Structural insights into the stabilization of the human immunodeficiency virus type 1 capsid protein by the cyclophilin-binding domain and implications on the virus cycle

• Cyclophilin-binding domain confers stability to the HIV-1 capsid protein (CA).
• SAXS data reveal that WT and Δ87–97 capsid proteins have elongated conformations.
• Trp residues are more accessible to solvent in the mutant protein Δ87–97.
• The HIV CA mutant Δ87–97 has lower pressure and chemical stabilities than WT CA.

During infection, human immunodeficiency virus type 1 (HIV-1) interacts with the cellular host factor cyclophilin A (CypA) through residues 85–93 of the N-terminal domain of HIV-1's capsid protein (CA). The role of the CA:CypA interaction is still unclear. Previous studies showed that a CypA-binding loop mutant, Δ87–97, has increased ability to assemble in vitro. We used this mutant to infer whether the CypA-binding region has an overall effect on CA stability, as measured by pressure and chemical perturbation. We built a SAXS-based envelope model for the dimer of both WT and Δ87–97. A new conformational arrangement of the dimers is described, showing the structural plasticity that CA can adopt. In protein folding studies, the deletion of the loop drastically reduces CA stability, as assayed by high hydrostatic pressure and urea. We hypothesize that the deletion promotes a rearrangement of helix 4, which may enhance the heterotypic interaction between the N- and C-terminal domains of CA dimers. In addition, we propose that the cyclophilin-binding loop may modulate capsid assembly during infection, either in the cytoplasm or near the nucleus by binding to the nuclear protein Nup385.