Purification and characterization of HIV-1 reverse transcriptase having a 1:1 ratio of p66 and p51 subunits

Wild-type and several mutant forms of recombinant human immunodeficiency virus type-1 reverse transcriptase were overexpressed as either the p66 or the p51 subunit in a protease-deficient strain of Escherichia coli. Immediately prior to cell lysis, p51 cell paste was mixed with cell paste containing the corresponding overexpressed p66 subunit in a ratio resulting in an excess of the smaller subunit with respect to the larger. During the subsequent chromatography steps stable heterodimer p66/p51 was purified to homogeneity. This protein was characterized by amino acid analysis, denaturing sodium dodecyl sulfate-polyacrylamide gel electrophoresis, analytical gel filtration HPLC, laser desorption mass spectroscopy, and isoelectric focusing. In addition, we were able to obtain crystals of the purified enzyme complexed with a quinazolinone class nonnucleoside inhibitor that diffracted to 3.2 A resolution. A potential application of this expression/purification methodology is the ability to alter specific amino acids residues, by site-directed-mutagenesis, of only one subunit of the RT-dimer.

Susceptibilities of human immunodeficiency virus type 1 enzyme and viral variants expressing multiple resistance-engendering amino acid substitutions to reserve transcriptase inhibitors

To evaluate the potential that multiply resistant human immunodeficiency virus type 1 variants may arise during combination nucleoside and nonnucleoside reverse transcriptase inhibitor therapy, we constructed a series of mutant reverse transcriptase enzymes and viruses that coexpressed various combinations of resistance-associated amino acid substitutions. Substitutions at residues 100 (Leu-->Ile) and 181 (Tyr-->Cys), which mediate resistance to the nonnucleosides, suppressed resistance to 3'-azido-3'-deoxythymidine (AZT) when coexpressed with AZT-specific substitutions. However, a number of viral variants that exhibited significantly reduced susceptibilities to both classes of inhibitors were constructed.

Human immunodeficiency virus type 1 protease inhibitors: evaluation of resistance engendered by amino acid substitutions in the enzyme's substrate binding site

The human immunodeficiency virus type 1 (HIV-1) protease is a homodimeric aspartyl endopeptidase that is required for virus replication. A number of specific, active-site inhibitors for this enzyme have been described. Many of the inhibitors exhibit significant differences in activity against the HIV-1 and HIV type 2 (HIV-2) enzymes. An initial study was conducted to ascertain the HIV-1 protease's potential to lose sensitivity to several test inhibitors while retaining full enzymatic activity. The substrate binding sites of the HIV-1 and HIV-2 enzymes are almost fully conserved, except for four amino acid residues at positions 32, 47, 76, and 82. Accordingly, recombinant mutant type 1 proteases were constructed that contained the cognate type 2 residue at each of these four positions. The substitution at position 32 resulted in a significant adverse effect on inhibitor potency. However, this substitution also mediated a noted increase in the Km of the substrate. Individual substitutions at the remaining three positions, as well as a combination of all four substitutions, had very little effect on enzyme activity or inhibitor susceptibility. Hence, the four studied active site residues are insufficient to be responsible for differences in inhibitor sensitivity between the HIV-1 and HIV-2 proteases and are unlikely to contribute to the generation of inhibitor-resistant mutant HIV-1 protease.

Inhibition of HIV-1 reverse transcriptase by pyridinone derivatives. Potency, binding characteristics, and effect of template sequence

The inhibition of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase by pyridinone compounds has been investigated using as templates synthetic RNA with sequences based on the HIV-1 genome sequence. In reactions catalyzed by the enzyme that incorporated more than one nucleotide per primer, inhibition by a representative pyridinone inhibitor, 3-[2-(1,3-benzoxazol-2-yl)ethyl]-5-ethyl-6-methyl-pyridin-2(1H)one (L-696,229), was noncompetitive against deoxynucleotide triphosphate. For reactions that incorporated one deoxynucleotide per primer, IC50 values ranged from 20 to 200 nM, depending on the position of incorporation of the incoming deoxynucleotide base on the template. Inhibition of synthesis on a set of four templates differing only at the template base complementary to the incoming nucleotide had similar IC50 values. These results demonstrate that inhibitory potency is dependent on the primary structure of the template and that inhibitory potency is largely independent of the identity of the incoming nucleotide base. The inhibition of HIV-1 reverse transcription by L-696,229 also displayed slow-binding characteristics. The slow-binding aspect was exploited to gauge the interaction between inhibitor and enzyme. By titrating the reduction in the extent of the burst of synthesis observed in a reaction incorporating dideoxythymidine monophosphate into poly(rA)-oligo(dT)18, the apparent equilibrium constant for dissociation of the reverse transcriptase-L-696,229 complex was estimated to be 400 nM.

Functional analysis of HIV-1 reverse transcriptase amino acids involved in resistance to multiple nonnucleoside inhibitors

Several novel, structurally distinct classes of specific human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) nonnucleoside inhibitors have been described recently. These include the pyridinone derivatives L-697,639, L-697,661, and L-696,229 as well as BI-RG-587 and the tetrahydroimidazo[4,5,1-j,k]-benzodiazepin-2(1H)-one and -thione compounds. Previous studies have implicated involvement of the RT amino acid residues at positions 103, 181, and 188 in the activity of the compounds. Accordingly, HIV-1 RT mutants containing a series of amino acid substitutions at these positions were constructed. The relative resistance of purified mutant enzymes to each of the inhibitors was assessed. This analysis established the functional equivalence of the three inhibitor classes and provided evidence for the interaction of the 103 site with the 181/188 region. Amino acid substitutions at these positions were also found to influence RT sensitivity to inhibition by phosphonoformate, thereby suggesting a close association between this pyrophosphate analog's binding site in RT and the binding site of the nonnucleoside inhibitors. In addition, aromatic stacking of the amino acid side groups at residues 181 and 188 was suggested to be required for inhibitor activity.

Identification of the human immunodeficiency virus reverse transcriptase residues that contribute to the activity of diverse nonnucleoside inhibitors

The reverse transcriptase (RT) of human immunodeficiency virus type 1 (HIV-1) is potently inhibited by a structurally diverse group of nonnucleoside compounds. These include pyridinone derivatives, tetrahydroimadazo[4,5,1-j,k][1,4]-benzodiazepin-2(1H)-one and -thione, and BI-RG-587 (nevirapine). The compounds act noncompetitively, by an unknown mechanism, with respect to template-primer and nucleotide substrates. Despite a high degree of similarity between the HIV-1 and HIV-2 RTs, the HIV-2 enzyme is totally insensitive to these inhibitors. Using a novel method for joining DNA sequences, we have exploited this difference between the two enzymes to identify the regions of the RT that contribute to the compounds' inhibitory activities. The relative in vitro sensitivities of HIV-1/HIV-2 chimeric and site-specific mutant enzymes were determined. Sensitivity to inhibition was largely, though not exclusively, dependent upon the RT region defined by amino acid residues 176 to 190, with specific contributions by residues 181 and 188. The region defined by residues 101 to 106 was found to functionally interact with the domain from 155 to 217. In addition, the functional equivalence of the three inhibitor groups was shown.

Avian cells expressing the murine Mx1 protein are resistant to influenza virus infection

The cDNA encoding the murine Mx1 protein, a mediator of resistance to influenza virus, was inserted into a replication-competent avian retroviral vector in either the sense (referred to as Mx+) or the antisense (referred to as Mx-) orientation relative to the viral structural genes. Both vectors produced virus retaining the Mx insert (Mx recombinant viruses referred to as Mx+ and Mx-) following transfection into chicken embryo fibroblasts (CEF). Mx protein of the appropriate size and nuclear localization was expressed only in CEF cells infected with the Mx+ virus. Mx expression was observed in all Mx(+)-infected cells and was stable during long-term culture. Cells infected with the Mx+ virus were resistant to infection by human influenza A/WSN/33 (H1N1) and avian influenza viruses A/Turkey/Wisconsin/68 (H5N9) and A/Turkey/Massachusetts/65 (H6N2), but were susceptible to infection by the enveloped RNA viruses Sindbis and vesicular stomatitis virus (VSV). Normal CEF and cells infected with the Mx virus were susceptible to influenza A, Sindbis, and VSV. The synthesis of influenza proteins, especially the larger polymerase and hemagglutinin proteins, was reduced in Mx+ retrovirus-infected cells superinfected by influenza A.

Sequence-specific interaction of Tat protein and Tat peptides with the transactivation-responsive sequence element of human immunodeficiency virus type 1 in vitro

Bacterially expressed Tat protein of human immunodeficiency virus type 1 binds selectively to short RNA transcripts containing the viral transactivation-responsive element (TAR). Sequences sufficient for Tat interaction map to the distal portion of the TAR stem-loop. We show that critical sequences for Tat binding are located in the single-stranded "bulge," but no requirement for specific "loop" sequences could be demonstrated. TAR RNA competed for complex formation, and TAR mutants exhibited up to 10-fold reduced affinity for Tat. Synthetic peptides containing the basic region of Tat bound selectively to TAR RNA and exhibited the same sequence requirements and similar relative affinities for mutant TAR RNA as the intact protein. These results suggest that Tat contains a small RNA-binding domain capable of recognizing TAR and implicate functional relevance for direct Tat-TAR interaction in transactivation.

Bacterial expression of antibody fragments that block human rhinovirus infection of cultured cells

Human rhinoviruses are the major causative agents of the common cold in humans and have been divided into major and minor groups based on receptor specificity. cDNAs encoding the light and heavy chains of a murine monoclonal antibody that recognizes the major group receptor were cloned, abundantly expressed in Escherichia coli, and renatured into Fab fragments that blocked virus binding and protected HeLa cell monolayers from rhinovirus infection. Elimination of the cysteines normally bridging the heavy and light chains yielded molecules indistinguishable from wild-type Fab fragments in virus binding assays. Single-chain antibodies with covalently linked light and heavy variable domains were also expressed and showed receptor binding and cell protection activities. These recombinant antibody fragments are potentially useful in preventing or treating common colds in humans.

Multiple (alpha-NH-ubiquitin)protein endoproteases in cells

Ubiquitin is encoded as a variable, spacerless repeat of the gene terminating with an additional amino acid or as a gene coding for a single ubiquitin with a carboxyl-terminal extension of 52 to 80 amino acids. We report the identification and partial purification of enzymes that specifically hydrolyze the peptide bond between ubiquitin-ubiquitin conjugate (Ub-Ubase) or ubiquitin fusion proteins (Ub-Xase). The Ub-Ubase was separated from the Ub-Xase by dye-ligand-Sepharose chromatography. The Ub-Xase was purified further by affinity chromatography on ubiquitin-Sepharose. The fidelity of the endoprotease reaction was assessed by measuring the ability of the released ubiquitin to be activated by ubiquitin-activating enzyme (E1) which requires intact ubiquitin and by sequence analysis of the released carboxyl extension protein with 52 amino acids after endoproteolysis of human ubiquitin with 52-amino acid carboxyl extension. The failure of both Ub-Ubase and Ub-Xase to cleave a mutant ubiquitin-Gly-76----Ala-metallothionein showed that the endoproteases distinguish Gly-X from an Ala-X peptide bonds.

Gene synthesis, expression, and processing of human ubiquitin carboxyl extension proteins

In order to study 1) the mechanisms responsible for generating free ubiquitin monomer and 2) the function of ubiquitin carboxyl extension proteins in eukaryotes, we have developed a system for expression of human ubiquitin carboxyl extension proteins in prokaryotic and eukaryotic hosts. When expressed in Saccharomyces cerevisiae, the intact ubiquitin carboxyl extension proteins were rapidly processed to free ubiquitin monomer and extension protein. Furthermore, expression in this host conferred a slow growth phenotype mediated by the extension protein. Expression in Escherichia coli did not result in processing of the fusion proteins. However, when the expressed fusion proteins were purified from E. coli and incubated with a rabbit reticulocyte extract, the proteins were rapidly processed to free ubiquitin monomer and extension protein. These results show that human ubiquitin carboxyl extension proteins are processed to ubiquitin and extension protein when expressed in eukaryotic but not prokaryotic cells and that pre- and co-translational events are not necessary for their processing. Establishment of this system will allow for large scale purification of these proteins which will aid future studies on the function and structure of ubiquitin carboxyl extension proteins, as well as the mechanisms responsible for their processing.

Immuno cross-reactivity suggests that catecholamine biosynthesis enzymes and beta-adrenergic receptors may be related

Turkey red blood cell, beta 1-adrenergic receptors (BARs) were prepared to electrophoretic homogeneity by affinity chromatography, size exclusion high performance liquid chromatography, and preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis and used to prepare rabbit polyclonal anti-BAR antibodies. Anti-BAR activity was confirmed by immunoadsorption of [125I]cyanopindolol-labeled BAR to a protein A affinity column using the anti-BAR antibodies. BAR was compared to the catecholamine biosynthetic enzyme dopamine B-hydroxylase (DBH) by anti-BAR antibody cross-reactivity. DBH was purified from bovine adrenal medullae chromaffin vesicles by ion exchange, size exclusion, and concanavalin A-Sepharose chromatography. Final DBH specific activities were 42 +/- 4 units/mg of protein. Homogeneity was confirmed by nondenaturing polyacrylamide gel electrophoresis. Both DBH and BAR were recognized by the anti-BAR antibodies on Western transfer and immunoblotting. No interactions were observed with preimmune controls. Similar results were obtained with glycosylated and deglycosylated DBH, suggesting that the anti-BAR antibodies recognize specific portions of DBH amino acid sequence and not associated carbohydrate. DBH-cross-reactive antibodies were also purified by affinity chromatography using immobilized DBH and shown to immunoadsorb [125I]cyanopindolol-labeled BAR by protein A affinity chromatography. These results suggest that the catecholamine biosynthetic enzyme DBH and BAR may be related in structure.

Evidence for tyrosine at the ligand binding center of beta-adrenergic receptors

Beta 1- and beta 2-adrenergic receptor radioligand antagonist binding activities in plasma membrane preparations from mammalian lung, as well as amphibian and avian red blood cells, have been shown to be inactivated by agents which specifically alkylate tyrosine. In membrane preparations, protection against inactivation was afforded by both agonists and antagonists. In soluble purified preparations, antagonists but not agonists protected against inactivation. These results suggest that tyrosine is located at or near the ligand binding site of both beta 1- and beta 2-adrenergic receptors.

Diffusion rates of cell surface antigens of mouse-human heterokaryons. III. Regulation of lateral diffusion rates by calcium ions

In mouse-human heterokaryons, the lateral diffusion of major histocompatibility (MHC) antigens in the plasma membrane is enhanced by treatment of parent cells with ouabain. Ouabain treatment is ineffective if the medium lacks calcium ion, or if Verapamil, a blocker of calcium channels, is present. The divalent ionophore A23187 also enhances lateral diffusion of MHC antigens, to the same extent as ouabain, A23187 is effective only if calcium is present in the medium. Thus it appears that increased levels of cell calcium release constraints to lateral diffusion of MHC antigens.