NMR studies on the conformation of the CD4 36-59 peptide bound to HIV-1 gp120

A peptide containing residues 36-59 of the human CD4 receptor includes most of the residues thought to be involved in binding the HIV surface glycoprotein, gp120. This peptide was synthesized and inhibited the binding of gp120 to soluble CD4. NMR relaxation experiments indicated that the peptide was in fast exchange between the free and gp120-bound states. Transferred NOESY NMR showed a number of long-range NOEs, from the gp120-bound state, between residues 38, 40, 45, 48, and 49 of the peptide. NMR evidence also suggested that the Phe43 in the peptide, which corresponds to a critical residue in CD4 for the binding of gp120, makes intimate contact with gp120. The Tr-NOESY cross-peak intensities provided proton-proton distance constraints on the conformation of the gp120-bound peptide. The distance constraints were used in simulated annealing, and a set of 20 very similar structures was obtained for the central region of the gp120-bound peptide. Residues 42-49 of the peptide formed a loop with the side chain of Phe43 pointing away from the rest of the peptide. This Phe43 ring points away from the protein surface in two structures of the amino-terminal domain of CD4 found by X-ray crystallography. Differences in the conformation of CD4 in the two crystal forms suggest that the 36-59 region might be flexible. The NMR data on the 36-59 CD4 peptide predicts a gp120-bound conformation different from either of the CD4 crystal forms in the absence of gp120.