Structure of a dihydroxycoumarin active-site inhibitor in complex with the RNase H domain of HIV-1 reverse transcriptase and structure-activity analysis of inhibitor analogs.

Human immunodeficiency virus (HIV) encodes four essential enzymes: protease, integrase, reverse transcriptase (RT)-associated DNA polymerase, and RT-associated ribonuclease H (RNase H). Current clinically approved anti-AIDS drugs target all HIV enzymatic activities except RNase H, which has proven to be a very difficult target for HIV drug discovery. Our high-throughput screening activities identified the dihydroxycoumarin compound F3284-8495 as a specific inhibitor of RT RNase H, with low micromolar potency in vitro. Optimization of inhibitory potency can be facilitated by structural information about inhibitor-target binding. Here, we report the crystal structure of F3284-8495 bound to the active site of an isolated RNase H domain of HIV-1 RT at a resolution limit of 1.71Å. From predictions based on this structure, compounds were obtained that showed improved inhibitory activity. Computational analysis suggested structural alterations that could provide additional interactions with RT and thus improve inhibitory potency. These studies established proof of concept that F3284-8495 could be used as a favorable chemical scaffold for development of HIV RNase H inhibitors.

Detecting allosteric sites of HIV-1 reverse transcriptase by X-ray crystallographic fragment screening.

HIV-1 reverse transcriptase (RT) undergoes a series of conformational changes during viral replication and is a central target for antiretroviral therapy. The intrinsic flexibility of RT can provide novel allosteric sites for inhibition. Crystals of RT that diffract X-rays to better than 2 Å resolution facilitated the probing of RT for new druggable sites using fragment screening by X-ray crystallography. A total of 775 fragments were grouped into 143 cocktails, which were soaked into crystals of RT in complex with the non-nucleoside drug rilpivirine (TMC278). Seven new sites were discovered, including the Incoming Nucleotide Binding, Knuckles, NNRTI Adjacent, and 399 sites, located in the polymerase region of RT, and the 428, RNase H Primer Grip Adjacent, and 507 sites, located in the RNase H region. Three of these sites (Knuckles, NNRTI Adjacent, and Incoming Nucleotide Binding) are inhibitory and provide opportunities for discovery of new anti-AIDS drugs.

Crystallographic study of a novel subnanomolar inhibitor provides insight on the binding interactions of alkenyldiarylmethanes with human immunodeficiency virus-1 reverse transcriptase.

Two crystal structures have been solved for separate complexes of alkenyldiarylmethane (ADAM) nonnucleoside reverse transcriptase inhibitors (NNRTI) 3 and 4 with HIV-1 reverse transcriptase (RT). The structures reveal inhibitor binding is exclusively hydrophobic in nature and the shape of the inhibitor-bound NNRTI binding pocket is unique among other reported inhibitor-RT crystal structures. Primarily, ADAMs 3 and 4 protrude from a large gap in the back side of the binding pocket, placing portions of the inhibitors unusually close to the polymerase active site and allowing 3 to form a weak hydrogen bond with Lys223. The lack of additional stabilizing interactions, beyond the observed hydrophobic surface contacts, between 4 and RT is quite perplexing given the extreme potency of the compound (IC(50)

Crystal engineering of HIV-1 reverse transcriptase for structure-based drug design.

HIV-1 reverse transcriptase (RT) is a primary target for anti-AIDS drugs. Structures of HIV-1 RT, usually determined at approximately 2.5-3.0 A resolution, are important for understanding enzyme function and mechanisms of drug resistance in addition to being helpful in the design of RT inhibitors. Despite hundreds of attempts, it was not possible to obtain the structure of a complex of HIV-1 RT with TMC278, a nonnucleoside RT inhibitor (NNRTI) in advanced clinical trials. A systematic and iterative protein crystal engineering approach was developed to optimize RT for obtaining crystals in complexes with TMC278 and other NNRTIs that diffract X-rays to 1.8 A resolution. Another form of engineered RT was optimized to produce a high-resolution apo-RT crystal form, reported here at 1.85 A resolution, with a distinct RT conformation. Engineered RTs were mutagenized using a new, flexible and cost effective method called methylated overlap-extension ligation independent cloning. Our analysis suggests that reducing the solvent content, increasing lattice contacts, and stabilizing the internal low-energy conformations of RT are critical for the growth of crystals that diffract to high resolution. The new RTs enable rapid crystallization and yield high-resolution structures that are useful in designing/developing new anti-AIDS drugs.

High-resolution structure of the p53 core domain: implications for binding small-molecule stabilizing compounds.

The p53 transcriptional regulator is the most frequently mutated protein in human cancers and the majority of tumor-derived p53 mutations map to the central DNA-binding core domain, with a subset of these mutations resulting in reduced p53 stability. Here, the 1.55 A crystal structure of the mouse p53 core domain with a molecule of tris(hydroxymethyl)aminomethane (Tris) bound through multiple hydrogen bonds to a region of p53 shown to be important for repair of a subset of tumor-derived p53-stability mutations is reported. Consistent with the hypothesis that Tris binding stabilizes the p53 core domain, equilibrium denaturation experiments are presented that demonstrate that Tris binding increases the thermodynamic stability of the mouse p53 core domain by 3.1 kJ mol(-1) and molecular-dynamic simulations are presented revealing an overall reduction in root-mean-square deviations of the core domain of 0.7 A when Tris is bound. It is also shown that these crystals of the p53 core domain are suitable for the multiple-solvent crystal structure approach to identify other potential binding sites for possible core-domain stabilization compounds. Analysis of the residue-specific temperature factors of the high-resolution core-domain structure, coupled with a comparison with other core-domain structures, also reveals that the L1, H1-S5 and S7-S8 core-domain loops, also shown to mediate various p53 activities, harbor inherent flexibility, suggesting that these regions might be targets for other p53-stabilizing compounds. Together, these studies provide a molecular scaffold for the structure-based design of p53-stabilization compounds for development as possible therapeutic agents.

Structure of the p53 core domain dimer bound to DNA.

The p53 tumor suppressor protein binds to DNA as a dimer of dimers to regulate transcription of genes that mediate responses to cellular stress. We have prepared a cross-linked trapped p53 core domain dimer bound to decamer DNA and have determined its structure by x-ray crystallography to 2.3A resolution. The p53 core domain subunits bind nearly symmetrically to opposite faces of the DNA in a head-to-head fashion with a loophelix motif making sequence-specific DNA contacts and bending the DNA by about 20 degrees at the site of protein dimerization. Protein subunit interactions occur over the central DNA minor groove and involve residues from a zinc-binding region. Analysis of tumor derived p53 mutations reveals that the dimerization interface represents a third hot spot for mutation that also includes residues associated with DNA contact and protein stability. Residues associated with p53 dimer formation on DNA are poorly conserved in the p63 and p73 paralogs, possibly contributing to their functional differences. We have used the dimeric protein-DNA complex to model a dimer of p53 dimers bound to icosamer DNA that is consistent with solution bending data and suggests that p53 core domain dimer-dimer contacts are less frequently mutated in human cancer than intra-dimer contacts.

Acridine derivatives activate p53 and induce tumor cell death through Bax.

CP-31398 activates wild-type p53 by a novel mechanism that does not involve phosphorylation of the amino-terminus of p53 and disassociation of MDM2. To identify more potent CP-31398-like p53 activators, we synthesized 4 acridine derivatives with a similar structure to CP-31398. These four compounds induced strong p53 transcription in cells with wild-type p53. We also found that several randomly chosen acridine derivatives, including 9-aminoacridine, amsacrine, quinacrine and acridine orange, induced p53 transcriptional activity. All these acridine derivatives stabilized p53 protein by blocking its ubiquitination, without phosphorylation of ser15 or ser20 on p53. Furthermore, acridine derivatives induced p53-dependent cell death. Knockout of Bax, a p53 target and a key cell death inducer in both intrinsic and extrinsic apoptotic pathways, blocked acridine derivatives from inducing cell death. In addition, in vivo delivery of quinacrine and amsacrine induced p53 transcriptional activity in tumor xenografts. Our results reveal that DNA-intercalating acridine derivatives can induce p53 stabilization by a manner similar to CP-31398. These findings provide insights into p53 regulation in response to DNA intercalating drugs and may assist new anti-cancer drug design.