Characterization and autoprocessing of precursor and mature forms of human immunodeficiency virus type 1 (HIV 1) protease purified from Escherichia coli

HIV-1 protease substrate-groove: Role in substrate recognition and inhibitor resistance

A key target in the treatment of HIV-1/AIDS has been the viral protease. Here we first studied in silico the evolution of protease resistance. Primary active site resistance mutations were found to weaken interactions between protease and both inhibitor and substrate P4-P4' residues. We next studied the effects of secondary resistance mutations, often distant from the active site, on protease binding to inhibitors and substrates. Those secondary mutations contributed to the rise of multi-drug resistance while also enhancing viral replicative capacity. Here many secondary resistance mutations were found in the HIV-1 protease substrate-grooves, one on each face of the symmetrical protease dimer. The protease active site binds substrate P4-P4' residues, while the substrate-groove allows the protease to bind residues P12-P5/P5'-P12', for a total of twenty-four residues. The substrate-groove secondary resistance mutations were found to compensate for the loss of interactions between the inhibitor resistant protease active site and substrate P4-P4' residues, due to primary resistance mutations, by increasing interactions with substrate P12-P5/P5'-P12' residues. In vitro experiments demonstrated that a multi-drug resistant protease with substrate-groove resistance mutations was slower than wild-type protease in cleaving a peptide substrate, which did not allow for substrate-groove interactions, while it had similar activity as wild-type protease when using a Gag polyprotein in which cleavage-site P12-P5/P5'-P12' residues could be bound by the protease substrate-grooves. When the Gag MA/CA cleavage site P12-P5/P5'-P12' residues were mutated the multi-drug resistant protease cleaved the mutant Gag significantly slower, indicating the importance of the protease S-grooves in binding to substrate.

Gag-Pol processing during HIV-1 virion maturation: a systems biology approach

Proteolytic processing of Gag and Gag-Pol polyproteins by the viral protease (PR) is crucial for the production of infectious HIV-1, and inhibitors of the viral PR are an integral part of current antiretroviral therapy. The process has several layers of complexity (multiple cleavage sites and substrates; multiple enzyme forms; PR auto-processing), which calls for a systems level approach to identify key vulnerabilities and optimal treatment strategies. Here we present the first full reaction kinetics model of proteolytic processing by HIV-1 PR, taking into account all canonical cleavage sites within Gag and Gag-Pol, intermediate products and enzyme forms, enzyme dimerization, the initial auto-cleavage of full-length Gag-Pol as well as self-cleavage of PR. The model allows us to identify the rate limiting step of virion maturation and the parameters with the strongest effect on maturation kinetics. Using the modelling framework, we predict interactions and compensatory potential between individual cleavage rates and drugs, characterize the time course of the process, explain the steep dose response curves associated with PR inhibitors and gain new insights into drug action. While the results of the model are subject to limitations arising from the simplifying assumptions used and from the uncertainties in the parameter estimates, the developed framework provides an extendable open-access platform to incorporate new data and hypotheses in the future.

Analysis and characterization of dimerization inhibition of a multi-drug-resistant human immunodeficiency virus type 1 protease using a novel size-exclusion chromatographic approach

Active-site inhibitors of HIV-1 PR (protease) block viral replication by preventing viral maturation. However, HIV-1 often develops resistance to active-site inhibitors through multiple mutations in PR and therefore recent efforts have focused on inhibiting PR dimerization as an alternative approach. Dimerization inhibitors have been identified using kinetic analysis, but additional characterization of the effect of these inhibitors on PR by physical methods has been difficult. In the present study, we identified a PR(MDR) (multi-drug-resistant HIV-1 PR) that was highly resistant to autoproteolysis. Using this PR and a novel size-exclusion chromatographic approach that incorporated fluorescence and MS detection, we were able to demonstrate inhibition of dimerization using P27 (peptide 27), a peptide dimerization inhibitor of PR previously identified on the basis of kinetic analysis. Incubation of PR(MDR) with P27, or other dimerization inhibitors, led to a dose- and time-dependent formation of PR monomers based on the change in elution time by size exclusion and its similar elution time to engineered forms of monomeric PR, namely PR(T26A) and glutathionylated PR. In contrast, incubation of PR(MDR) with a potent active-site inhibitor did not change the elution time for the PR(MDR) dimer. The monomeric PR induced by P27 had fluorescent characteristics which were consistent with unfolded PR. Structure-activity studies identified the active regions of P27 and experiments were performed to examine the effect of other dimerization inhibitors on PR. The present study is the first characterization of dimerization inhibition of PR(MDR), a prime target for these inhibitors, using a novel size-exclusion chromatographic approach.

Identification of a key target sequence to block human immunodeficiency virus type 1 replication within the gag-pol transframe domain

Although the full sequence of the human immunodeficiency virus type 1 (HIV-1) genome has been known for more than a decade, effective genetic antivirals have yet to be developed. Here we show that, of 22 regions examined, one highly conserved sequence (ACTCTTTGGCAACGA) near the 3' end of the HIV-1 gag-pol transframe region, encoding viral protease residues 4 to 8 and a C-terminal Vpr-binding motif of p6(Gag) protein in two different reading frames, can be successfully targeted by an antisense peptide nucleic acid oligomer named PNA(PR2). A disrupted translation of gag-pol mRNA induced at the PNA(PR2)-annealing site resulted in a decreased synthesis of Pr160(Gag-Pol) polyprotein, hence the viral protease, a predominant expression of Pr55(Gag) devoid of a fully functional p6(Gag) protein, and the excessive intracellular cleavage of Gag precursor proteins, hindering the processes of virion assembly. Treatment with PNA(PR2) abolished virion production by up to 99% in chronically HIV-1-infected H9 cells and in peripheral blood mononuclear cells infected with clinical HIV-1 isolates with the multidrug-resistant phenotype. This particular segment of the gag-pol transframe gene appears to offer a distinctive advantage over other regions in invading viral structural genes and restraining HIV-1 replication in infected cells and may potentially be exploited as a novel antiviral genetic target.

HIV protease: enzyme function and drug resistance

HIV protease is responsible for processing of the gag and gag-pol polyproteins during virion maturation. The activity of this enzyme is essential for virus infectivity, rendering the protein a major therapeutic target for AIDS treatment. This articles reviews the biochemical and biophysical properties of the enzyme. The clinical and in vitro observations of resistance to protease inhibitors are discussed from the perspective of drug resistance mechanisms of HIV protease mutants.

Kinetic analysis of the interaction between HIV-1 protease and inhibitors using optical biosensor technology

The interaction between HIV-1 protease and reversible inhibitors was studied by surface plasmon resonance biosensor technology. The steady-state binding level and the time course of association and dissociation could be observed by measuring the binding of inhibitors injected in a continuous flow of buffer to the immobilized enzyme. Fourteen low molecular weight inhibitors (500-700 Da), including the four clinically used HIV-1 protease inhibitors (indinavir, nelfinavir, ritonavir, and saquinavir), were analyzed. Affinities were estimated as B(50) values from a series of sensorgrams at different concentrations of inhibitors. These values were found to be correlated with inhibition constants (K(i)) determined by an enzyme inhibition assay (r(2) = 0.84, logarithmic values). Dissociation rates were estimated at a single saturating concentration of the inhibitors as t(1/2,obs), but these values did not correlate with K(i) (r(2) = 0.26, logarithmic values). Indinavir had the highest affinity (B(50) = 11 nM) and the fastest dissociation (t(1/2,obs) = 500 s) among the clinically used inhibitors while saquinavir had a lower affinity (B(50) = 25 nM) and the slowest dissociation rate (t(1/2,obs) = 6500 s). Since these two inhibitors have similar K(i) values, the differences in dissociation rates reveal important characteristics in the interaction that cannot be obtained by the inhibition studies. The biosensor data are expected to be of greater in vivo relevance since the experiments were performed in a buffer more similar to physiological conditions.

The 80's loop (residues 78 to 85) is important for the differential activity of retroviral proteases

The abundance of structural data available for retroviral proteases affords a unique opportunity to investigate structure activity relationships. Our approach attempts to genetically engineer an HIV (human immunodeficiency virus)-1 protease that is functionally equivalent to the HIV-2 and the SIV (simian immunodeficiency virus) enzymes and conversely to engineer an HIV-2 protease that is functionally equivalent to the HIV-1 enzyme. For this purpose, the HIV-2 and SIV proteases were cloned and characterized in an Escherichia coli (E. coli) assay system along with 33 engineered HIV-1 and HIV-2 enzymes. The results of these experiments show that a relatively large S1 or S1' subsite volume, which is likely determined by the conformation of the 80's loop (residues 78 to 85), is necessary to fully accommodate the HIV-1 protease specificity site AETF*YCDG (the asterisk indicates the location scissile bond) during productive binding.

HIV-I protease. Cloning, expression, and purification

A new method for obtaining HIV-I protease was suggested. Fusion proteins composed of the N-terminal fragment of human gamma-interferon and HIV-I protease connected with (Asp)4Lys (protein I) or Asp-Pro (protein II) linkers were expressed in Escherichia coli cells. The fusion proteins were produced as insoluble inclusion bodies in the 20% yield of total cell protein. Protein I was cleaved by enterokinase. The solubility of protein I was increased by treating with Na-sulfite/Na-tetrathionate under denaturing conditions. Optimal conditions for efficient acidic hydrolysis of protein II at Asp-Pro bond were found. The hydrolysis products were separated by reversed-phase FPLC. The amount of tryptophan and cysteine residues in the enzyme obtained was estimated. The activity of HIV-I protease was determined using the chromogenic peptide. AlaArgVal NleNphGluAlaNleNH2 and a high-mol-wt substrate consisting of beta-galactosidase and a fragment of gag proteins, including p17-p24 processing site.

Predicting human immunodeficiency virus protease cleavage sites in proteins by a discriminant function method

Based on the sequence-coupled (Markov chain) model and vector-projection principle, a discriminant function method is proposed to predict sites in protein substrates that should be susceptible to cleavage by the HIV-1 protease. The discriminant function is defined by delta = phi+ - phi-, where phi+ and phi- are the cleavable and noncleavable attributes for a given peptide, and they can be derived from two complementary sets of peptides, S+ and S-, known to be cleavable and noncleavable, respectively, by the enzyme. The rate of correct prediction by the method for the 62 cleavable peptides and 239 noncleavable peptides in the training set are 100 and 96.7%, respectively. Application of the method to the 55 sequences which are outside the training set and known to be cleaved by the HIV-1 protease accurately predicted 100% of the peptides as substrates of the enzyme. The method also predicted all but one of the sites hydrolyzed by the protease in native HIV-1 and HIV-2 reverse transcriptases, where the HIV-1 protease discriminates between nearly identical sequences in a very subtle fashion. Finally, the algorithm predicts correctly all of the HIV-1 protease processing sites in the native gag and gag/pol HIV-1 polyproteins, and all of the cleavage sites identified in denatured protease and reverse transcriptase. The new predictive algorithm provides a novel route toward understanding the specificity of this important therapeutic target.

Facile syntheses of C2-symmetrical HIV-1 protease inhibitors

With the goal of obtaining inexpensive yet potent anti-AIDS drugs, simple inhibitors of HIV-1 protease were synthesised. The C2-symmetrical pseudopeptidic substrate analogues can be prepared as inhibitors for HIV-1 protease starting from symmetrical ketones 3a-d by a facile four-step synthesis. After bromination of 3a-d to alpha,alpha'-dibromoketones 4a-d, we synthesised the diamino compounds 6a-c by Gabriel synthesis, which were then coupled with Z-valine to yield inhibitors including a central hydroxy group 8a-d a-i by azidation, reduction with LiAlH4 and coupling of the beta,beta'-diaminohydroxy compounds with appropriate peptides. The first set of compounds showed only weak inhibition whereas the latter reach Ki values of up to 3.0 microM.

Enhancement of HIV-1 proteinase activity by HIV-1 reverse transcriptase

HIV-1 reverse transcriptase (RT) was found to increase the activity of HIV-1 proteinase in vitro and in eukaryotic cells. The effect of RT on proteinase activity was dose-dependent and independent of pH or salt concentration. The cleavage of sequences corresponding to all the naturally occurring cleavage sites that could be tested in vitro was enhanced. The effect of RT on cleavage was greatest at the cleavage site between RT and integrase. The enhancement of viral proteinase activity by the virus RT may contribute to regulation of the order and/or efficiency of cleavage at different sites during virus replication and maturation.

Proteolytic activity of novel human immunodeficiency virus type 1 proteinase proteins from a precursor with a blocking mutation at the N terminus of the PR domain

The mature human immunodeficiency virus type 1 proteinase (PR; 11 kDa) can cleave all interdomain junctions in the Gag and Gag-Pol polyprotein precursors. To determine the activity of the enzyme in its precursor form, we blocked release of mature PR from a truncated Gag-Pol polyprotein by introducing mutations into the N-terminal Phe-Pro cleavage site of the PR domain. The mutant precursor autoprocessed efficiently upon expression in Escherichia coli. No detectable mature PR was released; however, several PR-related products ranging in size from approximately 14 to 18 kDa accumulated. Products of the same size were generated when mutant precursors were digested with wild-type PR. Thus, PR can utilize cleavage sites in the region upstream of the PR domain, resulting in the formation of extended PR species. On the basis of active-site titration, the PR species generated from mutated precursor exhibited wild-type activity on peptide substrates. However, the proteolytic activity of these extended enzymes on polyprotein substrates provided exogenously was low when equimolar amounts of extended and wild-type PR proteins were compared. Mammalian cells expressing the mutated precursor produced predominantly precursor and considerably reduced amounts of mature products. Released particles consisted mostly of uncleaved or partially cleaved polyproteins. Our results suggest that precursor forms of PR can autoprocess but are less efficient in processing of the Gag precursor for formation of mature virus particles.

Stability of dimeric retroviral proteases

The determination of dimer stabilities for the retroviral proteases has proved more challenging than anticipated, but it is a tractable problem when careful attention is made to potential interferences. For investigations of retroviral proteases not yet characterized, the fundamentally rigorous sedimentation equilibrium and other biophysical techniques may yet provide useful Kd values. They are preferable to the indirect methods emphasized in this chapter but nevertheless should be coupled with basic considerations such as recovery of activity at the end of an experiment and the relevance of values obtained to other situations. In the likely event that nanomolar Kd values are encountered in new investigations, the assay techniques provide the most readily available methods for many laboratories. Because these methods are sensitive to anything that affects enzyme activity, the use of complementary methods to verify dimerization constants is imperative. Inactivating reactions not due to monomer formation should be explored, and the potential impact of those reactions on the constants being measured should be estimated. Most of the Kd and dimerization rate data available for retroviral proteases are obtained with the HIV-1 protease, with each investigator choosing methods and solvent conditions different from the others. The confusing diversity of results should be the impetus for a direct comparison of methods for the identification of the sources of differences. If more comprehensive and rigorous measures of the kinetics and thermodynamics of subunit aggregation are obtained, they might be coupled with the large volume of detailed structural data accumulating for this class of protein to provide insights into more general problems of protein-folding chemistry.

Use of nitrogen-15 kinetic isotope effects to elucidate details of the chemical mechanism of human immunodeficiency virus 1 protease

We have used 15N kinetic isotope effects of the HIV-1 protease-catalyzed peptidolysis of Ac-Ser-Gln-Asn-Tyr-Pro-Val-Val-NH2 to characterize the chemical mechanism of this enzyme. In addition, the multiple isotope effects have been determined by measuring the 15N kinetic isotope effects in both H2O and D2O. The isotope effects, measured on values of V/K, were determined by the incorporation of a radiolabel (tritium and 14C in peptides bearing the heavy and light isotopes, respectively) at a position remote from the isotopically labeled scissile peptide bond, such that the isotope effect was determined by measurement of the change in the 14C/3H ratio in recovered substrates at various fractions of reaction. At pH = 6.0 (37 degrees C), the nitrogen isotope effects were slightly, but significantly, inverse in both solvents: 15(V/K)H2O = 0.995 +/- 0.002, and 15(V/K)D2O = 0.992 +/- 0.003. The observation of an inverse nitrogen kinetic isotope effect implies that bonding to the nitrogen atom is becoming stiffened in a reaction transition state, and since this inverse isotope effect is enhanced in D2O, this isotope effect likely arises from protonation of the proline nitrogen atom.(ABSTRACT TRUNCATED AT 250 WORDS)

Large scale purification and refolding of HIV-1 protease from Escherichia coli inclusion bodies

The protease encoded by the human immunodeficiency virus type 1 (HIV-1) was engineered in Escherichia coli as a construct in which the natural 99-residue polypeptide was preceded by an NH2-terminal methionine initiator. Inclusion bodies harboring the recombinant HIV-1 protease were dissolved in 50% acetic acid and the solution was subjected to gel filtration on a column of Sephadex G-75. The protein, eluted in the second of two peaks, migrated in SDS-PAGE as a single sharp band of M(r) approximately 10,000. The purified HIV-1 protease was refolded into an active enzyme by diluting a solution of the protein in 50% acetic acid with 25 volumes of buffer at pH 5.5. This method of purification, which has also been applied to the purification of HIV-2 protease, provides a single-step procedure to produce 100 mg quantities of fully active enzyme.

A symmetric inhibitor binds HIV-1 protease asymmetrically

Potential advantages of C2-symmetric inhibitors designed for the symmetric HIV-1 protease include high selectivity, potency, stability, and bioavailability. Pseudo-C2-symmetric monools and C2-symmetric diols, containing central hydroxymethylene and (R,R)-dihydroxyethylene moieties flanked by a variety of hydrophobic P1/P1' side chains, were studied as HIV-1 protease inhibitors. The monools and diols were synthesized in 8-10 steps from D-(+)-arabitol and D-(+)-mannitol, respectively. Monools with ethyl or isobutyl P1/P1' side chains were weak inhibitors of recombinant HIV-1 protease (Ki > 10 microM), while benzyl P1/P1' side chains afforded a moderately potent inhibitor (apparent Ki = 230 nM). Diols were 100-10,000x more potent than analogous monools, and a wider range of P1/P1' side chains led to potent inhibition. Both classes of compounds exhibited lower apparent Ki values under high-salt conditions. Surprisingly, monool and diol HIV-1 protease inhibitors were potent inhibitors of porcine pepsin, a prototypical asymmetric monomeric aspartic protease. These results were evaluated in the context of the pseudosymmetric structure of monomeric aspartic proteases and their evolutionary kinship with the retroviral proteases. The X-ray crystal structure of HIV-1 protease complexed with a symmetric diol was determined at 2.6 A. Contrary to expectations, the diol binds the protease asymmetrically and exhibits 2-fold disorder in the electron density map. Molecular dynamics simulations were conducted beginning with asymmetric and symmetric HIV-1 protease/inhibitor model complexes. A more stable trajectory resulted from the asymmetric complex, in agreement with the observed asymmetric binding mode. A simple four-point model was used to argue more generally that van der Waals and electrostatic force fields can commonly lead to an asymmetric association between symmetric molecules.

Hydroxyethylene isostere inhibitors of human immunodeficiency virus-1 protease: structure-activity analysis using enzyme kinetics, X-ray crystallography, and infected T-cell assays

Analogues of peptides ranging in size from three to six amino acids and containing the hydroxyethylene dipeptide isosteres Phe psi Gly, Phe psi Ala, Phe psi NorVal, Phe psi Leu, and Phe psi Phe, where psi denotes replacement of CONH by (S)-CH(OH)CH2, were synthesized and studied as HIV-1 protease inhibitors. Inhibition constants (Ki) with purified HIV-1 protease depend strongly on the isostere in the order Phe psi Gly greater than Phe psi Ala greater than Phe psi NorVal greater than Phe psi Leu greater than Phe psi Phe and decrease with increasing length of the peptide analogue, converging to a value of 0.4 nM. Ki values are progressively less dependent on inhibitor length as the size of the P1' side chain within the isostere increases. The structures of HIV-1 protease complexed with the inhibitors Ala-Ala-X-Val-Val-OMe, where X is Phe psi Gly, Phe psi Ala, Phe psi NorVal, and Phe psi Phe, have been determined by X-ray crystallography (resolution 2.3-3.2 A). The crystals exhibit symmetry consistent with space group P6(1) with strong noncrystallographic 2-fold symmetry, and the inhibitors all exhibit 2-fold disorder. The inhibitors bind in similar conformations, forming conserved hydrogen bonds with the enzyme. The Phe psi Gly inhibitor adopts an altered conformation that places its P3' valine side chain partially in the hydrophobic S1' pocket, thus suggesting an explanation for the greater dependence of the Ki value on inhibitor length in the Phe psi Gly series. From the kinetic and crystallographic data, a minimal inhibitor model for tight-binding inhibition is derived in which the enzyme subsites S2-S2' are optimally occupied. The Ki values for several compounds are compared with their potencies as inhibitors of proteolytic processing in T-cell cultures chronically infected with HIV-1 (MIC values) and as inhibitors of acute infectivity (IC50 values). There is a rank-order correspondence, but a 20-1000-fold difference, between the values of Ki and those of MIC or IC50. IC50 values can approach those of Ki but are highly dependent on the conditions of the acute infectivity assay and are influenced by physiochemical properties of the inhibitors such as solubility.

Constructing proteins by dovetailing unprotected synthetic peptides: backbone-engineered HIV protease

Backbone-engineered HIV-1 protease was prepared by a total chemical synthesis approach that combines the act of joining two peptides with the generation of an analog structure. Unprotected synthetic peptide segments corresponding to the two halves of the HIV-1 protease monomer polypeptide chain were joined cleanly and in high yield through unique mutually reactive functional groups, one on each segment. Ligation was performed in 6 molar guanidine hydrochloride, thus circumventing limited solubility of protected peptide segments, the principal problem of the classical approach to the chemical synthesis of proteins. The resulting fully active HIV-1 protease analog contained a thioester replacement for the natural peptide bond between Gly51-Gly52 in each of the two active site flaps, a region known to be highly sensitive to mutational changes of amino acid side chains.

The HIV-1 protease as a therapeutic target for AIDS

HIV produces a small , dimeric aspartyl protease which specifically cleaves the polyprotein precursors encoding the structural proteins and enzymes of the virus. This proteolytic activity is absolutely required for the production of mature, infectious virions and is therefore an attractive target for therapeutic intervention. This review summarizes the strategies and multidisciplinary efforts that have been applied to date to the identification of specific inhibitors of this critical viral enzyme. These inhibitors include rationally designed peptide substrate analogs, compounds conceived from tertiary structure information on the enzyme and natural products. Future directions in the discovery and development of HIV-1 protease inhibitors are also discussed.

Inhibitors of HIV-1 protease

The human immunodeficiency virus (HIV), the etiological agent for the acquired immune deficiency syndrome (AIDS), is a retrovirus which makes use of a virally-encoded aspartic protease to perform specific proteolytic processing of two of its gene products in order to form active enzymes and structural proteins within the mature virion. Accordingly, specific, exogenous inhibition of the HIV-1 protease is thought to be a viable approach for the development of novel therapeutics for the treatment of AIDS. Indeed, this hypothesis has been validated in virally-infected cell culture with synthetic inhibitors of HIV-1 protease. This chapter reviews the current status of the development of inhibitors of this enzyme.

Large scale expression and purification of recombinant HIV-1 proteinase from Escherichia coli

The availability of target proteins in sufficient quantity is a limiting factor in crystallographic studies and therefore in rational drug design. Even after optimisation, expression of recombinant proteins may be low and the only way to produce enough protein is by large scale cell growth/purification. HIV-1 proteinase in Escherichia coli, which due to its toxicity is expressed as a soluble protein only at around 0.1% of total protein, is a paradigm for this. In this paper a detailed process for large scale expression and purification of HIV-1 proteinase which delivers material of suitable quantity (30 mg from 500 g of wet weight of cells) and quality for crystallographic studies is described.

Adaptation of the plasma renin radioimmunoassay for use with HIV-1 protease

We have demonstrated the use of a radioimmunoassay to quantitate the peptidolytic activity of human immunodeficiency virus, type 1 (HIV-1) protease using a tetradecapeptide substrate of porcine renin, Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu-Leu-Val-Tyr-Ser. HIV-1 protease catalyzes cleavage of this substrate at the same Leu-Leu bond as does porcine renin, resulting in the formation of authentic angiotensin-I. The angiotensin-I product is then detected by use of a commercially available renin plasma assay kit, which constitutes the basis of the RIA. The radioimmunoassay provides detection of the protease-catalyzed formation of angiotensin-I at picomolar concentrations in vitro. We demonstrate the use of this assay in determining IC50 values for two HIV-1 protease inhibitors present in cell culture media and in standard assay buffer. An example of the potential development of this assay for the quantitation of these inhibitors present in ex vivo plasma samples is also presented.

Expression of an autoprocessing CAT-HIV-1 proteinase fusion protein: purification to homogeneity of the release 99 residue proteinase

The 99 residue human immunodeficiency virus type 1 proteinase has been expressed in Escherichia coli as part of an autocleaving fusion protein. Expression of the fusion protein is toxic to the host cells, however yields of the released proteinase have been improved by optimising induction nad harvest times to increase culture biomass, and decrease degradation of the proteinase. Soluble proteinase was extracted from these cells by a simple and highly efficient three step process. N-terminal sequence analysis confirms that the enzyme preparation is highly pure and correctly autoprocessed. The proteinase cleaves peptide substrate IGCTLNFPISPIETV between F and P at pH 6.0 with a Km of 310 microM and a Kcat of 14s-1. The enzyme is sensitive to its ionic environment, showing stimulation of activity at high salt concentrations, and shows a pH optimising 5.5.

High-yield purification of HIV-1 proteinase expressed by a synthetic gene in Escherichia coli

A rapid and simple purification procedure for human immunodeficiency virus type 1 (HIV-1) proteinase from a synthetic gene expressed in Escherichia coli has been developed. The synthetic gene was constructed from oligonucleotides containing several restriction enzyme sites in order to allow simple construction of homologous genes. The protein was translated as a precursor which was autocatalytically processed into the mature protein as shown by N-terminal sequence analysis of the purified protein. Immunoblot analysis was used to verify the nature of the expression product and it was found that 2 of 10 anti-peptide antibodies, covering the whole proteinase sequence, were able to react with the enzyme in crude bacterial lysates. These two anti-peptide antibodies represent a continuous sequence partially overlapping the active site. The purification involves two initial precipitation steps followed by cation-exchange and size-exclusion chromatography. A high yield and a high specific activity were achieved.

The chronically infected cell as a target for the treatment of HIV infection and AIDS

Infection of the T lymphocyte with HIV results in a cytopathic effect and cell death that has been linked to a selective loss of the helper T-lymphocyte function of the immune system. In addition to acute infection, which leads to cell death, a chronic or persistent infection also occurs. The persistence of these viral reservoirs has been implicated in the progression of HIV infection and AIDS. Rational drug discovery targeted to late-stage events in HIV replication has the potential to yield antiviral agents capable of blocking virus spread by inhibiting the production of infectious virions from these chronic reservoirs. Steve Petteway and colleagues discuss antiviral strategies that target the chronically infected cell, with a focus on HIV protease inhibitors.

A role for the aspartyl protease from the human immunodeficiency virus type 1 (HIV-1) in the orchestration of virus assembly

Functional HIV-1 protease (PR) is required for the maturation of viral proteins, for the appearance of characteristic structural features in the virion (as determined by electron microscopy), and for the final assembly of mature virus. Most importantly, HIV-1 PR activity is required for the development of infectivity. Still largely undefined, however, is the timing and control of protease action in this assembly process. Based on the three-dimensional structure of HIV-1 PR2,3 and experimental data reported in the literature, we propose a comprehensive virus assembly model that highlights the role of HIV-1 PR, suggests further experiments to verify the validity of the model, and poses specific questions relevant to the ultimate exploitation of HIV-1 protease as a therapeutic target.

A radiometric assay for HIV-1 protease

A rapid, high-throughput radiometric assay for HIV-1 protease has been developed using ion-exchange chromatography performed in 96-well filtration plates. The assay monitors the activity of the HIV-1 protease on the radiolabeled form of a heptapeptide substrate, [tyrosyl-3,5-3H]Ac-Ser-Gln-Asn-Tyr-Pro-Val-Val-NH2, which is based on the p17-p24 cleavage site found in the viral polyprotein substrate Pr55gag. Specific cleavage of this uncharged heptapeptide substrate by HIV-1 protease releases the anionic product [tyrosyl-3,5-3H]Ac-Ser-Gln-Asn-Tyr, which is retained upon minicolumns of the anion-exchange resin AG1-X8. Protease activity is determined from the recovery of this radiolabeled product following elution with formic acid. This facile and highly sensitive assay may be utilized for steady-state kinetic analysis of the protease, for measurements of enzyme activity during its purification, and as a routine assay for the evaluation of protease inhibitors from natural product or synthetic sources.

Chromophoric peptide substrates for the spectrophotometric assay of HIV-1 protease

Purified HIV-1 protease hydrolyzes H-Ser-Gln-Asn-Leu-Phe(NO2)-Leu-Asp-Gly-NH2 (Peptide 1) and acetyl-Arg-Lys-Ile-Leu-Phe(NO2)-Leu-Asp-Gly-NH2 (Peptide 2) between the (p-nitro)phenylalanyl and leucyl residues. The cleavage of Peptides 1 and 2 resulted in a decrease in uv absorbance at 310 nm. The HIV-1 protease-catalyzed peptidolysis of Peptides 1 and 2 was characterized by a linear time course at substrate turnover of less than 20%. The solubilities of these substrates at pH 5.0 were sufficient to provide initial rate measurements over a concentration range of 0.05-0.5 mM. Steady-state kinetic data and inhibition constants using both spectrophotometric and high performance liquid chromatography (HPLC) analysis of the peptidolysis of these substrates resulted in comparable values.