Oligopeptides derived from the gag polyprotein (Pr55gag) of human immunodeficiency virus type 1 (HIV-1) segment were used to evaluate the extension of the putative binding region for the complex of Pr55gag and the human cytosolic peptidyl prolyl cisltrans isomerase (PPIase) 18 kDa cyclophilin (Cyp18). Five N-terminally acetylated, C-terminally amidated oligopeptides containing one (HIV-1 Gag 218–224; 1), two (HIV-1 Gag 218–226 and HIV-1 Gag 217−224; 2 and 3, respectively), three (HIV-1 Gag 217–226; 4) or four (HIV-1 Gag213−237; 5) proline residues were synthesized. Using competition experiments with a standard substrate the binding affinities to Cyp18 of the synthesized peptides were determined. The IC50 value of 184 μ.M for the 25-mer peptide 5 was fivefold or more lower than those of the peptides 1–4 lacking one or more prolines. Failure of competition in assays containing enzymes of other PPIase families by millimolar concentrations of 5 revealed a Cyp18 specific interaction involving the active site of the enzyme. In its far UV circular dichroism, aqueous solutions of 5 display properties of random coil conformation, but spectra were also consistent with a small contribution of proline specific secondary structures. However, a proline-rich peptide typical of forming left-handed polyproline II helices did not compete for the active site of Cyp18. The results demonstrate that the putative binding region of HIV-1 gag polyprotein has a certain degree of binding affinity to the PPIase site of Cyp18, and may add a previously unrecognized topological component to the known subsite specificity of cyclophilins.
