Tsg101 control of human immunodeficiency virus type 1 Gag trafficking and release

The structural precursor polyprotein of human immunodeficiency virus type 1, Pr55(gag), contains a proline-rich motif (PTAP) called the "late domain" in its C-terminal p6 region that directs release of mature virus-like particles (VLPs) from the plasma membranes of gag-transfected COS-1 cells. The motif binds Tsg101 (vacuolar protein-sorting protein 23, or Vps23), which functions in endocytic trafficking. Here, we show that accumulation of the wild-type (wt) Gag precursor in a fraction of COS-1 cytoplasm enriched in multivesicular bodies and small particulate components of the plasma membrane (P100) is p6 dependent. Cleavage intermediates and mature CA mainly partitioned with more rapidly sedimenting larger material enriched in components of lysosomes and early endosomes (P27), and this also was p6 dependent. Expression of truncated or full-length Tsg101 proteins interfered with VLP assembly and Gag accumulation in the P100 fraction. This correlated with reduced accumulation of Gag tagged with green fluorescent protein (Gag-GFP) at the plasma membrane and colocalization with the tagged Tsg101 in perinuclear early endosomes, as visualized by confocal microscopy. Fractionation analysis and confocal examination both indicated that the N-terminal region of Tsg101, which contains binding sites for PTAP and ubiquitin (Ub), was required for Gag trafficking to the plasma membrane. Expression of FLAG-tagged Tsg101 with a deletion in the Ub-binding pocket inhibited VLP release almost completely and to a significantly greater extent than expression of the wt tagged Tsg101 protein or Tsg101-FLAG containing a deletion in the PTAP-binding region. The results demonstrate that Gag associates with endosomal trafficking compartments and indicate that efficient release of virus particles from the plasma membrane requires both the PTAP- and Ub-binding functions of Tsg101 to recruit the cellular machinery required for budding.

HIV-1 capsid protein forms spherical (immature-like) and tubular (mature-like) particles in vitro: structure switching by pH-induced conformational changes

The viral genome and replicative enzymes of the human immunodeficiency virus are encased in a shell consisting of assembled mature capsid protein (CA). The core shell is a stable, effective protective barrier, but is also poised for dissolution on cue to allow transmission of the viral genome into its new host. In this study, static light scattering (SLS) and dynamic light scattering (DLS) were used to examine the entire range of the CA protein response to an environmental cue (pH). The CA protein assembled tubular structures as previously reported but also was capable of assembling spheres, depending on the pH of the protein solution. The switch from formation of one to the other occurred within a very narrow physiological pH range (i.e., pH 7.0 to pH 6.8). Below this range, only dimers were detected. Above this range, the previously described tubular structures were detected. The ability of the CA protein to form a spherical structure that is detectable by DLS but not by electron microscopy indicates that some assemblages are inherently sensitive to perturbation. The dimers in equilibrium with these assemblages exhibited distinct conformations: Dimers in equilibrium with the spherical form exhibited a compact conformation. Dimers in equilibrium with the rod-like form had an extended conformation. Thus, the CA protein possesses the inherent ability to form metastable structures, the morphology of which is regulated by an environmentally-sensitive molecular switch. Such metastable structures may exist as transient intermediates during the assembly and/or disassembly of the virus core.

Structural consequences of cyclophilin A binding on maturational refolding in human immunodeficiency virus type 1 capsid protein

While several cellular proteins are incorporated in the human immunodeficiency virus type 1 virion, cyclophilin (CyP) A is the only one whose absence has been demonstrated to impair infectivity. Incorporation of the cytosolic protein results from interaction with a highly exposed Pro-rich loop in the N-terminal region of the capsid (CA) domain of the precursor polyprotein, Pr55(Gag). Even when prevented from interacting with CyP A, Pr55Gag still forms particles that proceed to mature into morphologically wild-type virions, suggesting that CyP A influences a postassembly event. The nature of this CyP A influence has yet to be elucidated. Here, we show that while CyP A binds both Gag and mature CA proteins, the two binding interactions are actually different. Tryptophan 121 (W121) in CyP A distinguished the two proteins: a phenylalanine substitution (W121F) impaired binding of mature CA protein but not of Gag. This indicates the occurrence of a maturation-dependent switch in the conformation of the Pro-rich loop. A structural consequence of Gag binding to CyP A was to block this maturational refolding, resulting in a 24-kDa CA protein retaining the immature Pro-rich loop conformation. Using trypsin as a structure probe, we demonstrate that the conformation of the C-terminal region in mature CA is also a product of maturational refolding. Binding to wild-type CyP A altered this conformation, as indicated by a reduction in the accessibility of Cys residue(s) in the region to chemical modification. Hence, the end result of binding to CyP A, whether the Pro-rich loop is in the context of Gag or mature CA protein, is a structurally modified mature CA protein. The postassembly role of CyP A may be mediated through these modified mature CA proteins.

Membrane-induced alterations in HIV-1 Gag and matrix protein-protein interactions

The matrix (MA) domain of human immunodeficiency virus type 1 (HIV-1) contains sequences that direct association with the nucleus at early times in the virus replication cycle and with the plasma membrane at late times in the cycle. Localization to these sites is critical for functions related to the establishment of the infecting provirus and viral assembly, respectively. Mutational and structural analyses indicate that the opposing targeting signals which mediate these subcellular localization events include the same basic residues found in the N-terminal region of the protein. Here, we examined protein multimerization as a determinant of membrane association. Under high ionic strength conditions, Gag, but not MA, binds phospholipid membranes with high affinity. The oligomerization state of the protein per se did not appear to be a prerequisite for stable membrane binding, as Gag and MA were both capable of forming oligomers in high ionic strength buffer. To determine the fate of Gag and MA multimers in the presence of phospholipid membranes in real time, we measured resonance energy transfer between oligomer subunits in the presence and absence of lipid. The presence of phospholipid significantly increased the efficiency of resonance energy transfer between Gag molecules, consistent with enhanced Gag multimerization. This suggests that Gag oligomers assembled on the membrane surface and correlated with the observed stability of membrane binding. In contrast, the efficiency of resonance energy transfer between MA molecules decreased, indicating that MA oligomers dissociated in the presence of membrane, consistent with observed unstable binding. Identical results were obtained whether the probes were covalently attached to a Lys residue in Gag or to residues specifically within the MA domain of Gag; whether the fluorophore was rhodamine or fluorescein; or whether hetero- or homotransfer was measured. The results suggest that phospholipid induces alterations in Gag and MA protein-protein interactions that may contribute to the puzzling ability of MA to direct targeting functions requiring alternately membrane binding and membrane dissociation. The results also suggest that regions downstream of the MA domain in the precursor, or conformations formed after maturation of MA, play a critical role in oligomerization-modulated membrane binding.

Crystal structure of dimeric HIV-1 capsid protein

X-ray diffraction analysis of a human immunodeficiency virus (HIV-1) capsid (CA) protein shows that each monomer within the dimer consists of seven alpha-helices, five of which are arranged in a coiled coil-like structure. Sequence assignments were made for two of the helices, and tentative connectivity of the remainder of the protein was confirmed by the recent solution structure of a monomeric N-terminal fragment. The C-terminal third of the protein is mostly disordered in the crystal. The longest helices in the coiled coil-like structure are separated by a long, highly antigenic peptide that includes the binding site of an antibody fragment complexed with CA in the crystal. The site of binding of the Fab, the position of the antigenic loop and the site of cleavage between the matrix protein and CA establish the side of the dimer that would be on the exterior of the retroviral core.

Partitioning of HIV-1 Gag and Gag-related proteins to membranes

The binding of HIV-1 Gag and Gag-related proteins to model membranes was examined using three experimental systems: (i) large unilamellar phospholipid vesicles (LUVs) and recombinant Gag purified from Escherichia coli; (ii) LUVs added to a mammalian cell extract in which Gag proteins were expressed by a coupled transcription/translation system; and (iii) inside-out plasma membrane vesicles purified from human red blood cells (RBC) and recombinant, purified Gag from E. coli. Several novel aspects of HIV-1 Gag membrane interactions were observed: (i) Gag proteins bound with high affinity to both model membranes with a negatively charged surface and to RBC membranes. (ii) Binding of the Gag precursor and mature Gag proteins exhibited different sensitivities to ionic strength indicating that the precursor directed membrane binding through interactions that were qualitatively and quantitatively distinct from those of any of its individual domains. Studies using energy transfer between tryptophan residues in the proteins and anthroyloxy-containing probes inserted in the LUVs indicated that the orientation of the precursor and of the mature proteins on the membrane surface were distinct; (iii) Gag oligomers appear to have facilitated high-affinity binding under high salt conditions, suggesting that protein-protein interactions led to formation of stronger electrostatic or new hydrophobic membrane binding determinants. Since binding studies with model membranes permit quantitative analysis, these experimental approaches may permit identification of interactions that drive Gag assembly on the membrane.

A preliminary evaluation of hydroxyurea for the treatment of rheumatoid arthritis

OBJECTIVE

To obtain preliminary evidence on the safety and efficacy of low dose hydroxyurea as a treatment for rheumatoid arthritis (RA).

METHODS

Five patients with active RA unresponsive to conventional therapy were entered into a 12 week, open label trial of hydroxyurea followed by a one month postdrug evaluation.

RESULTS

Three of the 4 patients completing the study had a decrease in morning stiffness and number of swollen and tender joints. All 4 patients had a decrease in pain and an increase in function as measured by a modified health assessment questionnaire. No patient had improvement in sedimentation rate, C-reactive protein, or subjective measures of global well being. However, 3 of the 4 patients had disease flare after the drug was withdrawn, demonstrated by increased number of swollen and tender joints.

CONCLUSION

Low dose hydroxyurea may be effective in the treatment of RA, but confirmation will require further testing by a randomized double blind placebo controlled trial of patients with a broader spectrum of disease severity over a longer period of therapeutic intervention.

Spectral analysis and tryptic susceptibility as probes of HIV-1 capsid protein structure

The structures of HIV-1 capsid protein (CA, p24) isolated from mature virions and CA protein autoprocessed from a recombinant Gag-Pol precursor expressed in Escherichia coli were compared using circular dichroic (CD) spectral analysis. The spectra obtained for the intact recombinant and viral proteins were indistinguishable, indicating that the backbone configurations directed by the primary amino acid sequences of the proteins were similar or identical. The structure predictions derived from CD were, in general, inconsistent with a model proposing the eight-stranded beta barrel motif found in several RNA viruses. However, aspects of the model were supported by experiments that identified surface-exposed regions. Biochemical analysis indicated that the recombinant CA protein formed nonrandom higher-ordered structures in vitro. Under physiological conditions, the protein assembled into oligomers containing subunits in two packing arrangements. In one arrangement, the central region near Arg100 was exposed and susceptible to tryptic digestion at low enzyme concentrations (enzyme:substrate ratios = 1:5000 to 1:100). Also, in this arrangement, the proteins were susceptible to crosslinking by the bifunctional agent DTSSP. Proteins in the other arrangement were resistant to proteolysis at low enzyme concentrations. The central region of these resistant molecules was inaccessible to monospecific antibodies that recognized antigenic sites between residues 94 and 107 and these proteins were not crosslinked by DTSSP or EGS. Following incubation with trypsin, both the resistant molecules and the fragments derived from the susceptible proteins in the oligomer migrated as smaller complexes, suggesting that the regions digested by trypsin stabilized the oligomer unit. The results indicate that the central region of the HIV-1 CA protein plays a role in formation of higher-ordered structures. Moreover, the relative stability of the N- and C-terminal partial digestion fragments arising from cleavage at Arg100/Gly101 suggests that this exposed central region separates two structural domains of the protein.

Transformation of NIH/3T3 to anchorage independence by H-ras is accompanied by loss of suppressor activity

Despite their familiar sensitivity to transformation by dominant-acting ras oncogenes, NIH/3T3 cells carry a ras suppressor. When tested by cell fusion they were able to suppress the anchorage-independent phenotype of both mouse and human cells transformed by activated H-ras or N-ras. This suppression occurred without a decrease in expression of the activated ras oncogene. Ras-transformed NIH/3T3 clones cured of their oncogene by benzamide treatment reverted to a nontransformed phenotype, but had lost the ability to suppress other ras transformants, indicating that their initial transformation was accompanied by suppressor loss. In hamster cells an active ras oncogene increased the rate of chromosome segregation by > 100-fold. These results suggest that in vitro transformation of NIH/3T3 cells by ras may be more similar to multistep in vivo tumor development than previously suspected, involving not only expression of an active oncogene but also loss of a suppressor activity, perhaps induced by the clastogenic oncogene.

Inactivation of the human immunodeficiency virus by hypericin: evidence for photochemical alterations of p24 and a block in uncoating

Following attachment and entry of human immunodeficiency virus (HIV) into a host cell, the HIV genomic RNA is reverse transcribed to cDNA. This step may be inhibited by hypericin, a compound that induces alterations of the retroviral capsid. Incubation of HIV with hypericin rendered the virus noninfectious. The replication of HIV was blocked early; HIV cDNA could not be detected in cells challenged with hypericin-treated HIV. Hypericin did not inhibit the binding of recombinant gp120 to CD4+ cells, nor did hypericin inhibit syncytium formation. However, reverse transcriptase activity could not be released from hypericin-treated virions. Western blot analysis revealed altered mobility of the HIV major capsid protein (p24) following hypericin treatment. Hypericin-treated recombinant HIV p24 exhibited similar altered mobility. The inactivation of HIV infectivity and the alterations in p24 mobility required hypericin incubations in the presence of visible light. Collectively, these data suggest that photochemical alterations of the HIV capsid may contribute to the hypericin-mediated inactivation of HIV. Such alterations may inhibit the release of RT activity from treated HIV, and prevent uncoating and subsequent reverse transcription of the HIV genome within a target cell.

Assembly of recombinant human immunodeficiency virus type 1 capsid protein in vitro

The capsid protein (CA) (p24) of human immunodeficiency virus (HIV) type 1 expressed in Escherichia coli and purified to greater than 90% homogeneity was used to examine assembly in vitro and to probe the nature of interactions involved in the formation of capsid structures. The protein was detected in dimeric and oligomeric forms as indicated by molecular size measurements by gel filtration column chromatography, sedimentation through sucrose, and nondenaturing gel electrophoresis. Chemical cross-linking of CA molecules was observed with several homobifunctional reagents. Oligomer size was dependent on cross-linker concentration and exhibited a nonrandom pattern in which dimers and tetramers were more abundant than trimers and pentamers. Oligomers as large as dodecamers were detected in native polyacrylamide gels. These were stable in solutions of high ionic strength or in the presence of nonionic detergent, indicating that strong interactions were involved in oligomer stabilization. Limited tryptic digestion converted the putative dodecamers to octamers, suggesting that a region involved in CA protein multimerization was exposed in the structure. This region was mapped to the central portion of the protein. The recombinant CA proteins assembled in vitro into long rodlike structures and were disassembled into small irregular spheres by alterations in ionic strength and pH. The observation that assembly and disassembly of purified HIV type 1 CA protein can be induced in vitro suggests an approach for identifying possible control mechanisms involved in HIV viral core assembly.

Preparation and crystallization of a human immunodeficiency virus p24-Fab complex

A recombinant form of human immunodeficiency virus capsid protein, p24, expressed in Escherichia coli has been purified to homogeneity and separated into distinct isoelectric forms. A monoclonal antibody, mAb25.4, which recognizes an epitope in the amino-terminal region of p24, has been purified to homogeneity from ascites fluid and digested with papain to produce the respective antigen-binding fragment (Fab). The Fab25.4 was purified from the digestion mixture and separated into two distinct isoelectric forms. The two Fab species were each complexed with one isoelectric form of the recombinant p24 by incubating equimolar quantities of the two proteins. Two different crystal morphologies of the p24-Fab25.4 complex were obtained by the vapor-diffusion method with 12-24% PEG 3350 as the precipitant. One of these crystal forms has unit-cell parameters of a = 92.1 A, b = 85.4 A, c = 54.0 A, alpha = gamma = 90.0 degrees and beta = 90.4 degrees and belongs to the monoclinic space group P2(1), with one molecule of the complex per asymmetric unit. These crystals strongly diffracted x-rays to at least 2.7-A resolution.

Expression in Escherichia coli and purification of human immunodeficiency virus type 1 capsid protein (p24)

Capsid protein (p24;CA) of human immunodeficiency virus type 1 (HIV-1) was synthesized in Escherichia coli strain BL21 (DE3) using a plasmid encoding a truncated HIV-1 gag/pol gene. The plasmid, which contained a mutation in the frameshift region, expressed viral proteinase (PR), a pol gene product, in the gag reading frame, resulting in efficient processing of mature CA and other gag-related products. The expressed CA is soluble, recognized by monoclonal antibodies directed against HIV CA and has an N-terminal sequence identical to that of CA purified from HIV. Purification was done under mild conditions where coexpressed HIV PR retained enzymatic activity. Milligram quantities of 90% pure CA protein were obtained after chromatography on DEAE cellulose followed by facilitated aggregation of the CA in the unbound fraction. The precipitated CA was readily dissolved in low ionic strength aqueous buffer. Gel exclusion chromatography results indicated that, in solution, CA existed in oligomeric form.