The process of integrating the reverse-transcribed HIV-1 DNA into the host chromosomal DNA is catalyzed by the virally encoded enzyme integrase (IN). Integration requires two metal-dependent reactions, 3 end processing and strand transfer. Compounds that contain a diketo acid moiety have been shown to selectively inhibit the strand transfer reaction of IN in vitro and in infected cells and are effective as inhibitors of HIV-1 replication. To characterize the molecular basis of inhibition, we used functional assays and binding assays to evaluate a series of structurally related analogs. These studies focused on investigating the role of the conserved carboxylate and metal binding. We demonstrate that an acidic moiety such as a carboxylate or isosteric heterocycle is not required for binding to the enzyme complex but is essential for inhibition and confers distinct metaldependent properties on the inhibitor. Binding requires divalent metal and resistance is metal dependent with active site mutants displaying resistance only when the enzymes are evaluated in the context of Mg 2؉ . The mechanism of action of these inhibitors is therefore likely a consequence of the interaction between the acid moiety and metal ion(s) in the IN active site, resulting in a functional sequestration of the critical metal cofactor(s). These studies thus have implications for modeling active site inhibitors of IN, designing and evaluating analogs with improved efficacy, and identifying inhibitors of other metal-dependent phosphotransferases.A n essential step in HIV replication is the integration of the reverse-transcribed viral genome into host chromosomal DNA by the virally encoded integrase (IN) protein (1-3). Integration is required for efficient long terminal repeat-driven transcription of the provirus for the production of viral proteins and RNA progeny. IN represents an important chemotherapeutic target, as its inactivation, either by mutagenesis or inhibition, blocks productive infection by HIV-1 (4-7).Integration is carried out in the cell in a series of distinct steps (8-10). First, IN cleaves the two terminal nucleotides from each 3Ј end of the viral DNA. The 3Ј processing reaction is carried out concurrently with or soon after reverse transcription in the cytoplasm. In the second step, strand transfer, IN catalyzes staggered nicking of the target chromosomal DNA and joining of each 3Ј end of the viral DNA to the 5Ј ends of the host DNA. Strand transfer is temporally and spatially separated from 3Ј processing and occurs after transport of the preintegration complex from the cytoplasm into the nucleus.Divalent metals such as Mg 2ϩ or Mn 2ϩ are required for both 3Ј processing and strand transfer and for the assembly of IN onto specific viral donor DNA to form a complex competent to carry out either function (11)(12)(13) (18), L-731,988 and related DKAs inhibit integration and viral replication in cell culture (7). Mutations that confer resistance to DKAs have been identified and map to IN residues adjacent to D64 and E152. The in...
Abnormal production and accumulation of amyloid-β peptide (Aβ) plays a major role in the pathogenesis of Alzheimer's disease (AD). β-secretase (BACE1) is responsible for the cleavage at the β-site in amyloid β protein precursor (AβPP/APP) to generate the N-terminus of Aβ. Here we report the stepwise identification and characterization of a novel APP-β-site mutant, "NFEV" (APP NFEV) in vitro and in cells. In vitro, the APP NFEV exhibits 100-fold enhanced cleavage rate relative to the "wild-type" substrate (APPwt) and 10-fold increase relative to the Swedish-type mutation variant (APPsw). In cells, it was preferably cleaved among 24 APP β-site mutations tested. More importantly, the APP NFEV mutant failed to generate any detectable Aβ peptides in BACE1-KO mouse fibroblast cells. The production of Aβ peptides was restored by co-transfecting human BACE1, demonstrating that BACE1 is the only enzyme responsible for the processing of APP NFEV in these cells. Analysis of APP NFEV cleavage products secreted in the media revealed that in cells BACE1 cleaves APP NFEV at the position between NF and EV, identical to that observed in vitro. A BACE inhibitor blocked the processing of the APP NFEV β-site in vitro and in cells. Our data indicates that the "NFEV" mutant is not only an enhanced substrate for BACE1 in vitro, but also a specific substrate for BACE1 in cells.
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