Influence of the amino-terminal sequence on the structure and function of HIV integrase

Structural Impact of Ex Vivo Resistance Mutations on HIV-1 Integrase Polymers Induced by Allosteric Inhibitors

HIV-1 integrase (IN) is targeted by two classes of antivirals: integrase strand transfer inhibitors (INSTIs), which bind to the active site within the catalytic core domain (CCD), and allosteric integrase inhibitors (ALLINIs), which bind at the CCD dimer interface. ALLINIs were initially designed to disrupt interactions with the cellular cofactor LEDGF/p75, but it has become clear that ALLINIs primarily act by promoting formation of aberrant integrase polymers. The ALLINIs achieve this by stabilizing ectopic intermolecular interactions between the CCD dimer and the integrase carboxy-terminal domain (CTD), which disrupts viral maturation. Previously, we determined the structure of full-length HIV-1 IN bound to the ALLINI GSK1264 at 4.4 Å resolution, revealing its polymerization mechanism. More recently, we reported the X-ray crystal structure of a minimal ternary complex between CCD, CTD, and the ALLINI BI-224436 at a higher resolution. In this study, we improve the original 4.4 Å structure using this higher-resolution information and report two new structures of full-length HIV-1 IN harboring escape mutations in the CCD (Trp131Cys) or CTD (Asn222Lys) bound with the prototype ALLINI BI-D at 4.5 Å. These structures reveal perturbations to the tertiary organization associated with escape substitutions, which correlate with their reduced ability to form ectopic ALLINI-induced polymers in vitro. These findings suggest a general structural mechanism of ALLINI resistance and provide insights for the design of improved ALLINIs.

Fluorescent HIV-1 integrases for a suite of new user-friendly stability, nucleic acid binding and strand transfer activity assays

Human immunodeficiency virus (HIV) infects CD4+ T-cells, causing acquired immunodeficiency syndrome. Despite advances in antiretroviral therapy, drug resistance remains a critical issue. HIV integrase is a key therapeutic target. Resistance to integrase strand transfer inhibitors requires development of new drugs with distinct mechanisms. Integrases tethered with GFP (IN-GFP) and mCherry (IN-mCherry) were evaluated for the development of a comprehensive suite of user-friendly assays. A new fluorescent protein-based stability assay (FP-Basta) effectively assessed protein thermal stability, revealing aggregation midpoints of 45.0 °C for IN-GFP and 45.4 °C for IN-mCherry. FP-Basta showed that IN-mCherry was stabilized by a target DNA and viral LTR, confirming protein-DNA interactions. A new qPCR-based integrase activity assay demonstrated robust detection of strand transfer activity, with a ∼21,500-fold sensitivity over background. Manganese ions were essential, enhancing integrase activity 56-fold compared to magnesium ions, while Zn2+ impaired functionality. The integrase activity assay can distinguish 3'-processing and strand transfer activities and was validated for inhibitor screening. The combination of FP-Basta and qPCR-based integrase activity assay provides a comprehensive, cost-effective platform for evaluating IN function and inhibitor efficacy. These tools, leveraging GFP- and mCherry-tagged IN, offer potential for future high-throughput applications in HIV drug discovery and the development of therapies addressing resistance challenges.

HIV-1 Integrase Assembles Multiple Species of Stable Synaptic Complex Intasomes That Are Active for Concerted DNA Integration In vitro

Retroviral DNA integration is mediated by nucleoprotein complexes (intasomes) in which a pair of viral DNA ends are bridged by a multimer of integrase (IN). Most of the high-resolution structures of HIV-1 intasomes are based on an HIV-1 IN with an Sso7d protein domain fused to the N-terminus. Sso7d-IN aggregates much less than wild-type IN and has been critical for structural studies of HIV-1 intasomes. Unexpectedly, these structures revealed that the common core architecture that mediates catalysis could be assembled in various ways, giving rise to both tetrameric and dodecameric intasomes, together with other less well-characterized species. This differs from related retroviruses that assemble unique multimeric intasomes, although the number of protomers in the intasome varies between viruses. The question of whether the additional Sso7d domain contributes to the heterogeneity of HIV-1 intasomes is therefore raised. We have addressed this by biochemical and structural studies of intasomes assembled with wild-type HIV-1 IN. Negative stain and cryo-EM reveal a similar range of multimeric intasome species as with Sso7d-IN with the same common core architecture. Stacks of intasomes resulting from domain swapping are also seen with both wild-type and Sso7d-IN intasomes. The propensity to assemble multimeric intasome species is, therefore, an intrinsic property of HIV-1 IN and is not conferred by the presence of the Sso7d domain. The recently solved intasome structures of different retroviral species, which have been reported to be tetrameric, octameric, dodecameric, and hexadecameric, highlight how a common intasome core architecture can be assembled in different ways for catalysis.

Molecular determinants for Rous sarcoma virus intasome assemblies involved in retroviral integration

Integration of retroviral DNA into the host genome involves the formation of integrase (IN)-DNA complexes termed intasomes. Further characterization of these complexes is needed to understand their assembly process. Here, we report the single-particle cryo-EM structure of the Rous sarcoma virus (RSV) strand transfer complex (STC) intasome produced with IN and a preassembled viral/target DNA substrate at 3.36 Å resolution. The conserved intasome core region consisting of IN subunits contributing active sites interacting with viral/target DNA has a resolution of 3 Å. Our structure demonstrated the flexibility of the distal IN subunits relative to the IN subunits in the conserved intasome core, similar to results previously shown with the RSV octameric cleaved synaptic complex intasome produced with IN and viral DNA only. An extensive analysis of higher resolution STC structure helped in the identification of nucleoprotein interactions important for intasome assembly. Using structure-function studies, we determined the mechanisms of several IN-DNA interactions critical for assembly of both RSV intasomes. We determined the role of IN residues R244, Y246, and S124 in cleaved synaptic complex and STC intasome assemblies and their catalytic activities, demonstrating differential effects. Taken together, these studies advance our understanding of different RSV intasome structures and molecular determinants involved in their assembly.

Structure of a HIV-1 IN-Allosteric inhibitor complex at 2.93 Å resolution: Routes to inhibitor optimization

HIV integrase (IN) inserts viral DNA into the host genome and is the target of the strand transfer inhibitors (STIs), a class of small molecules currently in clinical use. Another potent class of antivirals is the allosteric inhibitors of integrase, or ALLINIs. ALLINIs promote IN aggregation by stabilizing an interaction between the catalytic core domain (CCD) and carboxy-terminal domain (CTD) that undermines viral particle formation in late replication. Ongoing challenges with inhibitor potency, toxicity, and viral resistance motivate research to understand their mechanism. Here, we report a 2.93 Å X-ray crystal structure of the minimal ternary complex between CCD, CTD, and the ALLINI BI-224436. This structure reveals an asymmetric ternary complex with a prominent network of π-mediated interactions that suggest specific avenues for future ALLINI development and optimization.

Advances in the development of HIV integrase strand transfer inhibitors

HIV-1 integrase (IN) is a key enzyme in viral replication that catalyzes the covalent integration of viral cDNA into the host genome. Currently, five HIV-1 IN strand transfer inhibitors (INSTIs) are approved for clinical use. These drugs represent an important addition to the armamentarium for antiretroviral therapy. This review briefly illustrates the development history of INSTIs. The characteristics of the currently approved INSTIs, as well as their future perspectives, are critically discussed.

Retroviral integrase: Structure, mechanism, and inhibition

The retroviral protein Integrase (IN) catalyzes concerted integration of viral DNA into host chromatin to establish a permanent infection in the target cell. We learned a great deal about the mechanism of catalytic integration through structure/function studies over the previous four decades of IN research. As one of three essential retroviral enzymes, IN has also been targeted by antiretroviral drugs to treat HIV-infected individuals. Inhibitors blocking the catalytic integration reaction are now state-of-the-art drugs within the antiretroviral therapy toolkit. HIV-1 IN also performs intriguing non-catalytic functions that are relevant to the late stages of the viral replication cycle, yet this aspect remains poorly understood. There are also novel allosteric inhibitors targeting non-enzymatic functions of IN that induce a block in the late stages of the viral replication cycle. In this chapter, we will discuss the function, structure, and inhibition of retroviral IN proteins, highlighting remaining challenges and outstanding questions.

Redondovirus Diversity and Evolution on Global, Individual, and Molecular Scales

Redondoviridae is a newly established family of circular Rep-encoding single-stranded (CRESS) DNA viruses found in the human ororespiratory tract. Redondoviruses were previously found in ∼15% of respiratory specimens from U.S. urban subjects; levels were elevated in individuals with periodontitis or critical illness. Here, we report higher redondovirus prevalence in saliva samples: four rural African populations showed 61 to 82% prevalence, and an urban U.S. population showed 32% prevalence. Longitudinal, limiting-dilution single-genome sequencing revealed diverse strains of both redondovirus species (Brisavirus and Vientovirus) in single individuals, persistence over time, and evidence of intergenomic recombination. Computational analysis of viral genomes identified a recombination hot spot associated with a conserved potential DNA stem-loop structure. To assess the possible role of this site in recombination, we carried out in vitro studies which showed that this potential stem-loop was cleaved by the virus-encoded Rep protein. In addition, in reconstructed reactions, a Rep-DNA covalent intermediate was shown to mediate DNA strand transfer at this site. Thus, redondoviruses are highly prevalent in humans, found in individuals on multiple continents, heterogeneous even within individuals and encode a Rep protein implicated in facilitating recombination. IMPORTANCE Redondoviridae is a recently established family of DNA viruses predominantly found in the human respiratory tract and associated with multiple clinical conditions. In this study, we found high redondovirus prevalence in saliva from urban North American individuals and nonindustrialized African populations in Botswana, Cameroon, Ethiopia, and Tanzania. Individuals on both continents harbored both known redondovirus species. Global prevalence of both species suggests that redondoviruses have long been associated with humans but have remained undetected until recently due to their divergent genomes. By sequencing single redondovirus genomes in longitudinally sampled humans, we found that redondoviruses persisted over time within subjects and likely evolve by recombination. The Rep protein encoded by redondoviruses catalyzes multiple reactions in vitro, consistent with a role in mediating DNA replication and recombination. In summary, we identify high redondovirus prevalence in humans across multiple continents, longitudinal heterogeneity and persistence, and potential mechanisms of redondovirus evolution by recombination.