Following entry of the HIV-1 core into target cells, productive
infection depends on the proper disassembly of the viral capsid (uncoating). Although much is known regarding HIV-1 entry, the actions of host cell
proteins that HIV-1 utilizes during early postentry steps are poorly understood. One such factor,
transportin SR2 (TRN-SR2)/
transportin 3 (TNPO3), promotes
infection by HIV-1 and some other lentiviruses, and recent studies have genetically linked TNPO3 dependence of
infection to the viral
capsid protein (CA). Here we report that purified recombinant TNPO3 stimulates the uncoating of HIV-1 cores in vitro. The stimulatory effect was reduced by RanGTP, a known
ligand for
transportin family members. Depletion of TNPO3 in target cells rendered HIV-1 less susceptible to inhibition by PF74, a small-molecule HIV-1 inhibitor that induces premature uncoating. In contrast to the case for TNPO3, addition of the CA-binding host
protein cyclophilin A (CypA) inhibited HIV-1 uncoating and reduced the stimulatory effect of TNPO3 on uncoating in vitro. In cells in which TNPO3 was depleted, HIV-1
infection was enhanced 4-fold by addition of
cyclosporine, indicating that the requirement for TNPO3 in HIV-1
infection is modulated by CypA-CA interactions. Although TNPO3 was localized primarily to the cytoplasm, depletion of TNPO3 from target cells inhibited HIV-1
infection without reducing the accumulation of nuclear proviral
DNA, suggesting that TNPO3 facilitates a stage of the virus life cycle subsequent to nuclear entry. Our results suggest that TNPO3 and
cyclophilin A facilitate HIV-1
infection by coordinating proper uncoating of the core in target cells.