Determination of equilibrium constants of associating protein systems. Graphical analysis for discrete and indefinite association

NMR observation of HIV-1 gp120 conformational flexibility resulting from V3 truncation

The envelope spike of HIV-1, which consists of three external gp120 and three transmembrane gp41 glycoproteins, recognizes its target cells by successively binding to its primary CD4 receptor and a coreceptor molecule. Until recently, atomic-resolution structures were available primarily for monomeric HIV-1 gp120, in which the V1, V2 and V3 variable loops were omitted (gp120core ), in complex with soluble CD4 (sCD4). Differences between the structure of HIV gp120core in complex with sCD4 and the structure of unliganded simian immunodeficiency virus gp120core led to the hypothesis that gp120 undergoes a major conformational change upon sCD4 binding. To investigate the conformational flexibility of gp120, we generated two forms of mutated gp120 amenable for NMR studies: one with V1, V2 and V3 omitted ((mut) gp120core ) and the other containing the V3 region [(mut) gp120core (+V3)]. The TROSY-(1)H-(15)N-HSQC spectra of [(2)H, (13)C, (15)N]Arg-labeled and [(2)H, (13)C, (15)N]Ile-labeled unliganded (mut) gp120core showed many fewer crosspeaks than the expected number, and also many fewer crosspeaks in comparison with the labeled (mut) gp120core bound to the CD4-mimic peptide, CD4M33. This finding suggests that in the unliganded form, (mut) gp120core shows considerable flexibility and motions on the millisecond time scale. In contrast, most of the expected crosspeaks were observed for the unliganded (mut) gp120core (+V3), and only a few changes in chemical shift were observed upon CD4M33 binding. These results indicate that (mut) gp120core (+V3) does not show any significant conformational flexibility in its unliganded form and does not undergo any significant conformational change upon CD4M33 binding, underlining the importance of V3 in stabilizing the gp120core conformation.

The conformation and orientation of a 27-residue CCR5 peptide in a ternary complex with HIV-1 gp120 and a CD4-mimic peptide

Interaction of CC chemokine receptor 5 (CCR5) with the human immunodeficiency virus type 1 (HIV-1) gp120/CD4 complex involves its amino-terminal domain (Nt-CCR5) and requires sulfation of two to four tyrosine residues in Nt-CCR5. The conformation of a 27-residue Nt-CCR5 peptide, sulfated at Y10 and Y14, was studied both in its free form and in a ternary complex with deglycosylated gp120 and a CD4-mimic peptide. NMR experiments revealed a helical conformation at the center of Nt-CCR5(1-27), which is induced upon gp120 binding, as well as a helical propensity for the free peptide. A well-defined structure for the bound peptide was determined for residues 7-23, increasing by 2-fold the length of Nt-CCR5's known structure. Two-dimensional saturation transfer experiments and measurement of relaxation times highlighted Nt-CCR5 residues Y3, V5, P8-T16, E18, I23 and possibly D2 as the main binding determinant. A calculated docking model for Nt-CCR5(1-27) suggests that residues 2-22 of Nt-CCR5 interact with the bases of V3 and C4, while the C-terminal segment of Nt-CCR5(1-27) points toward the target cell membrane, reflecting an Nt-CCR5 orientation that differs by 180° from that of a previous model. A gp120 site that could accommodate (CCR5)Y3 in a sulfated form has been identified. The present model attributes a structural basis for binding interactions to all gp120 residues previously implicated in Nt-CCR5 binding. Moreover, the strong interaction of sulfated (CCR5)Tyr14 with (gp120)Arg440 revealed by the model and the previously found correlation between E322 and R440 mutations shed light on the role of these residues in HIV-1 phenotype conversion, furthering our understanding of CCR5 recognition by HIV-1.

The membrane proximal external region of the HIV-1 envelope glycoprotein gp41 contributes to the stabilization of the six-helix bundle formed with a matching N' peptide

The HIV-1 envelope glycoprotein gp41 undergoes a sequence of extensive conformational changes while participating in the fusion of the virus with the host cell. Since the discovery of its postfusion conformation, the structure and function of the protease-resistant six-helix bundle (6-HB) have been the subject of extensive investigation. In this work, we describe additional determinants (S528-Q540 and W666-N677) in the fusion peptide proximal region (FP-PR) and the membrane proximal external region (MPER) that stabilize the six-helix bundle and are involved in the interaction of T-20 (FUZEON, an anti-HIV-1 fusion inhibitor drug) with the gp41 FP-PR. Circular dichroism and sedimentation equilibrium measurements indicate that the 1:1 mixture of N' and C' peptides comprising residues A541-T569 and I635-K665 from the gp41 first and second helical repeats, HR1 and HR2, respectively, fail to form a stable six-helix bundle. Triglutamic acid and triarginine tags were added to these N' and C' peptides, respectively, at the termini distant from the FP-PR and the MPER to alter their pI and increase their solubility at pH 3.5. The tagged HR1 and HR2 peptides were elongated by addition of residues S528-Q540 from the FP-PR and residues W666-N677 from the MPER, respectively. A 1:1 complex of the elongated peptides formed a stable six-helix bundle which melted at 60 degrees C. These results underscore the importance of a detailed high-resolution characterization of MPER interactions, the results of which may improve our understanding of the structure-function relationship of gp41 and its role in HIV-1 fusion.

Thermodynamic compensation process in interacting protein systems: definition of thermodynamic compensatory temperature, (Tc)

In our thermodynamic analysis of the non-linear Van't Hoff expression as applied to several self-associating systems -- specifically in the cases of bovine liver L-glutamate dehydrogenase (GDH), glucagon and S-carboxymethylated apo A-II protein from human high density lipoprotein -- we have examined the interrelationships of a number of thermodynamic temperatures as they affect the association process. We found the principal determinants of the linear thermodynamic compensation process to be delta S0(T)/delta C0p(T) = (delta T'C)/(Texp), where (delta T'C) = Texp). We have defined the unique compensatory temperature, (TC), for any interacting system, at which the contributions of enthalpy and entropy to the association process are balanced.

Determination of the equilibrium constants of self-associating protein systems. XI. The application of C-(r) in graphical analysis and the enumeration of interacting species in the ultracentrifuge

A greatly simplified procedure is proposed which employs C equals f(r) as determined from sedimentation equilibrium measurements in graphical analysis of self-associating protein systems and in the enumeration of interacting species in the ultracentrifuge. Basic equations given here are applicable to any self-associating system. A procedure is outlined for enumeration of interacting components independent of non-ideal behavior, using principal component analysis.