Antiviral inhibition of the HIV-1 capsid protein

Structure of full-length HIV-1 CA: a model for the mature capsid lattice

The capsids of mature retroviruses perform the essential function of organizing the viral genome for efficient replication. These capsids are modeled as fullerene structures composed of closed hexameric arrays of the viral CA protein, but a high-resolution structure of the lattice has remained elusive. A three-dimensional map of two-dimensional crystals of the R18L mutant of HIV-1 CA was derived by electron cryocrystallography. The docking of high-resolution domain structures into the map yielded the first unambiguous model for full-length HIV-1 CA. Three important protein-protein assembly interfaces are required for capsid formation. Each CA hexamer is composed of an inner ring of six N-terminal domains and an outer ring of C-terminal domains that form dimeric linkers connecting neighboring hexamers. Interactions between the two domains of CA further stabilize the hexamer and provide a structural explanation for the mechanism of action of known HIV-1 assembly inhibitors.

The 3-O-(3’,3’-dimethylsuccinyl) derivative of betulinic acid (DSB) inhibits the assembly of virus-like particles in HIV-1 Gag precursor-expressing cells

Background

The 3- O-(3’,3’-dimethylsuccinyl) derivative of betulinic acid (DSB) blocks HIV-1 maturation by interfering with viral protease (PR) at the capsid (CA)-SP1 cleavage site, a crucial region in HIV-1 morphogenesis.

Methods

We analysed the effect of DSB on the assembly of HIV-1 Gag precursor (Pr55GagHIV) into membrane-enveloped virus-like particles (VLP) in baculovirus-infected cells expressing Pr55GagHIV, in a cellular context devoid of viral PR.

Results

DSB showed a dose-dependent negative effect on VLP assembly, with an IC50∼10 μM. The DSB inhibitory effect was p6-independent and was also observed for intracellular assembly of non-N-myristoylated Gag core-like particles. HIV-1 VLP assembled in the presence of DSB exhibited a lower stability of their inner cores upon membrane delipidation compared with control VLP, suggesting weaker Gag-Gag interactions. DSB also inhibited the assembly of simian immunodeficiency virus SIVmac251 VLP, although with a twofold lower efficacy (IC50∼20 μM). No detectable inhibitory activity was observed for murine leukaemia virus (MLV) VLP; however, fusion of the SP1-NC-p6 domains from HIV-1 to the matrix (MA)-CA domains from MLV conferred DSB sensitivity to the chimaeric Gag precursor Pr72GagMLV–HIV (IC50=30 μM). This observation suggested that the main DSB target on Pr55Gag was the SP1 domain, but the higher degree of DSB resistance for Pr72GagMLV–HIV compared with Pr55GagHIV implied that other upstream Gag region(s) might contribute to DSB reactivity.

Conclusions

Sequence alignment and three-dimensional modelling by homology of the CA-SP1-NC junction in HIV-1, SIVmac251 and Pr72GagMLV–HIV suggested that a higher hydrophilic character of the CA region immediately upstream to the HIV-1 CA-SP1 junction, as occurred in Pr72GagMLV–HIV, correlated with a lower DSB sensitivity.

Structure of the antiviral assembly inhibitor CAP-1 complex with the HIV-1 CA protein

The CA domain of the human immunodeficiency virus type 1 (HIV-1) Gag polyprotein plays critical roles in both the early and late phases of viral replication and is therefore an attractive antiviral target. Compounds with antiviral activity were recently identified that bind to the N-terminal domain of CA (CA N) and inhibit capsid assembly during viral maturation. We have determined the structure of the complex between CA N and the antiviral assembly inhibitor N-(3-chloro-4-methylphenyl)-N'-{2-[({5-[(dimethylamino)-methyl]-2-furyl}-methyl)-sulfanyl]ethyl}-urea) (CAP-1) using a combination of NMR spectroscopy and X-ray crystallography. The protein undergoes a remarkable conformational change upon CAP-1 binding, in which Phe32 is displaced from its buried position in the protein core to open a deep hydrophobic cavity that serves as the ligand binding site. The aromatic ring of CAP-1 inserts into the cavity, with the urea NH groups forming hydrogen bonds with the backbone oxygen of Val59 and the dimethylamonium group interacting with the side-chains of Glu28 and Glu29. Elements that could be exploited to improve binding affinity are apparent in the structure. The displacement of Phe32 by CAP-1 appears to be facilitated by a strained main-chain conformation, which suggests a potential role for a Phe32 conformational switch during normal capsid assembly.

Characterization of the invariable residue 51 mutations of human immunodeficiency virus type 1 capsid protein on in vitro CA assembly and infectivity

BACKGROUND

The mature HIV-1 conical core formation proceeds through highly regulated protease cleavage of the Gag precursor, which ultimately leads to substantial rearrangements of the capsid (CAp24) molecule involving both inter- and intra-molecular contacts of the CAp24 molecules. In this aspect, Asp51 which is located in the N-terminal domain of HIV-1 CAp24 plays an important role by forming a salt-bridge with the free imino terminus Pro1 following proteolytic cleavage and liberation of the CAp24 protein from the Pr55Gag precursor. Thus, previous substitution mutation of Asp51 to alanine (D51A) has shown to be lethal and that this invariable residue was found essential for tube formation in vitro, virus replication and virus capsid formation.

RESULTS

We extended the above investigation by introducing three different D51 substitution mutations (D51N, D51E, and D51Q) into both prokaryotic and eukaryotic expression systems and studied their effects on in vitro capsid assembly and virus infectivity. Two substitution mutations (D51E and D51N) had no substantial effect on in vitro capsid assembly, yet they impaired viral infectivity and particle production. In contrast, the D51Q mutant was defective both for in vitro capsid assembly and for virus replication in cell culture.

CONCLUSION

These results show that substitutions of D51 with glutamate, glutamine, or asparagine, three amino acid residues that are structurally related to aspartate, could partially rescue both in vitro capsid assembly and intra-cellular CAp24 production but not replication of the virus in cultured cells.

HIV-1 assembly and cellular trafficking pathways: current understanding and potential for future therapeutics

HIV-1 assembly occurs at the plasma membrane of cells or on intracellular membranes as a result of the complex interplay of viral structural proteins and cellular vesicular transport systems. A number of recent discoveries have revealed direct interactions between the viral Gag and Env proteins and cellular molecules involved in trafficking. These direct interactions are often mediated by discrete motifs that may be suitable as targets for antiretroviral therapy in the future. Discrete motifs within the p6 region of Gag bind and recruit members of the endosomal sorting complex required for transport complex of proteins for regulation of particle budding. Gag interacts directly with the δ subunit of the cellular adaptin AP-3, and this interaction mediates movement of Gag to the multivesicular body (MVB). TIP47 has been identified as a linker between Gag and Env that directs vesicular movement of this complex within the cell, leading to Env incorporation into virions. Additional cellular determinants of assembly have been identified that are consistent with a model for assembly in which regulated vesicular transport plays a dominant role. Future discoveries should clarify the pathways utilized for the sequential movement of Gag and Env in the cell, including the differential movement of Gag and Env to the MVB or plasma membrane observed in T cells and macrophages. Research into the assembly pathway of HIV-1 is poised to reveal a number of additional targets for antiviral drug discovery in the coming years.

Biomedical Communications: Collaborative Research in Scientific Visualization, Online Learning, and Knowledge Translation

Collaborative research in biomedical communications investigates the role of visual media in scientific discovery and in patient and health professional education. The spectrum of work is broad and includes the visualization of scientific knowledge and simulation of hypothetical models of health and disease, as well as the design of audience-centered interactive visual media. The work cited supports the notion that research-based visual media can contribute to the core missions of science: discovery, communication, collaboration, and education.

Potent nonnucleoside reverse transcriptase inhibitors target HIV-1 Gag-Pol

Nonnucleoside reverse transcriptase inhibitors (NNRTIs) target HIV-1 reverse transcriptase (RT) by binding to a pocket in RT that is close to, but distinct, from the DNA polymerase active site and prevent the synthesis of viral cDNA. NNRTIs, in particular, those that are potent inhibitors of RT polymerase activity, can also act as chemical enhancers of the enzyme's inter-subunit interactions. However, the consequences of this chemical enhancement effect on HIV-1 replication are not understood. Here, we show that the potent NNRTIs efavirenz, TMC120, and TMC125, but not nevirapine or delavirdine, inhibit the late stages of HIV-1 replication. These potent NNRTIs enhanced the intracellular processing of Gag and Gag-Pol polyproteins, and this was associated with a decrease in viral particle production from HIV-1-transfected cells. The increased polyprotein processing is consistent with premature activation of the HIV-1 protease by NNRTI-enhanced Gag-Pol multimerization through the embedded RT sequence. These findings support the view that Gag-Pol multimerization is an important step in viral assembly and demonstrate that regulation of Gag-Pol/Gag-Pol interactions is a novel target for small molecule inhibitors of HIV-1 production. Furthermore, these drugs can serve as useful probes to further understand processes involved in HIV-1 particle assembly and maturation.

HIV-1 encodes reverse transcriptase (RT), an enzyme that is essential for virus replication. Nonnucleoside reverse transcriptase inhibitors (NNRTIs) are allosteric inhibitors of the HIV-1 RT. In HIV-1-infected cells NNRTIs block the RT-catalyzed synthesis of a double-stranded DNA copy of the viral genomic RNA, which is an early step in the virus life cycle. Potent NNRTIs have the novel feature of promoting the interaction between the two RT subunits. However, the importance of this effect on the inhibition of HIV-1 replication has not been defined. In this study, the authors show that potent NNRTIs block an additional step in the virus life cycle. NNRTIs increase the intracellular processing of viral polyproteins called Gag and Gag-Pol that express the HIV-1 structural proteins and viral enzymes. Enhanced polyprotein processing is associated with a decrease in viral particles released from NNRTI-treated cells. NNRTI enhanced polyprotein processing is likely due to the drug binding to RT, expressed as part of the Gag-Pol polyprotein and promoting the interaction between separate Gag-Pol polyproteins. This leads to premature activation of the Gag-Pol embedded HIV-1 protease, resulting in a decrease in full-length viral polyproteins available for assembly and budding from the host cell membrane. This study provides proof-of-concept that small molecules can modulate the interactions between Gag-Pol polyproteins and suggests a new target for the development of HIV-1 antiviral drugs.

A second-site suppressor significantly improves the defective phenotype imposed by mutation of an aromatic residue in the N-terminal domain of the HIV-1 capsid protein

The HIV-1 capsid (CA) protein plays an important role in virus assembly and infectivity. Previously, we showed that Ala substitutions in the N-terminal residues Trp23 and Phe40 cause a severely defective phenotype. In searching for mutations at these positions that result in a non-lethal phenotype, we identified one candidate, W23F. Mutant virions contained aberrant cores, but unlike W23A, also displayed some infectivity in a single-round replication assay and delayed replication kinetics in MT-4 cells. Following long-term passage in MT-4 cells, two second-site mutations were isolated. In particular, the W23F/V26I mutation partially restored the wild-type phenotype, including production of particles with conical cores and wild-type replication kinetics in MT-4 cells. A structural model is proposed to explain the suppressor phenotype. These findings describe a novel occurrence, namely suppression of a mutation in a hydrophobic residue that is critical for maintaining the structural integrity of CA and proper core assembly.

Effect of macromolecular crowding agents on human immunodeficiency virus type 1 capsid protein assembly in vitro

Previous studies on the self-assembly of capsid protein CA of human immunodeficiency virus type 1 (HIV-1) in vitro have provided important insights on the structure and assembly of the mature HIV-1 capsid. However, CA polymerization in vitro was previously observed to occur only at very high ionic strength. Here, we have analyzed the effects on CA assembly in vitro of adding unrelated, inert macromolecules (crowding agents), aimed at mimicking the crowded (very high macromolecular effective concentration) environment within the HIV-1 virion. Crowding agents induced fast and efficient polymerization of CA even at low (close to physiological) ionic strength. The hollow cylinders thus assembled were indistinguishable in shape and dimensions from those formed in dilute protein solutions at high ionic strength. However, two important differences were noted: (i) disassembly by dilution of the capsid-like particles was undetectable at very high ionic strength, but occurred rapidly at low ionic strength in the presence of a crowding agent, and (ii) a variant CA from a presumed infectious HIV-1 with mutations at the CA dimerization interface was unable to assemble at any ionic strength in the absence of a crowding agent; in contrast, this mutation allowed efficient assembly, even at low ionic strength, when a crowding agent was used. The use of a low ionic strength and inert macromolecules to mimic the crowded environment inside the HIV-1 virion may lead to a better in vitro evaluation of the effects of conditions, mutations or/and other molecules, including potential antiviral compounds, on HIV-1 capsid assembly, stability and disassembly.

A peptide inhibitor of HIV-1 assembly in vitro

Formation of infectious HIV-1 involves assembly of Gag polyproteins into immature particles and subsequent assembly of mature capsids after proteolytic disassembly of the Gag shell. We report a 12-mer peptide, capsid assembly inhibitor (CAI), that binds the capsid (CA) domain of Gag and inhibits assembly of immature- and mature-like capsid particles in vitro. CAI was identified by phage display screening among a group of peptides with similar sequences that bind to a single reactive site in CA. Its binding site was mapped to CA residues 169-191, with an additional contribution from the last helix of CA. This result was confirmed by a separate X-ray structure analysis showing that CAI inserts into a conserved hydrophobic groove and alters the CA dimer interface. The CAI binding site is a new target for antiviral development, and CAI is the first known inhibitor directed against assembly of immature HIV-1.

Cooperative effects in phospholipid monolayers induced by a peptide from HIV-1 capsid protein

The study of interactions between biological molecules and model membranes is essential for the understanding of a number of physiological mechanisms involved in viral infections and dissemination. In this paper, the analysis of the interaction between a peptide from the p24 protein of Human Immunodeficiency Virus type 1 (HIV-1) and a phospholipid monolayer has pointed to a cooperative response in which very small amounts of peptide p24-1 (e.g. 0.05 mol%) can lead to measurable effects. Monolayer surface pressure and surface potential isotherms were affected for peptide concentrations as low as 0.05 mol%, with saturation at 0.5 mol%. The expansion effect from p24-1 is confirmed by changes in morphology of the monolayers using Brewster angle microscopy. Even though p24-1 is disordered in aqueous solutions, the interaction with dipalmitoyl phosphatidylcholine (DPPC) causes it to adopt an alpha-helix structure, as shown by circular dichroism (CD) data for multilamellar vesicles (MLV). The expansion of the phospholipid monolayer in a cooperative way may imply that p24-1 has potential antiviral activity, by participating in the cell rupture, with no need of specific receptors in the membrane.

Heterologous expression, characterization and structural studies of a hydrophobic peptide from the HIV-1 p24 protein

Proteins from the inner core of HIV-1, such as the capsid protein (p24), are involved in crucial processes during the virus life cycle. The p24 protein plays an active structural role in the Gag protein and in its mature form. This work describes the production of a peptide derived from the p24 C-terminal, TLRAEQASQEVKNWMTETLLVQNA, using recombinant technology. This region (p24-3) is involved in interfaces during the p24 dimerization, which occurs during capsid assembly. The p24-3 sequence was obtained by a synthetic gene strategy and inserted into the pET 32a expression vector to produce soluble fusion protein in Escherichia coli BL21(DE3). This strategy leads to an incorporation of three amino acid residues (AMA) in the N-terminal of the native sequence to form the recombinant p24-3 (rp24-3). The rp24-3 was purified by reverse phase chromatography to homogeneity, as inferred by mass spectrometry and protein sequence analysis. Structural studies using circular dichroism and steady-state fluorescence showed that the rp24-3 is structured by helical and beta elements. As a function of its hydrophobic character it can self-associate forming oligomers. We present in this paper the first development of a suitable expression system for rp24-3, which provides high amounts of the peptide. This strategy will allow the development of new antiviral (HIV) agents.

Conformation of a synthetic antigenic peptide from HIV-1 p24 protein induced by ionic micelles

We studied the interaction of the peptide AAMQMLKETINEEAAEWDRVHPVHAGPIA from the HIV-1 p24 protein in the presence of SDS (anionic) and CTABr (cationic) micelles at pH 7.0 by circular dichroism, fluorescence, and electron spin resonance (ESR). The micelles induced secondary structure as well as a blue shift in the tryptophan fluorescence emission, indicating an interaction between the peptide and the micelles. However, different contents of secondary structure elements were found when the peptide interacts with SDS or CTABr micelles. Steady-state anisotropy indicates a constraint on the rotational mobility of the tryptophan residue of the peptide upon interaction with micelles. ESR studies pointed to different locations for the peptide in either micelle. Our results suggested that at least part of the peptide might be located at the hydrophobic core of the CTABr micelles, probably at the C-terminal region, while it is more inserted into the SDS micelles.

The dimerization domain of the HIV-1 capsid protein binds a capsid protein-derived peptide: a biophysical characterization

The type 1 HIV presents a conical capsid formed by approximately 1500 units of the capsid protein, CA. Homodimerization of CA via its C-terminal domain, CA-C, constitutes a key step in virion assembly. CA-C dimerization is largely mediated by reciprocal interactions between residues of its second alpha-helix. Here, we show that an N-terminal-acetylated and C-terminal-amidated peptide, CAC1, comprising the sequence of the CA-C dimerization helix plus three flanking residues at each side, is able to form a complex with the entire CA-C domain. Thermal denaturation measurements followed by circular dichroism (CD), NMR, and size-exclusion chromatography provided evidence of the interaction between CAC1 and CA-C. The apparent dissociation constant of the heterocomplex formed by CA-C and CAC1 was determined by several biophysical techniques, namely, fluorescence (using an anthraniloyl-labeled peptide), affinity chromatography, and isothermal titration calorimetry. The three techniques yielded similar values for the apparent dissociation constant, in the order of 50 microM. This apparent dissociation constant was only five times higher than was the dissociation constant of both CA-C and the intact capsid protein homodimers (10 microM).

Electrostatic Repulsion, Compensatory Mutations, and Long-range Non-additive Effects at the Dimerization Interface of the HIV Capsid Protein

In previous studies, thermodynamic dissection of the dimerization interface in CA-C, the C-terminal domain of the capsid protein of human immunodeficiency virus type 1, revealed that individual mutation to alanine of Ser178, Glu180, Glu187 or Gln192 led to significant increases in dimerization affinity. Four related aspects derived from this observation have been now addressed, and the results can be summarized as follows: (i) thermodynamic analyses indicate the presence of an intersubunit electrostatic repulsion between both Glu180 residues. (ii) The mutation Glu180 to Ala was detected in nearly all type 2 human immunodeficiency virus variants, and in several simian immunodeficiency viruses analyzed. However, this mutation was strictly co-variant with mutations Ser178Asp in a neighboring residue, and Glu187Gln. Thermodynamic analysis of multiple mutants showed that Ser178Asp compensated, alone or together with Glu187Gln, the increase in affinity caused by the mutation Glu180Ala, and restored a lower dimerization affinity. (iii) The increase in the affinity constant caused by the multiple mutation to Ala of Ser178, Glu180, Glu187 and Gln192 was more than one order of magnitude lower than predicted if additivity were present, despite the fact that the 178/180 pair and the two other residues were located more than 10A apart. (iv) Mutations in CA-C that caused non-additive increases in dimerization affinity also caused a non-additive increase in the capacity of the isolated CA-C domain to inhibit the assembly of capsid-like HIV-1 particles in kinetic assays. In summary, the study of a protein-protein interface involved in the building of a viral capsid has revealed unusual features, including intersubunit electrostatic repulsions, co-variant, compensatory mutations that may evolutionarily preserve a low association constant, and long-range, large magnitude non-additive effects on association.

Three-dimensional structure of HIV-1 virus-like particles by electron cryotomography

While the structures of nearly every HIV-1 protein are known in atomic detail from X-ray crystallography and NMR spectroscopy, many questions remain about how the individual proteins are arranged in the mature infectious viral particle. Here, we report the three-dimensional structures of individual HIV-1 virus-like particles (VLPs) as obtained by electron cryotomography. These reconstructions revealed that while the structures and positions of the conical cores within each VLP were unique, they exhibited several surprisingly consistent features, including similarities in the size and shape of the wide end of the capsid (the "base"), uniform positioning of the base and other regions of the capsid 11nm away from the envelope/MA layer, a cone angle that typically varied from 24 degrees to 18 degrees around the long axis of the cone, and an internal density (presumably part of the NC/RNA complex) cupped within the base. Multiple and nested capsids were observed. These results support the fullerene cone model for the viral capsid, indicate that viral maturation involves a free re-organization of the capsid shell rather than a continuous condensation, imply that capsid assembly is both concentration-driven and template-driven, suggest that specific interactions exist between the capsid and the adjacent envelope/MA and NC/RNA layers, and show that a particular capsid shape is favored strongly in-vivo.

The conserved carboxy terminus of the capsid domain of human immunodeficiency virus type 1 gag protein is important for virion assembly and release

The retroviral Gag precursor plays an important role in the assembly of virion particles. The capsid (CA) protein of the Gag molecule makes a major contribution to this process. In the crystal structure of the free CA protein of the human immunodeficiency virus type 1 (HIV-1), 11 residues of the C terminus were found to be unstructured, and to date no information exists on the structure of these residues in the context of the Gag precursor molecule. We performed phylogenetic analysis and demonstrated a high degree of conservation of these 11 amino acids. Deletion of this cluster or introduction of various point mutations into these residues resulted in significant impairment of particle infectivity. In this cluster, two putative structural regions were identified, residues that form a hinge region (353-VGGP-356) and those that contribute to an alpha-helix (357-GHKARVL-363). Overall, mutations in these regions resulted in inhibition of virion production, but mutations in the hinge region demonstrated the most significant reduction. Although all the Gag mutants appeared to have normal Gag-Gag and Gag-RNA interactions, the hinge mutants were characterized by abnormal formation of cytoplasmic Gag complexes. Gag proteins with mutations in the hinge region demonstrated normal membrane association but aberrant rod-like membrane structures. More detailed analysis of these structures in one of the mutants demonstrated abnormal trapped Gag assemblies. These data suggest that the conserved CA C terminus is important for HIV-1 virion assembly and release and define a putative target for drug design geared to inhibit the HIV-1 assembly process.

Small compounds targeted to subunit interfaces arrest maturation in a nonenveloped, icosahedral animal virus

Nudaurelia omega capensis virus (N omega V) capsids were previously characterized in two morphological forms, a T=4, 485-A-diameter round particle with large pores and a tightly sealed 395-A icosahedrally shaped particle with the same quasi-symmetric surface lattice. The large particle converts to the smaller particle when the pH is lowered from 7.6 to 5, and this activates an autocatalytic cleavage of the viral subunit at residue 570. Here we report that both 1-anilino-8 naphthalene sulfonate (ANS) and the covalent attachment of the thiol-reactive fluorophore, maleimide-ANS (MIANS), inhibit the structural transition and proteolysis at the lower pH. When ANS is exhaustively washed from the particles, the maturation proceeds normally; however, MIANS-modified particles are still inhibited after the same washing treatment, indicating that covalent attachment targets MIANS to a critical location for inhibition. Characterization of the low-pH MIANS product by electron cryo-microscopy (cryo-EM) and image reconstruction demonstrated a morphology intermediate between the two forms previously characterized. A pseudoatomic model of the intermediate configuration was generated by rigid body refinement of the X-ray structure of the subunits (previously determined in the assembled capsid) into the cryo-EM density, allowing a quantitative description of the inhibited intermediate and a hypothesis for the mechanism of the inhibition.

Key interactions in HIV-1 maturation identified by hydrogen-deuterium exchange

To characterize the intersubunit interactions underlying assembly and maturation in HIV-1, we determined the amide hydrogen exchange protection pattern of capsid protein in the immature virion and the mature virion using mass spectrometry. Alterations in protection upon maturation provide evidence for the maturation-induced formation of an interaction between the N- and C-terminal domains in half of the capsid molecules, indicating that only half of the capsid protein is assembled into the conical core.

The N-terminal half of the core protein of hepatitis C virus is sufficient for nucleocapsid formation

The core (C) protein of hepatitis C virus (HCV) appears to be a multifunctional protein that is involved in many viral and cellular processes. Although its effects on host cells have been extensively discussed in the literature, little is known about its main function, the assembly and packaging of the viral genome. We have studied the in vitro assembly of several deleted versions of recombinant HCV C protein expressed in E. coli. We demonstrated that the 75 N-terminal residues of the C protein were sufficient to assemble and generate nucleocapsid-like particles (NLPs) in vitro. However, homogeneous particles of regular size and shape were observed only when NLPs were produced from at least the first 79 N-terminal amino acids of the C protein. This small protein unit fused to the endoplasmic reticulum-anchoring domain also generated NLPs in yeast cells. These data suggest that the N-terminal half of the C protein is important for formation of NLPs. Similarities between the HCV C protein and C proteins of other members of the Flaviviridae are discussed.

Assembly properties of the human immunodeficiency virus type 1 CA protein

During retroviral maturation, the CA protein oligomerizes to form a closed capsid that surrounds the viral genome. We have previously identified a series of deleterious surface mutations within human immunodeficiency virus type 1 (HIV-1) CA that alter infectivity, replication, and assembly in vivo. For this study, 27 recombinant CA proteins harboring 34 different mutations were tested for the ability to assemble into helical cylinders in vitro. These cylinders are composed of CA hexamers and are structural models for the mature viral capsid. Mutations that diminished CA assembly clustered within helices 1 and 2 in the N-terminal domain of CA and within the crystallographically defined dimer interface in the CA C-terminal domain. These mutations demonstrate the importance of these regions for CA cylinder production and, by analogy, mature capsid assembly. One CA mutant (R18A) assembled into cylinders, cones, and spheres. We suggest that these capsid shapes occur because the R18A mutation alters the frequency at which pentamers are incorporated into the hexagonal lattice. The fact that a single CA protein can simultaneously form all three known retroviral capsid morphologies supports the idea that these structures are organized on similar lattices and differ only in the distribution of 12 pentamers that allow them to close. In further support of this model, we demonstrate that the considerable morphological variation seen for conical HIV-1 capsids can be recapitulated in idealized capsid models by altering the distribution of pentamers.

Entropic switch regulates myristate exposure in the HIV-1 matrix protein

The myristoylated matrix protein (myr-MA) of HIV functions as a regulator of intracellular localization, targeting the Gag precursor polyprotein to lipid rafts in the plasma membrane during virus assembly and dissociating from the membrane during infectivity for nuclear targeting of the preintegration complex. Membrane release is triggered by proteolytic cleavage of Gag, and it has, until now, been believed that proteolysis induces a conformational change in myr-MA that sequesters the myristyl group. NMR studies reported here reveal that myr-MA adopts myr-exposed [myr(e)] and -sequestered [myr(s)] states, as anticipated. Unexpectedly, the tertiary structures of the protein in both states are very similar, with the sequestered myristyl group occupying a cavity that requires only minor conformational adjustments for insertion. In addition, myristate exposure is coupled with trimerization, with the myristyl group sequestered in the monomer and exposed in the trimer (K(assoc) = 2.5 +/- 0.6 x 10(8) M(-2)). The equilibrium constant is shifted approximately 20-fold toward the trimeric, myristate-exposed species in a Gag-like construct that includes the capsid domain, indicating that exposure is enhanced by Gag subdomains that promote self-association. Our findings indicate that the HIV-1 myristyl switch is regulated not by mechanically induced conformational changes, as observed for other myristyl switches, but instead by entropic modulation of a preexisting equilibrium.

Human immunodeficiency virus type 1 N-terminal capsid mutants containing cores with abnormally high levels of capsid protein and virtually no reverse transcriptase

We previously described the phenotype associated with three alanine substitution mutations in conserved residues (Trp23, Phe40, and Asp51) in the N-terminal domain of human immunodeficiency virus type 1 capsid protein (CA). All of the mutants produce noninfectious virions that lack conical cores and, despite having a functional reverse transcriptase (RT), are unable to initiate reverse transcription in vivo. Here, we have focused on elucidating the mechanism by which these CA mutations disrupt virus infectivity. We also report that cyclophilin A packaging is severely reduced in W23A and F40A virions, even though these residues are distant from the cyclophilin A binding loop. To correlate loss of infectivity with a possible defect in an early event preceding reverse transcription, we modeled disassembly by generating viral cores from particles treated with mild nonionic detergent; cores were isolated by sedimentation in sucrose density gradients. In general, fractions containing mutant cores exhibited a normal protein profile. However, there were two striking differences from the wild-type pattern: mutant core fractions displayed a marked deficiency in RT protein and enzymatic activity (<5% of total RT in gradient fractions) and a substantial increase in the retention of CA. The high level of core-associated CA suggests that mutant cores may be unable to undergo proper disassembly. Thus, taken together with the almost complete absence of RT in mutant cores, these findings can account for the failure of the three CA mutants to synthesize viral DNA following virus entry into cells.

PA-457: a potent HIV inhibitor that disrupts core condensation by targeting a late step in Gag processing

New HIV therapies are urgently needed to address the growing problem of drug resistance. In this article, we characterize the anti-HIV drug candidate 3-O-(3',3'-dimethylsuccinyl) betulinic acid (PA-457). We show that PA-457 potently inhibits replication of both WT and drug-resistant HIV-1 isolates and demonstrate that the compound acts by disrupting a late step in Gag processing involving conversion of the capsid precursor (p25) to mature capsid protein (p24). We find that virions from PA-457-treated cultures are noninfectious and exhibit an aberrant particle morphology characterized by a spherical, acentric core and a crescent-shaped, electron-dense shell lying just inside the viral membrane. To identify the determinants of compound activity we selected for PA-457-resistant virus in vitro. Consistent with the effect on Gag processing, we found that mutations conferring resistance to PA-457 map to the p25 to p24 cleavage site. PA-457 represents a unique class of anti-HIV compounds termed maturation inhibitors that exploit a previously unidentified viral target, providing additional opportunities for HIV drug discovery.