HIV infection is initiated by the selective interaction between the cellular receptor CD4 and gp120, the external envelope glycoprotein of the virus. We used analytical ultracentrifugation, titration calorimetry, and surface plasmon resonance biosensor analysis to characterize the assembly state, thermodynamics, and kinetics of the CD4-gp120 interaction. The binding thermodynamics were of unexpected magnitude; changes in enthalpy, entropy, and heat capacity greatly exceeded those described for typical proteinprotein interactions. These unusual thermodynamic properties were observed with both intact gp120 and a deglycosylated and truncated form of gp120 protein that lacked hypervariable loops V1, V2, and V3 and segments of its N and C termini. Together with previous crystallographic studies, the large changes in heat capacity and entropy reveal that extensive structural rearrangements occur within the core of gp120 upon CD4 binding. CD spectral studies and slow kinetics of binding support this conclusion. These results indicate considerable conformational flexibility within gp120, which may relate to viral mechanisms for triggering infection and disguising conserved receptor-binding sites from the immune system. E ntry of enveloped viruses into cells requires transformation of the protective envelope into a fusion-competent state. In the case of the human immunodeficiency virus (HIV-1), infection is initiated by the selective interaction between the viral exterior envelope glycoprotein, gp120, and receptors on the target cell, CD4, and obligatory chemokine receptors (CCR5 or CXCR4). Accumulating biochemical and structural evidence indicates that, in addition to contributing to viral attachment, CD4 triggers conformational alterations in the HIV envelope that promote recognition of the chemokine receptors and ultimately lead to membrane fusion (1, 2). Evidence of CD4-induced conformational changes includes enhanced protease sensitivity in gp120 variable loops (3) and release of gp120 from virus and virus-infected cells (4, 5), as well as exposure or formation of the chemokine receptor site (1, 2) and of the epitopes for neutralizing antibodies that can block chemokine receptor binding (1, 2, 6, 7). By their nature, these biochemical studies primarily probe the peripheral regions of gp120 and indicate that CD4-induced movement of variable loops occurs.Recently, a structure was solved for a truncated core gp120 protein in a ternary complex with soluble CD4 and a Fab fragment of the neutralizing antibody 17b (8). In this complex, gp120 is organized into an inner and outer domain connected by a bridging sheet. Several elements of this structure appear to depend on association with CD4 for their stabilization. In particular, the extended conformation of the bridging sheet, which constitutes key elements of the chemokine receptorbinding site, is stabilized by direct contacts with CD4. In contrast, the structure of CD4 (D1D2) (9, 10) is virtually unchanged in the presence of gp120.Although the crystallographic data provi...

P0, the major protein of peripheral nerve myelin, mediates membrane adhesion in the spiral wraps of the myelin sheath. We have determined the crystal structure of the extracellular domain from P0 (P0ex) at 1.9 A resolution. P0ex is folded like a typical immunoglobulin variable-like domain; five residues at the C-terminus are disordered, suggesting a flexible linkage to the membrane. The requirements for crystallization of P0ex are similar to those for maintaining the native extracellular spacing of adjacent myelin lamellae; thus, given the self-adhesive character of P0ex, the crystal itself may reveal some of the natural interactions that occur between P0 molecules in myelin. The structure leads to the suggestion that P0 extracellular domains may emanate from the membrane surface as tetramers that link to tetramers on the opposing membrane surface, to result in the formation of networks of molecules. We report analytical ultracentrifugation data for P0ex that support this idea.

Cholesteryl ester transfer protein (CETP) shuttles various lipids between lipoproteins, resulting in the net transfer of cholesteryl esters from atheroprotective, high-density lipoproteins (HDL) to atherogenic, lower-density species. Inhibition of CETP raises HDL cholesterol and may potentially be used to treat cardiovascular disease. Here we describe the structure of CETP at 2.2-A resolution, revealing a 60-A-long tunnel filled with two hydrophobic cholesteryl esters and plugged by an amphiphilic phosphatidylcholine at each end. The two tunnel openings are large enough to allow lipid access, which is aided by a flexible helix and possibly also by a mobile flap. The curvature of the concave surface of CETP matches the radius of curvature of HDL particles, and potential conformational changes may occur to accommodate larger lipoprotein particles. Point mutations blocking the middle of the tunnel abolish lipid-transfer activities, suggesting that neutral lipids pass through this continuous tunnel.

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Herpesvirus entry mediator (HVEM),