Positive selection system to screen for inhibitors of human immunodeficiency virus-1 transcription.

We describe a transcription-based assay system to screen for antiviral drugs in vivo. The system consists of two transcription units, a cytomegalovirus promoter driving a reporter gene physically linked to an HIV-1 promoter oriented in the opposite direction. Based on the arrangement of the transcription units, enhanced HIV-1 promoter activity in the presence of the viral transactivating Tat protein downregulates reporter gene expression initiated from the CMV promoter. Inhibitors of HIV-1 transcription relieve the suppression and are identified by an increase in reporter gene expression. This positive selection system allows discrimination between drugs that nonspecifically block cellular functions by cytotoxicity and molecules that specifically repress HIV-1 promoter activity.

Receptor recognition and specificity of interleukin-8 is determined by residues that cluster near a surface-accessible hydrophobic pocket.

To determine the regions of interleukin-8 (IL-8) that allow high affinity and interleukin-8 receptor type 1 (IL8R1)-specific binding of chemokines, we produced chimeric proteins containing structural domains from IL-8, which binds to both IL8R1 and interleukin-8 receptor type 2 (IL8R2) with high affinity, and from GRO gamma, which does not bind to IL8R1 and binds to IL8R2 with reduced affinity. Receptor binding activity was tested by competition of 125I-IL-8 binding to recombinant IL8R1 and IL8R2 cell lines. Substitution into IL-8 of the GRO gamma sequences corresponding to either the amino-terminal loop (amino acids 1-18) or the first beta-sheet (amino acids 18-32) reduced binding to both IL8R1 and IL8R2. The third beta-sheet of IL-8 (amino acids 46-53) was required for binding to IL8R1 but not IL8R2. Exchanges of the second beta-sheet (amino acids 32-46) or the carboxyl-terminal alpha-helix (amino acids 53-72) had no significant effect. When IL-8 sequences were substituted into GRO gamma, a single domain containing the second beta-sheet of IL-8 (amino acids 18-32) was sufficient to confer high affinity binding for both IL8R1 and IL8R2. The amino-terminal loop (amino acids 1-18) and the third beta-sheet (amino acids 46-53) of IL-8 had little effect when substituted individually but showed increased binding to both receptors when substituted in combination. Individual amino acid substitutions were made at positions where IL-8 and GRO gamma sequences differ within the regions of residues 11-21 and 46-53. IL-8 mutations L49A or L49F selectively inhibited binding to IL8R1. Mutations Y13L and F21N enhanced binding to IL8R1 with little effect on IL8R2. A combined mutation Y13L/S14Q selectively decreased binding to IL8R2. Residues Tyr13, Ser14, Phe21, and Lys49 are clustered in and around a surface-accessible hydrophobic pocket on IL-8 that is physically distant from the previously identified ELR binding sequence. A homology model of GRO gamma, constructed from the known structure of IL-8 by refinement calculations, indicated that access to the hydrophobic pocket was effectively abolished in GRO gamma. These studies suggest that the surface hydrophobic pocket and/or adjacent residues participate in IL-8 receptor recognition for both IL8R1 and IL8R2 and that the hydrophobic pocket itself may be essential for IL8R1 binding. Thus this region contains a second site for IL-8 receptor recognition that, in combination with the Glu4-Leu5-Arg6 region, can modulate receptor binding affinity and IL8R1 specificity.