Mechanistic Investigation of HIV-1 Gag Association with Lipid Membranes

Human Immunodeficiency Virus Type 1 Gag Polyprotein Modulates Membrane Physical Properties like a Surfactant: Potential Implications for Virus Assembly

Human immunodeficiency virus (HIV) assembly at an infected cell's plasma membrane requires membrane deformation to organize the near-spherical shape of an immature virus. While the cellular expression of HIV Gag is sufficient to initiate budding of virus-like particles, how Gag generates membrane curvature is not fully understood. Using highly curved lipid nanotubes, we have investigated the physicochemical basis of the membrane activity of recombinant nonmyristoylated Gag-Δp6. Gag protein, upon adsorption onto the membrane, resulted in the shape changes of both charged and uncharged nanotubes. This shape change was more pronounced in the presence of charged lipids, especially phosphatidylinositol bisphosphate (PI(4,5)P2). We found that Gag modified the interfacial tension of phospholipid bilayer membranes, as judged by comparison with the effects of amphipathic peptides and nonionic detergent. Bioinformatic analysis demonstrated that a region of the capsid and SP1 domains junction of Gag is structurally similar to the amphipathic peptide magainin-1. This region accounts for integral changes in the physical properties of the membrane upon Gag adsorption, as we showed with the synthetic CA-SP1 junction peptide. Phenomenologically, membrane-adsorbed Gag could diminish the energetic cost of increasing the membrane area in a way similar to foam formation. We propose that Gag acts as a surface-active substance at the HIV budding site that softens the membrane at the place of Gag adsorption, lowering the energy for membrane bending. Finally, our experimental data and theoretical considerations give a lipid-centric view and common mechanism by which proteins could bend membranes, despite not having intrinsic curvature in their molecular surfaces or assemblies.

A Review of FDA-Approved Anti-HIV-1 Drugs, Anti-Gag Compounds, and Potential Strategies for HIV-1 Eradication

Acquired immunodeficiency syndrome (AIDS) is an enormous global health threat stemming from human immunodeficiency virus (HIV-1) infection. Up to now, the tremendous advances in combination antiretroviral therapy (cART) have shifted HIV-1 infection from a fatal illness into a manageable chronic disorder. However, the presence of latent reservoirs, the multifaceted nature of HIV-1, drug resistance, severe off-target effects, poor adherence, and high cost restrict the efficacy of current cART targeting the distinct stages of the virus life cycle. Therefore, there is an unmet need for the discovery of new therapeutics that not only bypass the limitations of the current therapy but also protect the body's health at the same time. The main goal for complete HIV-1 eradication is purging latently infected cells from patients' bodies. A potential strategy called "lock-in and apoptosis" targets the budding phase of the life cycle of the virus and leads to susceptibility to apoptosis of HIV-1 infected cells for the elimination of HIV-1 reservoirs and, ultimately, for complete eradication. The current work intends to present the main advantages and disadvantages of United States Food and Drug Administration (FDA)-approved anti-HIV-1 drugs as well as plausible strategies for the design and development of more anti-HIV-1 compounds with better potency, favorable pharmacokinetic profiles, and improved safety issues.

Inhibition of neutral sphingomyelinase 2 impairs HIV-1 envelope formation and substantially delays or eliminates viral rebound

Although HIV-1 Gag is known to drive viral assembly and budding, the precise mechanisms by which the lipid composition of the plasma membrane is remodeled during assembly are incompletely understood. Here, we provide evidence that the sphingomyelin hydrolase neutral sphingomyelinase 2 (nSMase2) interacts with HIV-1 Gag and through the hydrolysis of sphingomyelin creates ceramide that is necessary for proper formation of the viral envelope and viral maturation. Inhibition or depletion of nSMase2 resulted in the production of noninfectious HIV-1 virions with incomplete Gag lattices lacking condensed conical cores. Inhibition of nSMase2 in HIV-1-infected humanized mouse models with a potent and selective inhibitor of nSMase2 termed PDDC [phenyl(R)-(1-(3-(3,4-dimethoxyphenyl)-2, 6-dimethylimidazo[1,2-b]pyridazin-8-yl) pyrrolidin-3-yl)-carbamate] produced a linear reduction in levels of HIV-1 in plasma. If undetectable plasma levels of HIV-1 were achieved with PDDC treatment, viral rebound did not occur for up to 4 wk when PDDC was discontinued. In vivo and tissue culture results suggest that PDDC selectively kills cells with actively replicating HIV-1. Collectively, this work demonstrates that nSMase2 is a critical regulator of HIV-1 replication and suggests that nSMase2 could be an important therapeutic target with the potential to kill HIV-1-infected cells.

Second Harmonic Generation Interrogation of the Endonuclease APE1 Binding Interaction with G-Quadruplex DNA

The binding interaction between the DNA repair enzyme apurinic/apyrimidinic endonuclease-1 (APE1) with promoter G-quadruplex (G4) folds bearing an abasic site (AP) can serve as a gene regulatory switch during oxidative stress. Prior fluorescence-based analysis in solution suggested APE1 binds the VEGF promoter G4 but whether this interaction was specific or not remained an open question. Second harmonic generation (SHG) was used in this work to measure the noncanonical DNA-protein binding interaction in a label-free assay with high sensitivity to demonstrate the interaction is ordered and specific. The binding of APE1 to the VEGF promoter G4 with AP sites modeled by a tetrahydrofuran analogue produced dissociation constants of ∼100 nM that differed from duplex and single-stranded DNA control studies. The SHG measurements confirmed APE1 binds the VEGF G4 folds in a specific manner resolving a remaining question regarding how this endonuclease with gene regulatory features engages G4 folds. The studies demonstrate the power of SHG to interrogate noncanonical DNA-protein interactions providing a foundational example for the use of this analytical method in future biochemical analyses.

Relationship between HIV-1 Gag Multimerization and Membrane Binding

HIV-1 viral particle assembly occurs specifically at the plasma membrane and is driven primarily by the viral polyprotein Gag. Selective association of Gag with the plasma membrane is a key step in the viral assembly pathway, which is traditionally attributed to the MA domain. MA regulates specific plasma membrane binding through two primary mechanisms including: (1) specific interaction of the MA highly basic region (HBR) with the plasma membrane phospholipid phosphatidylinositol (4,5) bisphosphate [PI(4,5)P2], and (2) tRNA binding to the MA HBR, which prevents Gag association with non-PI(4,5)P2 containing membranes. Gag multimerization, driven by both CA–CA inter-protein interactions and NC-RNA binding, also plays an essential role in viral particle assembly, mediating the establishment and growth of the immature Gag lattice on the plasma membrane. In addition to these functions, the multimerization of HIV-1 Gag has also been demonstrated to enhance its membrane binding activity through the MA domain. This review provides an overview of the mechanisms regulating Gag membrane binding through the MA domain and multimerization through the CA and NC domains, and examines how these two functions are intertwined, allowing for multimerization mediated enhancement of Gag membrane binding.

Molecular Determinants in tRNA D-arm Required for Inhibition of HIV-1 Gag Membrane Binding

Plasma-membrane-specific localization of Gag, an essential step in HIV-1 particle assembly, is regulated by the interaction of the Gag MA domain with PI(4,5)P₂ and tRNA-mediated inhibition of non-specific or premature membrane binding. Different tRNAs inhibit PI(4,5)P₂-independent membrane binding to varying degrees in vitro; however, the structural determinants for this difference remain unknown. Here we demonstrate that membrane binding of full-length Gag synthesized in vitro using reticulocyte lysates is inhibited when RNAs that contain the anticodon arm of tRNA^Pro, but not that of tRNA^Lys3, are added exogenously. In contrast, in the context of a liposome binding assay in which the effects of tRNAs on purified MA were tested, full-length tRNA^Lys3 showed greater inhibition of MA membrane binding than full-length tRNA^Pro. While transplantation of the D loop sequence of tRNA^Lys3 into tRNA^Pro resulted in a modest increase in the inhibitory effect relative to WT tRNA^Pro, replacing the entire D arm sequence with that of tRNA^Lys3 was necessary to confer the full inhibitory effects upon tRNA^Pro. Together, these results demonstrate that the D arm of tRNA^Lys3 is a major determinant of strong inhibition of MA membrane binding and that this inhibitory effect requires not only the D loop, which was recently reported to contact the MA highly basic region, but the loop sequence in the context of the D arm structure.

Rendezvous at Plasma Membrane: Cellular Lipids and tRNA Set up Sites of HIV-1 Particle Assembly and Incorporation of Host Transmembrane Proteins

The HIV-1 structural polyprotein Gag drives the virus particle assembly specifically at the plasma membrane (PM). During this process, the nascent virion incorporates specific subsets of cellular lipids and host membrane proteins, in addition to viral glycoproteins and viral genomic RNA. Gag binding to the PM is regulated by cellular factors, including PM-specific phospholipid PI(4,5)P2 and tRNAs, both of which bind the highly basic region in the matrix domain of Gag. In this article, we review our current understanding of the roles played by cellular lipids and tRNAs in specific localization of HIV-1 Gag to the PM. Furthermore, we examine the effects of PM-bound Gag on the organization of the PM bilayer and discuss how the reorganization of the PM at the virus assembly site potentially contributes to the enrichment of host transmembrane proteins in the HIV-1 particle. Since some of these host transmembrane proteins alter release, attachment, or infectivity of the nascent virions, the mechanism of Gag targeting to the PM and the nature of virus assembly sites have major implications in virus spread.