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Robert Blumenthal, Ph.D.
|The Membrane Fusion Reaction|
|(From Chem Rev 103: 53-69 (2003)). Two lipid vesicles (A and B) form close contacts (AB) and then proceed to become one vesicle (C). The mixing of lipid and aqueous markers originally in the separate vesicles (green and red) report on the progress of the fusion reaction. The reaction may proceed spontaneously or via catalysis by a specific viral or cellular protein. In the latter case a conformational transition of the catalyst (E goes to E') is coupled to the transitions of the lipids. Many of the assays to study viral fusion have been developed in our laboratory (see Chem Phys Lipids 116: 39-55 (2002).|
|Viral Entry Into Cells|
|Influenza virus employs a complex cellular entry route that involves binding to cell surface receptors, endocytosis through clathrin-coated pits and vesicles, followed by delivery to a low pH endosomal compartment. The triggering signal that leads to influenza virus fusion is relatively simple, requiring only low pH. HIV/SIV, on the other hand, deliver their genetic material into the cell by direct fusion of the viral membrane with the plasma membrane of the host cells. The triggering mechanisms that activate HIV/SIV Env are quite complex involving target cell CD4, co-receptors and perhaps other cell surface components. These components are localized on distinct regions in the membrane, presumably microvilli (Viard et al, J Virol. 76, 11584-11595 (2002)). These interactions trigger a barrage of conformational changes in HIV/SIV Env that drive the membrane fusion process.|
|CD4 is a co-receptor in the cellular immune response (see http://www.ncbi.nlm.nih.gov/prow/guide/1246540099_g.htm). It increases the avidity of association between a T cell and an antigen-presenting cell by interacting with the class II major histocompatibility complex (MHC) and T-cell receptor (TCR) molecules, and it contributes directly to signal transduction through its cytoplasmic association with the lymphocyte kinase Lck. CD4 had been early on identified as the primary receptor for HIV (1-3). The extracellular portion of CD4 comprises four immunoglobulin-like domains (D1-D4). This part of human CD4 (residues 1-369) has been characterized as a recombinant soluble protein (sCD4). High-resolution crystal structures of intact sCD4 have been determined in three crystal lattices (4). These structures have a hinge-like variability at the D1D2 to D3D4 junction that might be important in immune recognition and HIV fusion, and a common dimeric association through D4 domains.
The chemokine receptors CCR5 and CXCR4, are the principal co-receptors used by macrophage M- tropic (R5) and T- cell line T-tropic (X4) HIV-1, respectively. CXCR4 (CD184) is a G protein-coupled receptor with selectivity for a single CXC-motif chemokine, called stromal cell-derived factor-1 (SDF1) (see http://www.ncbi.nlm.nih.gov/prow/guide/192999234_g.htm). CXCR4 had originally been identified as the co-receptor for certain (X4-tropic) HIV variants that are preferentially found in infected individuals with AIDS (5). HIV variants that are efficiently transmitted and predominate during the asymptomatic stages require the chemokine receptor CCR5 and are called R5-tropic (6). The structures are based on molecular modeling of seven trans-membrane helix G protein coupled receptor proteins by Stewart Durell (http://www-lmmb.ncifcrf.gov/~durell/).
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|Blumenthal Group Photo|
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