Cyclophilin A complexed with a fragment of HIV-1 gag protein: Insights into HIV-1 infectious activity

Background: Cyclophilin A (CyPA), a receptor of the immunosuppressive drug cyclosporin A, catalyzes the cis-irans isomerization of peptidyl-prolyl bonds and is required for the infectious activity of human immunodeficiency virus type 1 (HIV-1), The crystal structure of CyPA complexed with a fragment of the HIV-1 gag protein should provide insights into the nature of CyPA-gag interactions and may suggest a role for CyPA in HIV-I infectious activity. Results: The crystal structure of CyPA complexed with a 25 amino acid peptide of HIV-1 gag capsid protein (25-mer) was determined and refined to an R factor of 0.195 at 1.8 Angstrom resolution, The sequence Ala88-Gly89-Pro90-Ile91 of the gag fragment is the major portion to bind to the active site of CyPA. Two residues of the 25-mer (Pro90-Ile91) bind to CyPA in a similar manner to two residues (Pro-Phe) of the CyPA substrate, succinyl-Ala-Ala-Pro-Phe-p-nitroanilide (AAPF). However, the N-terminus of the 25-mer (Ala88-Gly89) exhibits a different hydrogen-bonding pattern and molecular conformation than AAPF. The peptidyl-prolyl bond between Gly89 and Pro90 of the 25-mer has a irans conformation, in contrast to the cis conformation observed in other known CyPA-peptide complexes, The residue preceding proline, Gly89, has an unfavorable backbone conformation usually only adopted by glycine. Conclusions: The unfavorable backbone conformation of Gly89 of the gag 25-mer fragment suggests that binding between HIV-1 gag protein and CyPA requires a special sequence, Gly-Pro. Thus, in HIV-1 infectivity, CyPA is likely to function as a chaperone, rather than as a cis-trans isomerase. However, the observation of similarities between the C termini of the 25-mer and the substrate AAPF means that the involvement of the cis-trans isomerase activity of CyPA cannot be completely ruled out.