Broadly neutralizing monoclonal antibodies (mAbs) target epitopes on the viral envelope spike at the surface of HIV-1. The spike is a heterotrimer containing the viral glycoproteins (gp120)3 (gp41)3. This model is generated by combining cryoelectron tomographic (13), crystallographic (14), and computational analyses. It shows the gp120 core structure (tan) fitted in the electron density map (gray) of the spike. The membrane proximal external region (MPER) and viral membrane are shown. The V1/V2 and V3 protein loops are represented as ovals (light green and light blue) at the top of the spike. Glycans (green and blue) are indicated. The model is derived from (5) with permission from Wolters Kluwer Health–Lippincott Williams and Wilkins; from (6) with permission from Elsevier; and from (13) with permission from Macmillan Publishers Ltd./Nature Publishing Group.

The cytoplasmic body component TRIM5a rh restricts HIV-1 infection in Old World monkeys. HIV-1 can enter the cells of Old World monkeys, but encounters a block prior to or during the early stages of reverse transcription. A tripartite motif protein called TRIM5a, which resides in cytoplasmic bodies, mediates this intracellular resistance (Stremlau M, et al., Nature 427, 848-853, 2004). HIV-1 infection is restricted more efficiently by rhesus monkey TRIM5a rh than by human TRIM5a hu . The green and orange shapes near the viral nucleic acids represent reverse transcriptase and integrase proteins, respectively, in the reverse transcription complex. Figure provided courtesy of Drs. Joseph Sodroski (Harvard Medical School), Eric Freed (NCI-Frederick), T. Schroyer (Scientific Publications, Graphics & Media; NCI-Frederick), and used with permission of authors and Cold Spring Harbor Laboratory, New York. Figure was redrawn by John Weddle.

This schematic model illustrates: (a) the normal trafficking of cellular molecules from the Golgi and plasma membrane to the lysosome, (b) HIV budding at both the plasma membrane and at the late endosomal compartments called multivesicular bodies (MVB), (c) internalization and endosomal trafficking of infectious viral particles via the DC-SIGN receptor, and (d) directional release of viral particles toward a target T cell through the virological synapse. Models in which viral particles can bud directly into the synapse or, alternatively, can be targeted there from late endosomes through the exosomal pathway are shown. Note that the virological synapse is created, at least in part through interactions between the viral envelope proteins (Env) on the surface of producer cell, CD4/chemokine receptors on the target T cell, and other cell surface molecules (Morita E and Sundquist WI, Annu. Rev. Cell Dev.Biol., 20, 395-425, 2004). Figure provided courtesy of Dr. Wesley Sundquist (University of Utah) and used with permission of the authors and the Annual Review of Cell and Developmental Biology, Volume 20(c) 2004 by Annual Reviews, http://www.annualreviews.org.

The V3 loop of the HIV-1 envelope glycoprotein gp120 is involved in bindingto the CCR5 and CXCR HIV-1 coreceptors. The structure of an HIV-1 V3MN peptide bound to the FV of the broadly neutralizing human anti-V3monoclonal antibody 447-52D was solved by NMR and found to be a ß-hairpin.The structure of V3MN was found to have conformation and sequencesimilarities to ß-hairpins in CCR5 ligands MIP-1a, MIP-1ß, andRANTES and with CD8, and was found to differ from the ß-hairpin of V3IIIB bound to the strain-specfic murine anti-gp120 antibody 0.5ß. In contrastto the structure of the bound V3MN peptide, the structure of thebound V3IIIB peptide resembles a ß-hairpin in SDF-1, a CXCR4ligand. These results indicate that the 447-52D-bound v3MN and the0.5ß-bound V3IIIB structures represent alternative V3 conformationsresponsible for selective interactions with CCR5 and CXCR4, respectively (Sharon,M. et al., Structure 11:225-236, 2003). This structural information is beingused to design immmunogens taht will induce broadly neutralizing anti-V3 antibodies.

Following synthesis, Gag polypeptides (p55) and the viral accessory protein Vif associate with a host protein, HP68. HP68, an ATP binding protein, appears to interact with the NC region of Gag and promotes progression of Gag-containing assembly intermediates into immature capsids at the host cell plasma membrane (Zimmerman, et al., Nature 415, 88-92, 2002). A cellular factor Tsg101, which functions in vacuolar protein sorting (Vps), is required for HIV-1 budding. Tsg101 binds to an essential tetrapeptide (PTAP) motif within the p6 "Late" domain of the Gag protein and also to ubiquitin. Depletion of cellular Tsg101 by small interfering RNA (RNAi) arrests HIV-1 budding at a late stage, and budding is rescued by reintroduction of Tsg101. Vps28 binds to Tsg101 and also appears to be essential for budding. Finally, dominant negative mutant Vps4 proteins that inhibit vacuolar protein sorting also arrest HIV-1 budding (Garrus JE, et al., Cell 107, 55-65, 2001). A similar role for Tsg101 in HIV and Ebola virus budding is observed by Martin-Serrano J, et al., Nature Medicine 7, 1313-1319, 2001. Figure provided by courtesy of Drs. Jaisri Lingappa and Wes Sundquist, and redrawn by John Weddle.

                                     Legend

Specialized membrane microdomains or lipid rafts are regions of host cell membraneenriched in glycosphingolipids, sphingomyelin and cholesterol. Lipid rafts arecrucial elements in organizing the viral envelope, CD4 and the appropriate chemokinereceptor into a membrane fusion complex leading to viral entry (Manes S et al.,EMBO Reports 1, 190-196, 2000; Hug P et al., J. Virology 74, 6377- 6385, 2000).They are also involved in virus budding (Nguyen DH and Hildreth JEF, J. Virology74, 3264-3272, 2000). The Figure shows HIV approaching target cell membranethat bears CD4 in rafts and coreceptor outside the rafts. Interaction of HIVenvelope with CD4 receptor results in recruiting of coreceptors into rafts andtriggering of conformational changes in HIV envelope, which allow the fusionof viral and cell membranes. Image provided courtesy of Dr. Robert Blumenthalat the National Cancer Institute, National Institutes of Health, USA.

Dendritic cells (DC) in mucosal tissues capture HIV via DC-SIGN, a transmembrane protein with an external mannose binding, C-type lectin domain. DC carry HIV from mucosal tissues to draining lymph nodes where DC-SIGN promotes HIV infection by delivering virus to CD4+ T cells. Image adopted from Geijtenbeek TBH et al., Cell 100:587-597 (2000), with permission of the authors and Cell Press.

Light blue, streptavidin; dark blue, MHC heavy chain; intermediate blue, b-2 microglobulin; red, the peptide (for details, see McMichael AJ and O'Callaghan CA. J Exp Med 187:1367 1371, 1998). Image provided courtesy of Drs. Chris O'Callaghan and Andrew J. McMichael. Reproduced with permission of the authors and Rockefeller University Press.

In the native state of the trimeric gp120/gp41 complex, the fusion peptide (not shown) is buried. Upon interaction of gp120 with cellular receptors, the envelope complex undergoes a conformational change to the pre-hairpin intermediate, in which the fusion peptide is inserted into the target membrane and the N peptide region is a trimeric coiled coil. The C peptide region has not yet associated with the N peptide coiled coil because of a kinetic block due to association with either another portion of gp41, or more likely, gp120. This intermediate is relatively long- lived (many minutes) and is vulnerable to C peptide inhibition (bottom). The pre-hairpin intermediate resolves to the fusion-active hairpin structure when the C peptide region binds to the N peptide coiled coil and adopts a helical conformation. This rearrangement results in membrane apposition. The interactions necessary for fusion are unknown (as indicated by "?"), but may involve aggregation of gp41 trimers to form fusion pores. After fusion is completed, the fusion peptide and the transmembrane segment of gp41 lie within the same membrane. The steps at which various HIV entry inhibitors act are shown (for details, see Chan DC and Kim PS. Cell 93:681 684, 1998). Imageprovided courtesy of Dr. Peter Kim. Reproduced with permission of the authors and Cell Press.

View of the HIV-1 gp120-CD4 complex, from the perspective of the target cell. The molecular surface of the HIV-1 gp120 core is shown in gray. The peptide-proximal carbohydrates added to gp120 in mammalian cells are depicted in blue. The conserved gp120 residues important for binding to the CCR5 chemokine receptor are depicted in green. The two domain amino-terminal region of CD4 is in red. Image courtesy of Dr. Joseph Sodroski. See Kwong PD, et al. Nature 393:648-659, 1998; Wyatt R, et al. Nature 393:705-711, 1998; and Rizzuto CD, et al. Science 280:1949-1953, 1998.

View of the HIV-1 gp120-CD4 complex, from the perspective of the virus. The molecular surface of the HIV-1 gp120 core is shown in gray, carbohydrate structures in blue. The two domain amino-terminal region of CD4 is in red. CD4 binds in a recession on the gp120 surface. The outer domain of the gp120 core, on the right side of the figure, is extensively glycosylated. Image courtesy of Dr. Joseph Sodroski. See Kwong PD, et al. Nature 393:648-659, 1998; Wyatt R, et al. Nature 393:705-711, 1998; and Rizzuto CD, et al. Science 280:1949-1953, 1998.

Image courtesy of Dr. Robert Doms. Reprinted with permission of Academic Press.                                                                                                                                                                                                                                                                                                                                           

This list of reagents are available only through Quality Biological. For further details on the virus reagents and their availability, please submit your request with the QBI number to HIVReagents@qualitybiological.com.

Image provided courtesy of Drs. Louis Henderson and Larry Arthur