, and S67D/S72D). We were particularly interested in substitutions at residue 6, since conservative mutations adjacent to this site strongly perturb the myristyl switch equilibrium, and this site had not been genetically tested due to its involvement in post-translational myristylation. Our studies reveal that none of these mutations, including S6D, influences the PIP(2)- or concentration-dependent myristyl switch equilibrium. In addition, all of the mutants bind liposomes with affinities that are only slightly reduced in comparison with the native protein. In contrast, the myristylated mutants bind liposomes with substantially greater affinity than that of the native, unmyristylated protein. These findings support the hypothesis that phosphorylation is unlikely to significantly influence membrane-mediated intracellular trafficking.

Characterization of replication defects induced by mutations in the basic domain and C-terminus of HIV-1 matrix

Extensive mutagenesis has defined distinct functional domains in the HIV-1 matrix domain (MA). In an attempt to more clearly define functions of regions of MA which affect viral entry, we analyzed mutations in the N-terminal basic and the C-terminal helical domains. Deletions of 8-10 amino acid residues of the C-terminal fifth helix of MA resulted in viruses that were only mildly defective in infectivity and fusion. The defect exhibited by these mutations could largely be attributed to a reduction in levels of viral envelope incorporated into mature virions. Truncation of the gp41 cytoplasmic tail (gp41CT) could rescue the phenotype of one of these mutants. In contrast, mutations of multiple basic residues in the N-terminus of MA were severely defective in both infectivity and fusion. While these mutations induce severe envelope incorporation defects, they also result in virus crippled at a post-entry step, since truncation of the gp41CT could not rescue the infectivity defect.

Antiviral activity of a Rac GEF inhibitor characterized with a sensitive HIV/SIV fusion assay

A virus-dependent fusion assay was utilized to examine the activity of a panel of HIV-1, -2, and SIV isolates of distinct coreceptor phenotypes. This assay allowed identification of entry inhibitors, and characterization of an antagonist of a Rac guanine nucleotide exchange factor, as an inhibitor of HIV-mediated fusion.

Regulating the functions of the HIV-1 matrix protein

The HIV-1 structural protein matrix (MA) is involved in a number of essential steps during infection and appears to possess multiple, seemingly conflicting targeting signals. Although MA has long been known to be crucial for virion assembly, details regarding this function, and the domains responsible for mediating it, are still emerging. MA has also been implicated in nuclear import of HIV cDNA and is purported to contain a nuclear targeting signal. Little is known about how these opposing plasma membrane and nuclear targeting signals are regulated and which signals predominate at various stages of infection. Additionally, MA has recently been implicated in a number of novel roles during infection including viral entry/uncoating, cytoskeletal-mediated transport, and targeting viral assembly to lipid rafts. Here we discuss our current understanding of MA's functions during infection and explore the recent advancements made in elucidating the mechanism of these processes. It appears that MA possesses a cache of targeting signals that are likely to be regulated throughout the infectious cycle by a combination of structural and biochemical modifications including phosphorylation, myristoylation, and multimerization. The ability of HIV to modify the properties of MA at specific stages of infection is central to the multifunctional behavior of MA and the efficiency of HIV infection. The recently reported success of drugs specifically designed to block MA function (Haffar O, Dubrovsky L, and Lowe R et al. J Virol 2005;79:13028-13036) confirms the importance of this protein for HIV infection and highlights a potentially new avenue in multivalent drug therapy.

An Envelope-determined Endocytic Route of Viral Entry Allows HIV-1 to Escape from Secreted Phospholipase A2 Entry Blockade

Secreted phospholipases A(2) (sPLA(2)s) represent a new class of human immunodeficiency virus (HIV) inhibitors that block the early steps of virus entry into cells. Here, we applied an in vitro evolution/selection procedure to select, from primary HIV isolates, an emerging variant (HIV(RBV-3)) able to actively infect cells in the presence of sPLA(2)s. HIV(RBV-3) represents a very atypical HIV-1 isolate because, in contrast to others, this virus requires a functional endocytic machinery to infect cells. Indeed, endocytosis inhibitors that affect endosome acidification (bafilomycin A(1), monensin) and/or endosomal trafficking (nocodazole, latrunculin A) drastically reduced HIV(RBV-3) replication. Using a standardized PCR-assay, we showed that endocytosis inhibitors block HIV(RBV-3) entry just before the reverse transcription step. Concurrently, to identify the viral proteins responsible for the HIV(RBV-3) atypical behaviour, we constructed a HIV-1 molecular chimera bearing different HIV(RBV-3) proteins. We demonstrated that the sole presence of the HIV(RBV-3) envelope glycoprotein is enough, not only to confer the resistance to sPLA(2)s, but also to direct HIV(RBV-3) to the endosomal-dependent entry pathway. Interestingly, HIV(RBV-3) envelope glycoprotein sequencing revealed an unusual structural pattern with the presence of rare mutations in the N-terminal region and V1-V2 envelope loop sequence extensions. Taken together, we conclude that HIV-1 may escape from entry inhibitors, such as sPLA2s, through the selection of a particular HIV-1 envelope glycoprotein that allows HIV to infect cells via an alternative entry route that relies on endosome trafficking.

Efficiency of human immunodeficiency virus type 1 postentry infection processes: evidence against disproportionate numbers of defective virions

The vast majority of human immunodeficiency virus type 1 particles are claimed to be noninfectious, but there is disagreement as to whether they are defective or simply lack the opportunity to initiate an infection. We have examined the efficiencies of reverse transcription and integration and find that approximately 1 of every 8 virions that initiate reverse transcription form proviruses, a quantity significantly different from the commonly reported ratio of 1 in 1,000. In addition, results from two different infectivity assays demonstrate that the titers are not equivalent to the number of infectious particles. The apparent predominance of noninfectious particles is due to infrequent occurrences of successful virus-cell interactions.

HIV-1 matrix protein: a mysterious regulator of the viral life cycle

Significant progress has been achieved in the last few years concerning the human immunodeficiency virus (HIV-1) life cycle, mostly in the fields of cellular receptors for the virus, virus assembly and budding of virus particles from the cell surface. Meanwhile, some aspects, such as postentry events, virus maturation and the regulatory role of individual viral proteins remain poorly defined. This review summarizes some recent findings concerning the role of Gag Pr55 and its proteolytic processing in the HIV-1 life cycle with particular emphasis on the functions of matrix protein p17 (MA), the protein which plays a key role in regulation of the early and late steps of viral morphogenesis. Based on our recent observations, the possibility is discussed that two subsets of MA exist, one cleaved from the Gag precursor in the host cell (cMA), and the other cleaved in the virions (vMA). It is suggested that two MA fractions possess diverse functions and are involved in different stages of virus morphogenesis as key regulators of the viral life cycle.

Protein methylation is required to maintain optimal HIV-1 infectivity

Background:

Protein methylation is recognized as a major protein modification pathway regulating diverse cellular events such as protein trafficking, transcription, and signal transduction. More recently, protein arginine methyltransferase activity has been shown to regulate HIV-1 transcription via Tat. In this study, adenosine periodate (AdOx) was used to globally inhibit protein methyltransferase activity so that the effect of protein methylation on HIV-1 infectivity could be assessed.

Results:

Two cell culture models were used: HIV-1-infected CEM T-cells and HEK293T cells transfected with a proviral DNA plasmid. In both models, AdOx treatment of cells increased the levels of virion in culture supernatant. However, these viruses had increased levels of unprocessed or partially processed Gag-Pol, significantly increased diameter, and displayed reduced infectivity in a MAGI X4 assay. AdOx reduced infectivity equally in both dividing and non-dividing cells. However, infectivity was further reduced if Vpr was deleted suggesting virion proteins, other than Vpr, were affected by protein methylation. Endogenous reverse transcription was not inhibited in AdOx-treated HIV-1, and infectivity could be restored by pseudotyping HIV with VSV-G envelope protein. These experiments suggest that AdOx affects an early event between receptor binding and uncoating, but not reverse transcription.

Conclusion:

Overall, we have shown for the first time that protein methylation contributes towards maximal virus infectivity. Furthermore, our results also indicate that protein methylation regulates HIV-1 infectivity in a complex manner most likely involving the methylation of multiple viral or cellular proteins and/or multiple steps of replication.

Central role of the respiratory syncytial virus matrix protein in infection

Respiratory syncytial virus is the major respiratory pathogen of infants and children worldwide, with no effective treatment or vaccine available. Steady progress has been made in understanding the respiratory syncytial virus life cycle and the consequences of infection, but many areas of respiratory syncytial virus biology remain poorly understood, including the role of subcellular localisation of respiratory syncytial virus gene products such as the matrix protein in the infected host cell. The matrix protein plays a central role in viral assembly and, intriguingly, has been observed to traffic into and out of the nucleus at specific times during the respiratory syncytial virus infectious cycle. Further, the matrix protein has been shown to be able to inhibit transcription, which may be a key to respiratory syncytial virus pathogenesis. This review will focus on the role of the matrix protein in respiratory syncytial virus infection and what is known of its nucleocytoplasmic trafficking, the understanding of which may lead to new therapeutic approaches to combat respiratory syncytial virus, and/or vaccine development.

The effect of capsid mutations on HIV-1 uncoating

Efficient uncoating requires not only an optimal cellular environment, but also some intrinsic properties of the viral capsid protein itself. Using an in vitro uncoating model, we demonstrated that substitution of each serine residue with alanine at the three major phosphorylation sites of HIV-1 capsid protein, i.e. Ser-109, Ser-149 and Ser-178, could significantly reduce uncoating activity of purified core particles. We also showed that the core stability of mutant viruses was lower than that of the wild-type virus so that the lack of efficient uncoating of each mutant could not be due to an increase in capsid physical stability. However, serine-to-aspartic acid mutation to mimic the negative charge of phosphor-serine could not restore either uncoating activity or infectivity, and treatment of purified core particles with a phosphatase did not alter the uncoating activity. Our data indicated that mutations at phosphoacceptor sites of capsid disturbed the uncoating mechanism, but the defect may not be directly caused by the lack of phosphate on the core particles undergoing uncoating.

HIV type 1 inhibition by protein kinase C modulatory compounds

The dichotomous effects of the protein kinase C (PKC) modulatory compounds 12-myristate 13-acetate (PMA), prostratin, and ingenol 3-angelate (I3A) on HIV-1 infection were investigated. PKC modulatory compounds were shown to be potent activators of cells latently infected with HIV-1 (I3A > prostratin). Conversely, PKC modulatory compounds inhibited infection of indicator cells (MAGI) with CXCR4-tropic HIV-1 (PMA > I3A > prostratin), and I3A also inhibited infection with CCR5-tropic virus (AD8-1). Pretreatment with the PKC inhibitors prior to treatment with either I3A or PMA resulted in increased infection, indicating inhibition is PKC mediated. Cell infections suggested that I3A rapidly inhibited the virus from infecting cells at an early point in infection. This observation was supported by the demonstration of inhibition at or before the synthesis of early reverse transcription products, and the inability of these compounds to block vesicular stomatitis virus (VSV) pseudotyped HIV-1 particles. As has already been shown with prostratin, treatment with I3A resulted in down-regulation of the CD4 receptor and CXCR4 coreceptor suggesting that this was a contributor to the infection inhibition. Intriguingly, 48 h pretreatment of unstimulated peripheral blood mononuclear cells (PBMC) prior to infection resulted in abrogation of virus production at concentrations where receptor/ coreceptor levels were not significantly reduced. This result hints at the possibility of inhibition by a PKC modulatory compound of an early pathway of viral entry in PBMC.

Characterization of human immunodeficiency virus type 1 (HIV-1) containing mutations in the nucleocapsid protein at a putative HIV-1 protease cleavage site

The HIV-1 nucleocapsid protein (NC) has been hypothesized to be cleaved by the viral protease (PR) during early infection. Characterization of viruses, with amino-acid substitutions that modulate PR cleavage of NC in vitro, was performed in cell culture. Two of the NC mutants, NCN17F and NCN17G, had decreased infectivity and exhibited severe H9 replication defects. Examination of viral DNA after infections revealed defects in reverse transcription and integration, although integration defects were cell-type dependent. However, while the defects in reverse transcription and integration correlate with lowered infectivity in a single-round of infection, they did not approach the magnitude of the replication defect measured in H9 cells over multiple rounds. Importantly, we fail to see evidence that H9 cells are re-infected with the NCN17G and NCN17F viruses 24 h after the initial infection, which suggests that the principal defect caused by these NC mutations occurs during late events of viral replication.

A mutation in the human immunodeficiency virus type 1 Gag protein destabilizes the interaction of the envelope protein subunits gp120 and gp41

The Gag protein of human immunodeficiency virus type 1 (HIV-1) associates with the envelope protein complex during virus assembly. The available evidence indicates that this interaction involves recognition of the gp41 cytoplasmic tail (CT) by the matrix protein (MA) region of Pr55(Gag). Here we show that substitution of Asp for Leu at position 49 (L49D) in MA results in a specific reduction in particle-associated gp120 without affecting the levels of gp41. Mutant virions were markedly reduced in single-cycle infectivity despite a relatively modest defect in fusion with target cells. Studies with HIV-1 particles containing decreased levels of envelope proteins suggested that the L49D mutation also inhibits a postentry step in infection. Truncation of the gp41 tail, or pseudotyping by vesicular stomatitis virus glycoprotein, restored both the fusion and infectivity of L49D mutant virions to wild-type levels. Truncation of gp41 also resulted in equivalent levels of gp120 on particles with and without the MA mutation and enhanced the replication of the L49D mutant virus in T cells. The impaired fusion and infectivity of L49D mutant particles were also complemented by a single point mutation in the gp41 CT that disrupted the tyrosine-containing endocytic motif. Our results suggest that an altered interaction between the MA domain of Gag and the gp41 cytoplasmic tail leads to dissociation of gp120 from gp41 during HIV-1 particle assembly, thus resulting in impaired fusion and infectivity.

Retroviral matrix domains share electrostatic homology: models for membrane binding function throughout the viral life cycle

The matrix domain (MA) of Gag polyproteins performs multiple functions throughout the retroviral life cycle. MA structures have an electropositive surface patch that is implicated in membrane association. Here, we use computational methods to demonstrate that electrostatic control of membrane binding is a central characteristic of all retroviruses. We are able to explain a wide range of experimental observations and provide a level of quantitative and molecular detail that has been inaccessible to experiment. We further predict that MA may exist in a variety of oligomerization states and propose mechanistic models for the effects of phosphoinositides and phosphorylation. The calculations provide a conceptual model for how non-myristoylated and myristoylated MAs behave similarly in assembly and disassembly. Hence, they provide a unified quantitative picture of the structural and energetic origins of the entire range of MA function and thus enhance, extend, and integrate previous observations on individual stages of the process.

Intracellular trafficking of retroviral vectors: obstacles and advances

Retroviruses are efficient vehicles for delivering transgenes in vivo. Their ability to integrate into the host genome, providing a permanent imprint of their genes in the host, is a key asset for gene therapy. Furthermore, the lentivirus subset of retroviruses can infect nondividing as well as dividing cells. This expands the cell types capable of gene therapy, driving the development of lentiviral vectors. However, the precise mechanisms used by different retroviruses to efficiently deliver their genes into cell nuclei remains largely unclear. Understanding these molecular mechanisms may reveal features to improve the efficacy of current retroviral vectors. Moreover, this knowledge may expose elements pliable to other gene therapy vehicles to improve their in vivo performance and circumvent the biosafety concerns of using retroviral vectors. Therefore, the mechanisms underlying the early trafficking of retroviral vectors in host cells are reviewed here, as understood from studying the native retroviruses. Events after virus entry up to nuclear delivery of the viral cDNA are discussed. Cellular obstacles faced by these retroviral vectors and how they advance beyond these barriers is emphasized.

Reconstitution of retroviral fusion and uncoating in a cell-free system

The molecular events underlying the immediate steps of retroviral uncoating, occurring after membrane fusion and leading to the formation of an active reverse transcription complex, are not known. To better understand these processes, we have developed a cell-free system that recapitulates these early steps of retroviral replication by using avian sarcoma and leukosis virus as a model retrovirus. The substrates used in this system are viral particles that are trapped before completing membrane fusion. These virions are induced to fuse out of endosomes and the viral cores are released into solution where they are amenable to biochemical manipulation. This system revealed that membrane fusion is not sufficient to stimulate the formation of a reverse transcription complex. Instead, ATP hydrolysis and cellular factors >5 kDa in size are required. Furthermore, later steps of avian sarcoma and leukosis virus reverse transcription were stimulated by nuclear factors. The cell-free system should now allow for the definition of retroviral uncoating mechanisms and facilitate the identification and characterization of the cellular factors involved.