Cyclophilin A facilitates HIV-1 integration

Area of Science:

• Virology
• Molecular Biology
• Immunology

Background:

• Cyclophilin A (CypA) is a host protein known to bind the HIV-1 capsid, aiding in reverse transcription and nuclear entry.
• CypA also antagonizes TRIM5α, an antiviral factor restricting HIV-1 infection.
• Recent findings suggest the HIV-1 capsid enters the nucleus intact before integration, highlighting potential nuclear roles for capsid-binding proteins.

Purpose of the Study:

• To investigate whether the CypA-capsid interaction influences HIV-1 integration, a post-nuclear entry step.
• To elucidate the mechanism by which CypA might regulate HIV-1 integration.
• To determine if CypA's role in integration is independent of its known functions in reverse transcription, nuclear entry, and TRIM5α interaction.

Main Methods:

• Compared HIV-1 integration levels in CypA-expressing (CypA+/+) versus CypA-depleted (CypA-/-) cells.
• Utilized cyclosporin A (CsA) to inhibit CypA-capsid binding.
• Tested HIV-1 capsid mutants (G89V, P90A) with impaired CypA binding.
• Assessed the in vitro integration activity of HIV-1 preintegration complexes (PICs) from infected cells.
• Examined the effect of CypA and TRIM5α depletion on PIC integration activity.

Main Results:

• CypA depletion significantly reduced HIV-1 proviral DNA integration, independent of reverse transcription, nuclear entry, or TRIM5α presence.
• Inhibition of CypA-capsid binding by CsA blocked integration in CypA+/+ cells but not CypA-/- cells.
• HIV-1 capsid mutants deficient in CypA binding were impaired in integration in CypA+/+ cells.
• PICs from CypA-/- cells exhibited lower in vitro integration activity compared to those from CypA+/+ cells.
• CypA specifically stimulated PIC integration activity, an effect blocked by CsA.

Conclusions:

• Cyclophilin A directly and positively regulates HIV-1 integration, a novel nuclear function.
• The CypA-capsid interaction is critical for efficient viral DNA integration.
• CypA's role in promoting HIV-1 integration is distinct from its effects on reverse transcription and TRIM5α antagonism.