Impact of the Vpr-L23F and -K27M substitutions on the three-dimensional structure of Vpr. A) 3D structure of HIV-1 Vpr , showing the three α-helices (residues 17–33, 38–50 and 54–77) represented in light blue, yellow and purple, respectively. The L23, K27, A30 and F34 residues are colored in red. The unstructured N- and C-terminal domains are represented in dark blue. B) CPK representation of Vpr. Residues are colored according to their hydrophobicity, except for L23 and K27 which are colored in yellow. The yellow box is enlarged in C), and this region shows a pocket that is organized around the L23 and K27 residues within the first α-helix and may represent a site for hCG1 binding. D) Helical-wheel diagram of the first α-helix of Vpr extending from a.a. D17 to F34. Residues L23, K27, A30 and F34 which have been mutated in the present study are indicated. Hydrophilic residues are in blue, whereas hydrophobic residues are in red.