How Do Viruses Enter Cells? The HIV Coreceptors Teach Us a Lesson of Complexity
1997; Cell Press; Volume: 91; Issue: 6 Linguagem: Inglês
10.1016/s0092-8674(00)80460-2
ISSN1097-4172
Autores Tópico(s)Mosquito-borne diseases and control
ResumoThe discovery that several chemokine receptors can serve as HIV-1 coreceptors (1Alkhatib G Combadiere C Broder C.C Feng Y Kennedy P.E Murphy P.M Berger E.A CC CKR5 a RANTES, MIP-1 alpha, MIP-1beta receptor as a fusion cofactor for macrophage-tropic HIV-1.Science. 1996; 272: 1955-1958Crossref PubMed Scopus (2443) Google Scholar, 11Choe H Farzan M Sun Y Sullivan N Rollins B Ponath P.D Wu L Mackay C.R LaRosa G Newman W et al.The beta-chemokine receptors CCR3 and CCR5 facilitate infection by primary HIV-1 isolates.Cell. 1996; 85: 1135-1148Abstract Full Text Full Text PDF PubMed Scopus (2090) Google Scholar, 14Deng H Liu R Ellmeier W Choe S Unutmaz D Burkhart M Di Marzio P Marmon S Sutton R.E Hill C.M et al.Identification of a major co-receptor for primary isolates of HIV-1.Nature. 1996; 381: 661-666Crossref PubMed Scopus (3194) Google Scholar, 18Doranz B.J Rucker J Yi Y Smyth R.J Samson M Peiper S.C Parmentier M Collman R.G Doms R.W A dual-tropic primary HIV-1 isolate that uses fusin and the beta-chemokine receptors CKR-5, CKR-3, and CKR-2b as fusion cofactors.Cell. 1996; 85: 1149-1158Abstract Full Text Full Text PDF PubMed Scopus (1684) Google Scholar, 20Dragic T Litwin V Allaway G.P Martin S.R Huang Y Nagashima K.A Cayanan C Maddon P.J Koup R.A Moore J.P Paxton W.A HIV-1 entry into CD4+ cells is mediated by the chemokine receptor CC-CKR-5.Nature. 1996; 381: 667-673Crossref PubMed Scopus (2813) Google Scholar) and the engineering of a recombinant virus that specifically targets cells expressing fusogenic HIV-1 envelope glycoprotein (Env) (40Schnell M.J Johnson J.E Buonocore L Rose J.K Construction of a novel virus that targets HIV-1-infected cells and controls HIV-1 infection.Cell. 1997; 90: 849-857Abstract Full Text Full Text PDF Scopus (198) Google Scholar) highlighted the importance of an old question: how do viruses enter cells? The identification and characterization of the HIV coreceptors not only resolved a long-standing puzzle and elucidated a number of phenomena that had been observed in the past decade (reviewed in a recent book17Dimitrov D.S Broder C.C HIV and Membrane Receptors. Landes Biosciences, Austin, TX1997Google Scholar) but it also demonstrated the complexity of the multifactorial and multistage process of virus entry. What are the molecular mechanisms determining the complexity of virus entry? How can knowledge of virus entry help in prevention and treatment of diseases? Possible answers to these fundamental and many other specific questions were discussed at a recent meeting on The Cell Biology of Viral Entry (September 21–24, Frederick, Maryland) organized by R. Blumenthal (National Cancer Institute, USA) and sponsored by the National Cancer Institute and the Office of AIDS Research, NIH. A major goal of the meeting was to bring together leading scientists from around the world to discuss different aspects of the mechanisms of virus entry, including three-dimensional (3-D) structures of viruses, virus receptors, and fusion kinetics, as well as the development of entry inhibitors and delivery systems targeting specific cell types. The mechanism of virus entry is critically dependent on the 3-D structure of the infecting virion. Many DNA viruses and some RNA viruses—both plus-stranded and double-stranded—do not have membranous envelopes but interact with host cells via their capsid proteins. For smaller viruses, the major (or only) capsid protein serves this purpose, but larger and more complex viruses (e.g., adenovirus, reovirus, DNA phages) have minor capsid proteins, called spikes or fibers, at particular capsid sites (e.g., vertices) that recognize host receptors. A. Steven (NIAMS-NIH, Bethesda) discussed how X-ray crystallography and cryo–electron microscopy (cryo-EM) complement each other as applied to viruses. Molecular details of large virions can be obtained by combining high-resolution X-ray structures of protein components with lower-resolution EM structures of whole virions; likewise, X-ray structures of virions may be computationally perturbed to fit EM-derived density maps of unstable transitional states that may be inaccessible to crystallography. The latter approach was applied to investigate the transition undergone by poliovirus (∼30 nm diameter, 160S particle) after interacting with host cell but prior to release of the RNA (135S particle, 33 nm diameter, "A particle") (J. Hogle, Harvard Medical School, Boston). The combined use of crystallography and cryo-EM was also applied to probe receptor-capsid interactions of rhinovirus (with soluble ICAM-1) and parvovirus (with globoside). In both cases, the receptor and its binding site were visualized: in neither case is the virion appreciably perturbed by this interaction. The structure of the native bovine papillomavirus was determined by cryo-EM and 3-D image reconstruction at 0.9 nm (F. Booy, NIAMSD, NIH, Bethesda) (45Trus B.L Roden R.B Greenstone H.L Vrhel M Schiller J.T Booy F.P Novel structural features of bovine papillomavirus capsid revealed by a three-dimensional reconstruction to 9 A resolution.Nat. Struct. Biol. 1997; 4: 413-420Crossref Scopus (160) Google Scholar). The high resolution allowed identification of intercapsomere protein connections (∼1 nm in diameter), which were not seen at 1.1 nm resolution. Whether these connections are important for entry remains to be determined. The initial stages of attachment and induction of conformational changes for both nonenveloped and enveloped viruses could be quite similar. However, for only a few envelope proteins has the 3-D structure been solved to allow for extensive comparison. For more than a decade, the only solved 3-D structure of fusion mediating envelope protein was that of the influenza hemagglutinin (HA) (49Wilson I.A Skehel J.J Wiley D.C Structure of the haemagglutinin membrane glycoprotein of influenza virus at 3 A resolution.Nature. 1981; 289: 366-373Crossref PubMed Scopus (1978) Google Scholar). In an explosion of recent developments, several new 3-D structures have been solved. Among these were fragments of: the major envelope glycoprotein (E) of the tick-borne encephalitis (TBE) virus (39Rey F.A Heinz F.X Mandl C Kunz C Harrison S.C The envelope glycoprotein from tick-borne encephalitis virus at 2 A resolution.Nature. 1995; 375: 291-298Crossref PubMed Scopus (1225) Google Scholar), the low-pH form of the HA transmembrane protein (6Bullough P.A Hughson F.M Skehel J.J Wiley D.C Structure of influenza haemagglutinin at the pH of membrane fusion.Nature. 1994; 371: 37-43Crossref PubMed Scopus (1376) Google Scholar), the ectodomain of Moloney murine leukemia virus (MoMuLV) (25Fass D Harrison S.C Kim P.S Retrovirus envelope domain at 1.7 angstrom resolution.Nat. Struct. Biol. 1996; 3: 465-469Crossref Scopus (302) Google Scholar), and the HIV-1 gp41 (47Weissenhorn W Dessen A Harrison S.C Skehel J.J Wiley D.C Atomic structure of the ectodomain from HIV-1 gp41.Nature. 1997; 387: 426-430Crossref PubMed Scopus (1460) Google Scholar, 8Chan D.C Fass D Berger J.M Kim P.S Core structure of gp41 from the HIV envelope glycoprotein.Cell. 1997; 89: 263-273Abstract Full Text Full Text PDF PubMed Scopus (1834) Google Scholar). P. Kim (Whitehead Institute for Biomedical Research, MIT, Cambridge) reported the latest in this series of achievements—the solution of the 3-D structure of a fragment of the Friend MuLV Env containing the receptor-binding site. The core of the receptor-binding domain is an antiparallel beta sandwich, with two interstrand loops forming a helical subdomain atop the sandwich. The residues in the helical region, but not in the beta sandwich, are highly variable among mammalian C-type retroviruses with distinct tropisms, indicating that the helical subdomain determines the receptor specificity of the virus (26Fass D Davey R.A Hamson C.A Kim P.S Cunningham J.M Berger J.M Structure of a murine leukemia virus receptor-binding glycoprotein at 2.0 angstrom resolution.Science. 1997; 277: 1662-1666Crossref PubMed Scopus (181) Google Scholar). It would be interesting to compare the details of this structure with the more extensively studied high-affinity receptor-binding sites of nonenveloped viruses, especially human rhinoviruses (M. Rossmann, Purdue University, West Lafayette). At this pace of research, we may soon be able to look at the HIV-1 gp120 structure. The known structures of nonenveloped virus coat and envelope proteins suggest that helical structures are involved in the binding specificity. Helices, but in coiled coils, are also involved in formation of oligomers and may serve as fusion intermediates. D. Wiley (Harvard University, Cambridge) discussed the evidence that the coiled coils of the low-pH form of HA are fusogenic intermediate structures. Experiments with HA2 polypeptide residues 38–175 expressed in Escherichia coli suggested that the structure of the low-pH-induced fold of viral HA2 (TBHA2) observed crystallographically is the lowest-energy-state fold of the HA2 polypeptide. This structure folds in vivo into a trimer. These results indicate that the HA2 conformation in viral HA is metastable at neutral pH and suggest that removal of the receptor-binding chain (HA1) is sufficient to allow HA2 to adopt the stable state. Furthermore, they provide direct evidence that low pH is not required to form the membrane-fusion conformation but acts to make this state kinetically accessible in viral HA. The striking similarity between the low pH (fusion) structure of the influenza HA and the fusogenic structures of the transmembrane proteins from MuLV and HIV-1 may indicate that not only influenza but also some retroviruses could use a "spring-loaded" mechanism (7Carr C.M Kim P.S A spring-loaded mechanism for the conformational changes of influenza hemagglutinin.Cell. 1993; 73: 823-832Abstract Full Text PDF PubMed Scopus (787) Google Scholar, 6Bullough P.A Hughson F.M Skehel J.J Wiley D.C Structure of influenza haemagglutinin at the pH of membrane fusion.Nature. 1994; 371: 37-43Crossref PubMed Scopus (1376) Google Scholar) to relocate the hydrophobic fusion peptides (P. Kim). Especially interesting is the possibility for design of small molecule inhibitors of HIV-1 entry based on the 3-D structure of the HIV-1 fusion core and the inhibitory properties of peptides derived from this core. The oligomeric structure of the HIV-1 Env was discussed extensively. The majority of the discussants supported the view that the Env is a trimer. D. Wiley pointed out that a misinterpretation of some previous experiments contributed to considerations of other Env oligomeric forms. Many viruses have evolved to utilize cell surface molecules for gaining access into the cell interior. The discovery of specific bacteriophage attachment to Shigella by d'Herelle in the beginning of this century was followed by extensive studies of the bacteriophages (D. Dimitrov, NCI-FCRDC, Frederick). These studies demonstrated that a variety of carbohydrates, lipopolysaccharides, and proteins (i.e., virtually every structure exposed at the bacterial surface) can serve as a phage receptor. As found with bacteria, any structure exposed at the cell surface could serve as an animal virus receptor. Although the 3-D structure of only a few virus receptors is yet known, their chemical composition precludes possible relationships between receptor and virus attachment protein (VAP) structures. Since the discovery that four chemokine receptors can also serve as HIV-1 coreceptors, five new human molecules have been identified as entry cofactors—GPR15, STRL33, GPR1, V28, and CCR8. STRL33 is a novel human seven-transmembrane domain orphan receptor that is related to chemokine receptors and is expressed in activated peripheral blood lymphocytes (PBLs) and T-cell lines (J. Farber, NIAID, NIH). It functions as an entry cofactor for Envs from macrophage-tropic (M-tropic), T-cell line–tropic (T-tropic), and dual tropic strains of HIV-1 and SIV (2Alkhatib G Liao F Berger E.A Farber J.M Peden K.W A new SIV co-receptor, STRL33.Nature. 1997; 388: 238Crossref Scopus (122) Google Scholar, 32Liao F Alkhatib G Peden K.W Sharma G Berger E.A Farber J.M STRL33, a novel chemokine receptor-like protein, functions as a fusion cofactor for both macrophage-tropic and T cell line-tropic HIV-1.J. Exp. Med. 1997; 185: 2015-2023Crossref PubMed Scopus (343) Google Scholar). Sodroski (Harvard Medical School, Boston) described two orphan seven-transmembrane segment receptors, GPR1 and GPR15, that serve as coreceptors for SIV and are expressed in human alveolar macrophages (24Farzan M Choe H Martin K.A Sun Y Sidelko M Mackay C.R Gerard N Sodroski J Gerard C HIV-1 entry and macrophage inflammatory protein-1beta-mediated signaling are independent functions of the chemokine receptor CCR5.J. Biol. Chem. 1997; 272 (b): 6854-6857Crossref Scopus (170) Google Scholar). The more efficient of these, GPR15, is also expressed in human CD4+ T lymphocytes and activated rhesus macaque peripheral blood mononuclear cells. GPR15 and STRL33 are the same as BOB and Bonzo, respectively, recently isolated by D. Littman and his colleagues (15Deng H.K Unutmaz D Kewalramani V.N Littman D.R Expression cloning of new receptors used by simian and human immunodeficiency viruses.Nature. 1997; 388: 296-300Crossref Scopus (599) Google Scholar). V28 and CCR8 were recently shown to serve as HIV coreceptors (R. Doms, E. Berger) (see also38Reeves J.D McKnight A Potempa S Simmons G Gray P.W Power C.A Wells T Weiss R.A Talbot S.J CD4-independent infection by HIV-2 (ROD/B) use of the 7-transmembrane receptors CXCR-4, CCR-3, and V28 for entry.Virology. 1997; 231: 130-134Crossref PubMed Scopus (132) Google Scholar). With the identification of US28, encoded by human cytomegalovirus (36Pleskoff O Treboute C Brelot A Heveker N Seman M Alizon M Identification of a chemokine receptor encoded by human cytomegalovirus as a cofactor for HIV-1 entry.Science. 1997; 276: 1874-1878Crossref Scopus (286) Google Scholar), the number of coreceptors used by HIV and/or SIV is now ten (CCR5, CXCR4, CCR3, CCR2b, STRL33, GPR15, GPR1, V28, CCR8, and US28). This number does not include nonhuman analogs and is likely to grow. How many molecules can serve as immunodeficiency virus coreceptors is presently unknown. An interesting possibility is that some HIV-1 entry cofactors may not be proteins at all but glycolipids (A. Puri; R. Blumenthal, NCI-FCRDC, Frederick). A neutral glycolipid, possibly with three sugar groups in the polar head group can serve as an alternative and/or additional cofactor in CD4-dependent HIV-1 fusion. This adds a new dimension to the ability of HIV-1 to use a variety of molecules as coreceptors. The 3-D dimensional structure of the HIV-1 coreceptors is presently unknown. A theoretical 3-D model of CXCR4 and CCR5 was developed by S. Durell (NCI, NIH, Bethesda), based on the physically determined structures of both bacteriorhodopsin and rhodopsin, as well as analysis of the amino acid sequences of related G-protein coupled receptors. The model highlighted differences in the electrostatic potentials of the extracellular portions of the molecules, which may be important for virus tropism. It appears that in the evolution of HIV, "coreceptors" may have initially served as receptors and the use of CD4 is a more recent adaptation, as was originally suggested by R. Weiss (reviewed in (17Dimitrov D.S Broder C.C HIV and Membrane Receptors. Landes Biosciences, Austin, TX1997Google Scholar)). The participants of the meeting supported that notion, particularly based on the observations that strains of SIV can infect CD4-negative cells by using CCR5 as a receptor molecule (R. Doms, University of Pennsylvania; J. Sodroski), and that some strains of HIV-2 enter CD4-negative cells (13Clapham P.R McKnight A Weiss R.A Human immunodeficiency virus type 2 infection and fusion of CD4-negative human cell lines induction and enhancement by soluble CD4.J. Virol. 1992; 66: 3531-3537Crossref Google Scholar) by using CXCR4 (22Endres M.J Clapham P.R Marsh M Ahuja M Turner J.D McKnight A Thomas J.F Stoebenau-Haggarty B Choe S Vance P.J et al.CD4-independent infection by HIV-2 is mediated by fusin/CXCR4.Cell. 1996; 87: 745-756Abstract Full Text Full Text PDF PubMed Scopus (587) Google Scholar) as observed by Clapham, Hoxie and colleagues. Whether other viruses can use so many coreceptors and other entry cofactors still remains unknown. However, entry cofactors have been identified for some viruses, e.g., adenoviruses as noted by A. Helenius (Yale University, New Haven) during the discussion. In some cases, the existence of entry cofactors has not been confirmed by further experimentation. For example, there is no evidence for poliovirus entry cofactors in spite of some early experiments indicating that CD44 may have such a function (V. Racaniello, Columbia University). This reminds us the lessons from the long story of the many HIV-1 candidate entry cofactors (17Dimitrov D.S Broder C.C HIV and Membrane Receptors. Landes Biosciences, Austin, TX1997Google Scholar). Viruses can be divided into two groups with respect to their interactions with receptor molecules: those with high affinity to their receptors (e.g., rhinovirus, poliovirus, and HIV) and those with low affinity (e.g., polyoma, SV40, and influenza). As a rule, the high affinity interactions serve a dual function; in addition to attachment they also induce conformational changes needed for the subsequent stages of entry. The low affinity interactions require a low pH trigger for entry. In a striking example of how much we know about structural details of virus-receptor interactions M. Rossman (Purdue University, West Lafayette) described the predicted 3-D structure of the ICAM-1 D1D2 fragment fitted into the human rhinovirus canyon. Similar considerations apply also to parvovirus and its receptor. In both cases, the CD4 molecule can be fitted into the nonenveloped virus canyon. This indicates similarities between the 3-D structure of the HIV Env CD4 binding site and the receptor binding site of these nonenveloped capsid proteins. Whether this similarity extends to details in molecular interactions may soon be evaluated when the 3-D structure of gp120 becomes available. Similar structures may be responsible for the attachment of poliovirus to its receptor (V. Racaniello, Columbia University, New York; J. Hogle). The identification of the major HIV-1 coreceptors and the recent solution of the crystal structure of the entire extracellular portion of CD4 (33Lu Z Berson J.F Chen Y Turner J.D Zhang T Sharron M Jenks M.H Wang Z Kim J Rucker J et al.Evolution of HIV-1 coreceptor usage through interactions with distinct CCR5 and CXCR4 domains.Proc. Natl. Acad. Sci. USA. 1997; 94: 6426-6431Crossref PubMed Scopus (168) Google Scholar) provided new opportunities to study the high affinity virus-receptor interactions in greater detail. The oligomeric HIV Env could "crosslink" the dimeric CD4 potentially leading to formation of large multimeric complexes (C. Broder, USUHS, Bethesda). While the oligomeric state of the HIV-1 coreceptors is currently unknown, it has been demonstrated that they associate with the gp120-CD4 complex and possibly with CD4 in the absence of gp120 (H. Golding, FDA, Bethesda; Q. Sattentau, Centre d'Immunologie de Marseille-Luminy, Marseille) (46Ugolini S Moulard M Mondor I Barois N Demandolx D Hoxie J Brelot A Alizon M Davoust J Sattentau Q.J HIV-1 gp120 induces an association between CD4 and the chemokine receptor CXCR4.J. Immunol. 1997; 159: 3000-3008Google Scholar, 44Trkola A Dragic T Arthos J Binley J.M Olson W.C Allaway G.P Cheng-Mayer C Robinson J Maddon P.J Moore J.P CD4-dependent, antibody-sensitive interactions between HIV-1 and its co-receptor CCR-5.Nature. 1996; 384: 184-187Crossref PubMed Scopus (957) Google Scholar, 50Wu L Gerard Wyatt R Choe H Parolin C Ruffing N Borsetti A Cardoso A.A Desjardin E Newman W Gerard C Sodroski J CD4-induced interaction of primary HIV-1 gp120 glycoproteins with the chemokine receptor CCR-5.Nature. 1996; 384: 179-183Crossref Scopus (1081) Google Scholar, 31Lapham C.K Ouyang J Chandrasekhar B Nguyen N.Y Dimitrov D.S Golding H Evidence for cell-surface association between fusin and the CD4-gp120 complex in human cell lines.Science. 1996; 274: 602-605Crossref Scopus (336) Google Scholar). Although the affinities of membrane-associated coreceptor-CD4 interactions are presently unknown, an interesting concept is the possibility that competition for use of CD4 leads to changes in tropism. C. Broder briefly described his experiments with macrophages where overexpression of CXCR4 seems to allow entry of T-cell line tropic (T-tropic) viruses. One possible interpretation of these results is that at low CD4 concentration CCR5 outcompetes CXCR4 for CD4 (Q. Sattentau) (5Broder C.C Dimitrov D.S HIV and the 7-transmembrane domain receptors.Pathobiology. 1996; 64: 171-179Crossref Scopus (62) Google Scholar). At high CXCR4 concentrations, CXCR4 could associate with CD4 allowing entry of T-tropic viruses. The delineation of the critical regions involved in the interactions within the HIV-1 Env-CD4-coreceptor complex are presently under intensive investigation (E. Berger, NIAID, NIH, Bethesda; R. Doms, University of Pennsylvania; T. Dragic, Aaron Diamond AIDS Research Center, New York; J. Sodroski, Harvard Medical School, Boston; P. Clapham, Institute of Cancer Research, London). The exchange of different portions between CCR3 and CCR1 indicates that the N-terminus does not affect the entry of the dual tropic virus 89.6 (E. Berger). By using a large number of mutants, chimeras, and homologs of CCR5, it has been convincingly demonstrated that the CCR5 N-terminus plays a critical role for entry of M-tropic HIV-1 and SIV (R. Doms) (19Doranz B.J Berson J.F Rucker J Doms R.W Chemokine receptors as fusion cofactors for human immunodeficiency virus type 1 (HIV-1).Immunol. Res. 1997; 16: 15-28Crossref Scopus (73) Google Scholar, 21Edinger A.L Amedee A Miller K Doranz B.J Endres M Sharron M Samson M Lu Z.H Clements J.E Murphey-Corb M et al.Differential utilization of CCR5 by macrophage and T cell tropic simian immunodeficiency virus strains.Proc. Natl. Acad. Sci. USA. 1997; 94: 4005-4010Crossref Scopus (203) Google Scholar). A second functional region which is important for entry includes the extracellular loops. Several individual residues in both functional regions, Asp-11, Lys-197, and Asp-276, contributed significantly to the coreceptor function. The T-tropic SIV required the second extracellular loop. This loop was also critical for entry of T-tropic and dual tropic HIV-1 and HIV-2 strains, but the first extracellular loop was also required for some strains (33Lu Z Berson J.F Chen Y Turner J.D Zhang T Sharron M Jenks M.H Wang Z Kim J Rucker J et al.Evolution of HIV-1 coreceptor usage through interactions with distinct CCR5 and CXCR4 domains.Proc. Natl. Acad. Sci. USA. 1997; 94: 6426-6431Crossref PubMed Scopus (168) Google Scholar). In agreement with these results T. Dragic found that the CCR5 amino-terminus is the major site of interaction with gp120, particularly the negatively charged aspartic acid residues at positions 2 and 11, and a glutamic acid residue at position 18. Since the CC-chemokine-mediated inhibition of HIV-1 entry is dependent mostly on the CCR5 second extracellular loop, it appears that the gp120 and CC-chemokine binding sites on CCR5 are only partially overlapping. J. Sodroski also noted the importance of the CCR5 N-terminus of CCR5 for interactions with M-tropic HIV strains and SIV. A motif rich in tyrosine residues was found, that is critical for the interactions with the immunodefficiency viruses, and was used for identification of new coreceptors (GPR1 and GPR15) (23Farzan M Choe H Martin K Marcon L Hofmann W Karlsson G Sun Y Barrett P Marchand N Sullivan N Gerard N Gerard C Sodroski J Two orphan seven-transmembrane segment receptors which are expressed in CD4-positive cells support simian immunodeficiency virus infection.J. Exp. Med. 1997; 186 (a): 405-411Crossref Scopus (253) Google Scholar). The coreceptor surface expression levels, particularly CCR5, affect significantly entry efficiency and, therefore, must be accurately quantitated in all cases when comparing effects of mutations (J. Sodroski). The HIV-1 Env regions interacting with the coreceptors may involve conformational epitopes related to the V3 loop of gp120 (J. Sodroski) (44Trkola A Dragic T Arthos J Binley J.M Olson W.C Allaway G.P Cheng-Mayer C Robinson J Maddon P.J Moore J.P CD4-dependent, antibody-sensitive interactions between HIV-1 and its co-receptor CCR-5.Nature. 1996; 384: 184-187Crossref PubMed Scopus (957) Google Scholar, 50Wu L Gerard Wyatt R Choe H Parolin C Ruffing N Borsetti A Cardoso A.A Desjardin E Newman W Gerard C Sodroski J CD4-induced interaction of primary HIV-1 gp120 glycoproteins with the chemokine receptor CCR-5.Nature. 1996; 384: 179-183Crossref Scopus (1081) Google Scholar). This result may explain numerous data relating the V3 loop to virus tropism (reviewed in (17Dimitrov D.S Broder C.C HIV and Membrane Receptors. Landes Biosciences, Austin, TX1997Google Scholar)) and is in aggreement with a conceptual model of the Env-CD4-coreceptor interactions suggested earlier (16Dimitrov D.S Fusin—a place for HIV-1 and T4 cells to meet. Identifying the coreceptor mediating HIV-1 entry raises new hopes in the treatment of AIDS.Nature Med. 1996; 2: 640-641Crossref Scopus (31) Google Scholar). Most of our knowledge on HIV-1 coreceptor interactions has been derived either by experiments with soluble components of the entry machinery or measurements of end results of those interactions, such as fusion, entry, or infection. By using a novel virion binding assay, Q. Sattentau and his colleagues directly measured the interactions between membrane-associated HIV-1 Env, CD4, and coreceptors. They found that HIV-1 may require both CD4 and coreceptor molecules for efficient binding to cells. Plasma membranes contain thousands of proteins, many of them being potentially capable of interacting with virus components and/or receptor molecules, especially at elevated surface concentrations. The human multidrug transporter (P-glycoprotein) is a large integral membrane protein that extrudes hydrophobic drugs and peptides from the plasma membrane. Interestingly, it also interferes with HIV-1 infection at the level of entry (M. Gottesman; C. Lee, NIH, Bethesda). A possible explanation for its inhibitory effect is the interaction with hydrophobic portions of the HIV Env, particularly the fusion peptide. However, it was found that another large integral membrane protein (CFTR), which serves as an ionic channel, also affects fusion. Therefore another possible inhibitory mechanism may involve interactions with receptor and coreceptor molecules (D. Dimitrov). Although the mechanisms of the inhibitory effects of large integral membrane proteins are currently unknown, their very existence again underlines the complexity of virus entry through the plasma membrane. Viruses are subjected to two conflicting requirements: stability, the requirement to survive in the environment, and destabilization, the need to enter the cell. Therefore, a trigger must exist to destabilize them at the right time and place. Currently, a leading hypothesis is that the virus entry proteins are in a metastable state. They require a trigger (receptor or low pH) to provide the needed activation energy to overcome the energy barrier and move to a more stable energetically favorable state. In line with this hypothesis, J. Hogle proposed that the poliovirus receptor serves as a catalyst that releases the mature virion from a metastable state. Thus, it promotes conformational changes in which VP4 and the amino terminus of the capsid protein VP1 are externalized, possibly in a way similar to the spring-loaded mechanism suggested for influenza (7Carr C.M Kim P.S A spring-loaded mechanism for the conformational changes of influenza hemagglutinin.Cell. 1993; 73: 823-832Abstract Full Text PDF PubMed Scopus (787) Google Scholar, 6Bullough P.A Hughson F.M Skehel J.J Wiley D.C Structure of influenza haemagglutinin at the pH of membrane fusion.Nature. 1994; 371: 37-43Crossref PubMed Scopus (1376) Google Scholar), leading to association with membranes. Notably, the same (or similar) conformational changes can be induced by elevation of temperature to 45°C–50°C or by hypotonic medium containing calcium. Therefore in this, and probably other cases too, the receptor functions to lower the activation barrier for the conformational changes at 37°C. It appears that the conformational changes in the poliovirus-receptor complex occur in several stages (V. Racaniello). Cold-adapted mutants were isolated that replicate efficiently, but formation of 135S particles, considered as possible entry intermediates, was not observed. The question of whether the 135S particles are entry intermediates needs further investigation. Human rhinoviruses and parvovirus are also destabilized by interactions with their receptors (M. Rossmann). That the primary HIV receptor CD4 induces conformational changes in the Env-CD4 complex has been known for many years (reviewed in17Dimitrov D.S Broder C.C HIV and Membrane Receptors. Landes Biosciences, Austin, TX1997Google Scholar). Another demonstration of the CD4 ability to "prime" the virus for the subsequent stages of entry by inducing conformational changes was the demonstration that soluble CD4 (sCD4) can promote HIV-1 fusion with CD4-negative cells (E. Berger). Although it has been previously reported that sCD4 can induce HIV-2 fusion with CD4-negative cells (13Clapham P.R McKnight A Weiss R.A Human immunodeficiency virus type 2 infection and fusion of CD4-negative human cell lines induction and enhancement by soluble CD4.J. Vir
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