BET Bromodomain Inhibitors: Novel Design Strategies and Therapeutic Applications

The mammalian bromodomain and extra-terminal domain (BET) family of proteins consists of four conserved members (Brd2, Brd3, Brd4, and Brdt) that regulate numerous cancer-related and immunity-associated genes. They are epigenetic readers of histone acetylation with broad specificity. BET proteins are linked to cancer progression due to their interaction with numerous cellular proteins including chromatin-modifying factors, transcription factors, and histone modification enzymes. The spectacular growth in the clinical development of small-molecule BET inhibitors underscores the interest and importance of this protein family as an anticancer target. Current approaches targeting BET proteins for cancer therapy rely on acetylation mimics to block the bromodomains from binding chromatin. However, bromodomain-targeted agents are suffering from dose-limiting toxicities because of their effects on other bromodomain-containing proteins. In this review, we provided an updated summary about the evolution of small-molecule BET inhibitors. The design of bivalent BET inhibitors, kinase and BET dual inhibitors, BET protein proteolysis-targeting chimeras (PROTACs), and Brd4-selective inhibitors are discussed. The novel strategy of targeting the unique C-terminal extra-terminal (ET) domain of BET proteins and its therapeutic significance will also be highlighted. Apart from single agent treatment alone, BET inhibitors have also been combined with other chemotherapeutic modalities for cancer treatment demonstrating favorable clinical outcomes. The investigation of specific biomarkers for predicting the efficacy and resistance of BET inhibitors is needed to fully realize their therapeutic potential in the clinical setting.

Identification and Optimization of a Novel HIV-1 Integrase Inhibitor

Human immunodeficiency virus-1 (HIV-1) is the causative agent of acquired immunodeficiency syndrome (AIDS). HIV-1, like all retroviruses, stably integrates its vDNA copy into host chromatin, a process allowing for permanent infection. This essential step for HIV-1 replication is catalyzed by viral integrase (IN) and aided by cellular protein LEDGF/p75. In addition, IN is also crucial for proper virion maturation as it interacts with the viral RNA genome to ensure encapsulation of ribonucleoprotein complexes within the protective capsid core. These key functions make IN an attractive target for the development of inhibitors with various mechanisms of action. We conducted a high-throughput screen (HTS) of ∼370,000 compounds using a homogeneous time-resolved fluorescence-based assay capable of capturing diverse inhibitors targeting multifunctional IN. Our approach revealed chemical scaffolds containing diketo acid moieties similar to IN strand transfer inhibitors (INSTIs) as well as novel compounds distinct from all current IN inhibitors including INSTIs and allosteric integrase inhibitors (ALLINIs). Specifically, our HTS resulted in the discovery of compound 12, with a novel IN inhibitor scaffold amenable for chemical modification. Its more potent derivative 14e similarly inhibited catalytic activities of WT and mutant INs containing archetypical INSTI- and ALLINI-derived resistant substitutions. Further SAR-based optimization resulted in compound 22 with an antiviral EC₅₀ of ∼58 μM and a selectivity index of >8500. Thus, our studies identified a novel small-molecule scaffold for inhibiting HIV-1 IN, which provides a promising platform for future development of potent antiviral agents to complement current HIV-1 therapies.

Development of Murine Leukemia Virus Integrase-Derived Peptides That Bind Brd4 Extra-Terminal Domain as Candidates for Suppression of Acute Myeloid Leukemia

The bromodomain and extra-terminal (BET) domain family of proteins, which include its prototypical member Brd4, is implicated in a variety of cancers and viral infections due to their interaction with cellular and viral proteins. BET proteins contain two bromodomains, a common protein motif that selectively binds acetylated lysine on histones. However, they are structurally distinct from other bromodomain-containing proteins because they encode a unique C-terminal extra-terminal (ET) domain that is important for the protein-protein interactions including jumonji C-domain-containing protein 6 (JMJD6) and histone-lysine N-methyltransferase NSD3 (NSD3). Brd4 functions primarily during transcription as a passive scaffold linking cellular and viral proteins to chromatin. The rapid development of clinical inhibitors targeting Brd4 highlights the importance of this protein as an anticancer target. Current therapeutic approaches focus on the development of small molecule acetylated lysine mimics of histone marks that block the ability of the bromodomains to bind their chromatin marks. Thus far, bromodomain-targeted agents have shown dose-limiting toxicities due to off-target effects on other bromodomain-containing proteins. Here, we exploited a viral-host protein interaction interface to design peptides for the disruption of BET protein function. A murine leukemia virus (MLV) integrase-derived peptide (ET binding motif, EBM) and its shorter minimal binding motif (pentapeptide LKIRL) were sufficient to directly bind the Brd4 ET domain and reduce cellular proliferation of an acute myeloid leukemia cell line. Using computational and biochemical approaches, we identified the minimal essential contacts between EBM and LKIRL peptides and the Brd4 ET domain. Our findings provide a structural foundation for inhibiting BET/Brd4-mediated cancers by targeting the ET domain with small peptide-based inhibitors.

Strategies for Targeting Retroviral Integration for Safer Gene Therapy: Advances and Challenges

Retroviruses are obligate intracellular parasites that must integrate a copy of the viral genome into the host DNA. The integration reaction is performed by the viral enzyme integrase in complex with the two ends of the viral cDNA genome and yields an integrated provirus. Retroviral vector particles are attractive gene therapy delivery tools due to their stable integration. However, some retroviral integration events may dysregulate host oncogenes leading to cancer in gene therapy patients. Multiple strategies to target retroviral integration, particularly to genetic safe harbors, have been tested with limited success. Attempts to target integration may be limited by the multimerization of integrase or the presence of host co-factors for integration. Several retroviral integration complexes have evolved a mechanism of tethering to chromatin via a host protein. Integration host co-factors bind chromatin, anchoring the complex and allowing integration. The tethering factor allows for both close proximity to the target DNA and specificity of targeting. Each retrovirus appears to have distinct preferences for DNA sequence and chromatin features at the integration site. Tethering factors determine the preference for chromatin features, but do not affect the subtle sequence preference at the integration site. The sequence preference is likely intrinsic to the integrase protein. New developments may uncouple the requirement for a tethering factor and increase the ability to redirect retroviral integration.

Sec24C is an HIV-1 host dependency factor crucial for virus replication

Early events of the human immunodeficiency virus 1 (HIV-1) lifecycle, such as post-entry virus trafficking, uncoating and nuclear import, are poorly characterized because of limited understanding of virus-host interactions. Here, we used mass spectrometry-based proteomics to delineate cellular binding partners of curved HIV-1 capsid lattices and identified Sec24C as an HIV-1 host dependency factor. Gene deletion and complementation in Jurkat cells revealed that Sec24C facilitates infection and markedly enhances HIV-1 spreading infection. Downregulation of Sec24C in HeLa cells substantially reduced HIV-1 core stability and adversely affected reverse transcription, nuclear import and infectivity. Live-cell microscopy showed that Sec24C co-trafficked with HIV-1 cores in the cytoplasm during virus ingress. Biochemical assays demonstrated that Sec24C directly and specifically interacted with hexameric capsid lattices. A 2.3-Å resolution crystal structure of Sec24C_228-242 in the complex with a capsid hexamer revealed that the Sec24C FG-motif bound to a pocket comprised of two adjoining capsid subunits. Combined with previous data^1-4, our findings indicate that a capsid-binding FG-motif is conserved in unrelated proteins present in the cytoplasm (Sec24C), the nuclear pore (Nup153; refs. ^3,4) and the nucleus (CPSF6; refs. ^1,2). We propose that these virus-host interactions during HIV-1 trafficking across different cellular compartments are crucial for productive infection of target cells.

Rationally Designed ACE2-Derived Peptides Inhibit SARS-CoV-2

Severe acute respiratory syndrome coronavirus (SARS-CoV)-2 is a novel and highly pathogenic coronavirus and is the causative agent of the coronavirus disease 2019 (COVID-19). The high morbidity and mortality associated with COVID-19 and the lack of an approved drug or vaccine for SARS-CoV-2 underscores the urgent need for developing effective antiviral therapies. Therapeutics that target essential viral proteins are effective at controlling virus replication and spread. Coronavirus Spike glycoproteins mediate viral entry and fusion with the host cell, and thus are essential for viral replication. To enter host cells, the Spike proteins of SARS-CoV-2 and related coronavirus, SARS-CoV, bind the host angiotensin-converting enzyme 2 (ACE2) receptor through their receptor binding domains (RBDs). Here, we rationally designed a panel of ACE2-derived peptides based on the RBD-ACE2 binding interfaces of SARS-CoV-2 and SARS-CoV. Using SARS-CoV-2 and SARS-CoV Spike-pseudotyped viruses, we found that a subset of peptides inhibits Spike-mediated infection with IC50 values in the low millimolar range. We identified two peptides that bound Spike RBD in affinity precipitation assays and inhibited infection with genuine SARS-CoV-2. Moreover, these peptides inhibited the replication of a common cold causing coronavirus, which also uses ACE2 as its entry receptor. Results from the infection experiments and modeling of the peptides with Spike RBD identified a 6-amino-acid (Glu37-Gln42) ACE2 motif that is important for SARS-CoV-2 inhibition. Our work demonstrates the feasibility of inhibiting SARS-CoV-2 with peptide-based inhibitors. These findings will allow for the successful development of engineered peptides and peptidomimetic-based compounds for the treatment of COVID-19.

HIV-1 integrase tetramers are the antiviral target of pyridine-based allosteric integrase inhibitors

Allosteric HIV-1 integrase (IN) inhibitors (ALLINIs) are a promising new class of antiretroviral agents that disrupt proper viral maturation by inducing hyper-multimerization of IN. Here we show that lead pyridine-based ALLINI KF116 exhibits striking selectivity for IN tetramers versus lower order protein oligomers. IN structural features that are essential for its functional tetramerization and HIV-1 replication are also critically important for KF116 mediated higher-order IN multimerization. Live cell imaging of single viral particles revealed that KF116 treatment during virion production compromises the tight association of IN with capsid cores during subsequent infection of target cells. We have synthesized the highly active (-)-KF116 enantiomer, which displayed EC₅₀ of ~7 nM against wild type HIV-1 and ~10 fold higher, sub-nM activity against a clinically relevant dolutegravir resistant mutant virus suggesting potential clinical benefits for complementing dolutegravir therapy with pyridine-based ALLINIs.

HIV-1 inserts its genetic code into human genomes, turning healthy cells into virus factories. To do this, the virus uses an enzyme called integrase. Front-line treatments against HIV-1 called “integrase strand-transfer inhibitors” stop this enzyme from working. These inhibitors have helped to revolutionize the treatment of HIV/AIDS by protecting the cells from new infections. But, the emergence of drug resistance remains a serious problem. As the virus evolves, it changes the shape of its integrase protein, substantially reducing the effectiveness of the current therapies. One way to overcome this problem is to develop other therapies that can kill the drug resistant viruses by targeting different parts of the integrase protein. It should be much harder for the virus to evolve the right combination of changes to escape two or more treatments at once.

A promising class of new compounds are “allosteric integrase inhibitors”. These chemical compounds target a part of the integrase enzyme that the other treatments do not yet reach. Rather than stopping the integrase enzyme from inserting the viral code into the human genome, the new inhibitors make integrase proteins clump together and prevent the formation of infectious viruses. At the moment, these compounds are still experimental. Before they are ready for use in people, researchers need to better understand how they work, and there are several open questions to answer. Integrase proteins work in groups of four and it is not clear how the new compounds make the integrases form large clumps, or what this does to the virus. Understanding this should allow scientists to develop improved versions of the drugs.

To answer these questions, Koneru et al. first examined two of the new compounds. A combination of molecular analysis and computer modelling revealed how they work. The compounds link many separate groups of four integrases with each other to form larger and larger clumps, essentially a snowball effect. Live images of infected cells showed that the clumps of integrase get stuck outside of the virus’s protective casing. This leaves them exposed, allowing the cell to destroy the integrase enzymes.

Koneru et al. also made a new compound, called (-)-KF116. Not only was this compound able to tackle normal HIV-1, it could block viruses resistant to the other type of integrase treatment. In fact, in laboratory tests, it was 10 times more powerful against these resistant viruses. Together, these findings help to explain how allosteric integrase inhibitors work, taking scientists a step closer to bringing them into the clinic. In the future, new versions of the compounds, like (-)-KF116, could help to tackle drug resistance in HIV-1.

An Isoquinoline Scaffold as a Novel Class of Allosteric HIV-1 Integrase Inhibitors

Allosteric HIV-1 integrase inhibitors (ALLINIs) are a new class of potential antiretroviral therapies with a unique mechanism of action and drug resistance profile. To further extend this class of inhibitors via a scaffold hopping approach, we have synthesized a series of analogues possessing an isoquinoline ring system. Lead compound 6l binds in the v-shaped pocket at the IN dimer interface and is highly selective for promoting higher-order multimerization of inactive IN over inhibiting IN-LEDGF/p75 binding. Importantly, 6l potently inhibited HIV-1_NL4-3 (A128T IN), which confers marked resistance to archetypal quinoline-based ALLINIs. Thermal degradation studies indicated that at elevated temperatures the acetic acid side chain of specific isoquinoline derivatives undergo decarboxylation reactions. This reactivity has implications for the synthesis of various ALLINI analogues.

Resistance to pyridine-based inhibitor KF116 reveals an unexpected role of integrase in HIV-1 Gag-Pol polyprotein proteolytic processing

The pyridine-based multimerization selective HIV-1 integrase (IN) inhibitors (MINIs) are a distinct subclass of allosteric IN inhibitors. MINIs potently inhibit HIV-1 replication during virion maturation by inducing hyper- or aberrant IN multimerization but are largely ineffective during the early steps of viral replication. Here, we investigated the mechanism for the evolution of a triple IN substitution (T124N/V165I/T174I) that emerges in cell culture with a representative MINI, KF116. We show that HIV-1 NL4-3(IN T124N/V165I/T174I) confers marked (>2000-fold) resistance to KF116. Two IN substitutions (T124N/T174I) directly weaken inhibitor binding at the dimer interface of the catalytic core domain but at the same time markedly impair HIV-1 replication capacity. Unexpectedly, T124N/T174I IN substitutions inhibited proteolytic processing of HIV-1 polyproteins Gag and Gag-Pol, resulting in immature virions. Strikingly, the addition of the third IN substitution (V165I) restored polyprotein processing, virus particle maturation, and significant levels of replication capacity. These results reveal an unanticipated role of IN for polyprotein proteolytic processing during virion morphogenesis. The complex evolutionary pathway for the emergence of resistant viruses, which includes the need for the compensatory V165I IN substitution, highlights a relatively high genetic barrier exerted by MINI KF116. Additionally, we have solved the X-ray structure of the drug-resistant catalytic core domain protein, which provides means for rational development of second-generation MINIs.

HIV-1 Integrase Binds the Viral RNA Genome and Is Essential during Virion Morphogenesis

While an essential role of HIV-1 integrase (IN) for integration of viral cDNA into human chromosome is established, studies with IN mutants and allosteric IN inhibitors (ALLINIs) have suggested that IN can also influence viral particle maturation. However, it has remained enigmatic as to how IN contributes to virion morphogenesis. Here, we demonstrate that IN directly binds the viral RNA genome in virions. These interactions have specificity, as IN exhibits distinct preference for select viral RNA structural elements. We show that IN substitutions that selectively impair its binding to viral RNA result in eccentric, non-infectious virions without affecting nucleocapsid-RNA interactions. Likewise, ALLINIs impair IN binding to viral RNA in virions of wild-type, but not escape mutant, virus. These results reveal an unexpected biological role of IN binding to the viral RNA genome during virion morphogenesis and elucidate the mode of action of ALLINIs.

The Competitive Interplay between Allosteric HIV-1 Integrase Inhibitor BI/D and LEDGF/p75 during the Early Stage of HIV-1 Replication Adversely Affects Inhibitor Potency

Allosteric HIV-1 integrase inhibitors (ALLINIs) have recently emerged as a promising class of antiretroviral agents and are currently in clinical trials. In infected cells, ALLINIs potently inhibit viral replication by impairing virus particle maturation but surprisingly exhibit a reduced EC50 for inhibiting HIV-1 integration in target cells. To better understand the reduced antiviral activity of ALLINIs during the early stage of HIV-1 replication, we investigated the competitive interplay between a potent representative ALLINI, BI/D, and LEDGF/p75 with HIV-1 integrase. While the principal binding sites of BI/D and LEDGF/p75 overlap at the integrase catalytic core domain dimer interface, we show that the inhibitor and the cellular cofactor induce markedly different multimerization patterns of full-length integrase. LEDGF/p75 stabilizes an integrase tetramer through the additional interactions with the integrase N-terminal domain, whereas BI/D induces protein-protein interactions in C-terminal segments that lead to aberrant, higher-order integrase multimerization. We demonstrate that LEDGF/p75 binds HIV-1 integrase with significantly higher affinity than BI/D and that the cellular protein is able to reverse the inhibitor induced aberrant, higher-order integrase multimerization in a dose-dependent manner in vitro. Consistent with these observations, alterations of the cellular levels of LEDGF/p75 markedly affected BI/D EC50 values during the early steps of HIV-1 replication. Furthermore, genome-wide sequencing of HIV-1 integration sites in infected cells demonstrate that LEDGF/p75-dependent integration site selection is adversely affected by BI/D treatment. Taken together, our studies elucidate structural and mechanistic details of the interplay between LEDGF/p75 and BI/D during the early stage of HIV-1 replication.

The mechanism of H171T resistance reveals the importance of Nδ-protonated His171 for the binding of allosteric inhibitor BI-D to HIV-1 integrase

BACKGROUND

Allosteric HIV-1 integrase (IN) inhibitors (ALLINIs) are an important new class of anti-HIV-1 agents. ALLINIs bind at the IN catalytic core domain (CCD) dimer interface occupying the principal binding pocket of its cellular cofactor LEDGF/p75. Consequently, ALLINIs inhibit HIV-1 IN interaction with LEDGF/p75 as well as promote aberrant IN multimerization. Selection of viral strains emerging under the inhibitor pressure has revealed mutations at the IN dimer interface near the inhibitor binding site.

RESULTS

We have investigated the effects of one of the most prevalent substitutions, H171T IN, selected under increasing pressure of ALLINI BI-D. Virus containing the H171T IN substitution exhibited an ~68-fold resistance to BI-D treatment in infected cells. These results correlated with ~84-fold reduced affinity for BI-D binding to recombinant H171T IN CCD protein compared to its wild type (WT) counterpart. However, the H171T IN substitution only modestly affected IN-LEDGF/p75 binding and allowed HIV-1 containing this substitution to replicate at near WT levels. The x-ray crystal structures of BI-D binding to WT and H171T IN CCD dimers coupled with binding free energy calculations revealed the importance of the Nδ- protonated imidazole group of His171 for hydrogen bonding to the BI-D tert-butoxy ether oxygen and establishing electrostatic interactions with the inhibitor carboxylic acid, whereas these interactions were compromised upon substitution to Thr171.

CONCLUSIONS

Our findings reveal a distinct mechanism of resistance for the H171T IN mutation to ALLINI BI-D and indicate a previously undescribed role of the His171 side chain for binding the inhibitor.

Molecular mechanisms of retroviral integration site selection

Retroviral replication proceeds through an obligate integrated DNA provirus, making retroviral vectors attractive vehicles for human gene-therapy. Though most of the host cell genome is available for integration, the process of integration site selection is not random. Retroviruses differ in their choice of chromatin-associated features and also prefer particular nucleotide sequences at the point of insertion. Lentiviruses including HIV-1 preferentially integrate within the bodies of active genes, whereas the prototypical gammaretrovirus Moloney murine leukemia virus (MoMLV) favors strong enhancers and active gene promoter regions. Integration is catalyzed by the viral integrase protein, and recent research has demonstrated that HIV-1 and MoMLV targeting preferences are in large part guided by integrase-interacting host factors (LEDGF/p75 for HIV-1 and BET proteins for MoMLV) that tether viral intasomes to chromatin. In each case, the selectivity of epigenetic marks on histones recognized by the protein tether helps to determine the integration distribution. In contrast, nucleotide preferences at integration sites seem to be governed by the ability for the integrase protein to locally bend the DNA duplex for pairwise insertion of the viral DNA ends. We discuss approaches to alter integration site selection that could potentially improve the safety of retroviral vectors in the clinic.

A critical role of the C-terminal segment for allosteric inhibitor-induced aberrant multimerization of HIV-1 integrase

Allosteric HIV-1 integrase (IN) inhibitors (ALLINIs) are a promising class of antiretroviral agents for clinical development. Although ALLINIs promote aberrant IN multimerization and inhibit IN interaction with its cellular cofactor LEDGF/p75 with comparable potencies in vitro, their primary mechanism of action in infected cells is through inducing aberrant multimerization of IN. Crystal structures have shown that ALLINIs bind at the IN catalytic core domain dimer interface and bridge two interacting subunits. However, how these interactions promote higher-order protein multimerization is not clear. Here, we used mass spectrometry-based protein footprinting to monitor surface topology changes in full-length WT and the drug-resistant A128T mutant INs in the presence of ALLINI-2. These experiments have identified protein-protein interactions that extend beyond the direct inhibitor binding site and which lead to aberrant multimerization of WT but not A128T IN. Specifically, we demonstrate that C-terminal residues Lys-264 and Lys-266 play an important role in the inhibitor induced aberrant multimerization of the WT protein. Our findings provide structural clues for exploiting IN multimerization as a new, attractive therapeutic target and are expected to facilitate development of improved inhibitors.

Altering murine leukemia virus integration through disruption of the integrase and BET protein family interaction

We report alterations to the murine leukemia virus (MLV) integrase (IN) protein that successfully result in decreasing its integration frequency at transcription start sites and CpG islands, thereby reducing the potential for insertional activation. The host bromo and extraterminal (BET) proteins Brd2, 3 and 4 interact with the MLV IN protein primarily through the BET protein ET domain. Using solution NMR, protein interaction studies, and next generation sequencing, we show that the C-terminal tail peptide region of MLV IN is important for the interaction with BET proteins and that disruption of this interaction through truncation mutations affects the global targeting profile of MLV vectors. The use of the unstructured tails of gammaretroviral INs to direct association with complexes at active promoters parallels that used by histones and RNA polymerase II. Viruses bearing MLV IN C-terminal truncations can provide new avenues to improve the safety profile of gammaretroviral vectors for human gene therapy.

Bimodal high-affinity association of Brd4 with murine leukemia virus integrase and mononucleosomes

The importance of understanding the molecular mechanisms of murine leukemia virus (MLV) integration into host chromatin is highlighted by the development of MLV-based vectors for human gene-therapy. We have recently identified BET proteins (Brd2, 3 and 4) as the main cellular binding partners of MLV integrase (IN) and demonstrated their significance for effective MLV integration at transcription start sites. Here we show that recombinant Brd4, a representative of the three BET proteins, establishes complementary high-affinity interactions with MLV IN and mononucleosomes (MNs). Brd4(1–720) but not its N- or C-terminal fragments effectively stimulate MLV IN strand transfer activities in vitro. Mass spectrometry- and NMR-based approaches have enabled us to map key interacting interfaces between the C-terminal domain of BRD4 and the C-terminal tail of MLV IN. Additionally, the N-terminal fragment of Brd4 binds to both DNA and acetylated histone peptides, allowing it to bind tightly to MNs. Comparative analyses of the distributions of various histone marks along chromatin revealed significant positive correlations between H3- and H4-acetylated histones, BET protein-binding sites and MLV-integration sites. Our findings reveal a bimodal mechanism for BET protein-mediated MLV integration into select chromatin locations.

The A128T resistance mutation reveals aberrant protein multimerization as the primary mechanism of action of allosteric HIV-1 integrase inhibitors

Allosteric HIV-1 integrase (IN) inhibitors (ALLINIs) are a very promising new class of anti-HIV-1 agents that exhibit a multimodal mechanism of action by allosterically modulating IN multimerization and interfering with IN-lens epithelium-derived growth factor (LEDGF)/p75 binding. Selection of viral strains under ALLINI pressure has revealed an A128T substitution in HIV-1 IN as a primary mechanism of resistance. Here, we elucidated the structural and mechanistic basis for this resistance. The A128T substitution did not affect the hydrogen bonding between ALLINI and IN that mimics the IN-LEDGF/p75 interaction but instead altered the positioning of the inhibitor at the IN dimer interface. Consequently, the A128T substitution had only a minor effect on the ALLINI IC50 values for IN-LEDGF/p75 binding. Instead, ALLINIs markedly altered the multimerization of IN by promoting aberrant higher order WT (but not A128T) IN oligomers. Accordingly, WT IN catalytic activities and HIV-1 replication were potently inhibited by ALLINIs, whereas the A128T substitution in IN resulted in significant resistance to the inhibitors both in vitro and in cell culture assays. The differential multimerization of WT and A128T INs induced by ALLINIs correlated with the differences in infectivity of HIV-1 progeny virions. We conclude that ALLINIs primarily target IN multimerization rather than IN-LEDGF/p75 binding. Our findings provide the structural foundations for developing improved ALLINIs with increased potency and decreased potential to select for drug resistance.

Structure of Desulfitobacterium hafniense PylSc, a pyrrolysyl-tRNA synthetase

Pyrrolysine, the 22nd genetically-encoded amino acid, is charged onto its specific tRNA by PylS, a pyrrolysyl-tRNA synthetase. While PylS is found as a single protein in certain archaeal methanogens, in the gram-positive bacterium Desulfitobacterium hafniense, PylS is divided into two separate proteins, PylSn and PylSc, corresponding to the N-terminal and C-terminal domains of the single PylS protein found in methanogens. Previous crystallographic studies have provided the structure of a truncated C-terminal portion of the archaeal Methanosarcina mazei PylS associated with catalysis. Here, we report the apo 2.1A resolution structure of the intact D. hafniense PylSc protein and compare it to structures of the C-terminal truncated PylS from methanogenic species. In PylSc, the hydrophobic pocket binding the ring of pyrrolysine is more constrained than in the archaeal enzyme; other structural differences are also apparent.

A natural genetic code expansion cassette enables transmissible biosynthesis and genetic encoding of pyrrolysine

Pyrrolysine has entered natural genetic codes by the translation of UAG, a canonical stop codon. UAG translation as pyrrolysine requires the pylT gene product, an amber-decoding tRNA(Pyl) that is aminoacylated with pyrrolysine by the pyrrolysyl-tRNA synthetase produced from the pylS gene. The pylTS genes form a gene cluster with pylBCD, whose functions have not been investigated. The pylTSBCD gene order is maintained not only in methanogenic Archaea but also in a distantly related Gram-positive Bacterium, indicating past horizontal gene transfer of all five genes. Here we show that lateral transfer of pylTSBCD introduces biosynthesis and genetic encoding of pyrrolysine into a naïve organism. PylS-based assays demonstrated that pyrrolysine was biosynthesized in Escherichia coli expressing pylBCD from Methanosarcina acetivorans. Production of pyrrolysine did not require tRNA(Pyl) or PylS. However, when pylTSBCD were coexpressed with mtmB1, encoding the methanogen monomethylamine methyltransferase, UAG was translated as pyrrolysine to produce recombinant monomethylamine methyltransferase. Expression of pylTSBCD also suppressed an amber codon introduced into the E. coli uidA gene. Strains lacking one of the pylBCD genes did not produce pyrrolysine or translate UAG as pyrrolysine. These results indicated that pylBCD gene products biosynthesize pyrrolysine using metabolites common to Bacteria and Archaea and, furthermore, that the pyl gene cluster represents a "genetic code expansion cassette," previously unprecedented in natural organisms, whose transfer allows an existing codon to be translated as a novel endogenously synthesized free amino acid. Analogous cassettes may have served similar functions for other amino acids during the evolutionary expansion of the canonical genetic code.

Characterization of a Methanosarcina acetivorans mutant unable to translate UAG as pyrrolysine

The methyltransferases initiating methanogenesis from trimethylamine, dimethylamine and monomethylamine possess a novel residue, pyrrolysine. Pyrrolysine is the 22nd amino acid, because it is encoded by a single amber (UAG) codon in methylamine methyltransferase transcripts. A dedicated tRNA(CUA) for pyrrolysine, tRNA(Pyl), is charged by a pyrrolysyl-tRNA synthetase with pyrrolysine. As the first step towards the genetic analysis of UAG translation as pyrrolysine, a 761 base-pair genomic segment in Methanosarcina acetivorans containing the pylT gene (encoding tRNA(Pyl)) was deleted and replaced by a puromycin resistance cassette. The DeltappylT mutant lacks detectable tRNA(Pyl), but grows as wild-type on methanol or acetate. Unlike wild-type, the DeltappylT strain cannot grow on any methylamine, nor use monomethylamine as sole nitrogen source. Wild-type cells, but not DeltappylT, have monomethylamine methyltransferase activity during growth on methanol. Immunoblot analysis indicated monomethylamine methyltransferase was absent in DeltappylT. The phenotype of DeltappylT reveals the deficiency in methylamine metabolism expected of a Methanosarcina species unable to decode UAG codons as pyrrolysine, but also that loss of pylT does not compromise growth on other substrates. These results indicate that in-depth genetic analysis of UAG translation as pyrrolysine is feasible, as deletion of pylT is conditionally lethal depending on growth substrate.

Direct charging of tRNA(CUA) with pyrrolysine in vitro and in vivo

Pyrrolysine is the 22nd amino acid. An unresolved question has been how this atypical genetically encoded residue is inserted into proteins, because all previously described naturally occurring aminoacyl-tRNA synthetases are specific for one of the 20 universally distributed amino acids. Here we establish that synthetic L-pyrrolysine is attached as a free molecule to tRNA(CUA) by PylS, an archaeal class II aminoacyl-tRNA synthetase. PylS activates pyrrolysine with ATP and ligates pyrrolysine to tRNA(CUA) in vitro in reactions specific for pyrrolysine. The addition of pyrrolysine to Escherichia coli cells expressing pylT (encoding tRNA(CUA)) and pylS results in the translation of UAG in vivo as a sense codon. This is the first example from nature of direct aminoacylation of a tRNA with a non-canonical amino acid and shows that the genetic code of E. coli can be expanded to include UAG-directed pyrrolysine incorporation into proteins.