Transportin 3 Promotes a Nuclear Maturation Step Required for Efficient HIV-1 Integration

Abstract

The HIV/AIDS pandemic is a major global health threat and understanding the detailed molecular mechanisms of HIV replication is critical for the development of novel therapeutics. To replicate, HIV-1 must access the nucleus of infected cells and integrate into host chromosomes, however little is known about the events occurring post-nuclear entry but before integration. Here we show that the karyopherin Transportin 3 (Tnp3) promotes HIV-1 integration in different cell types. Furthermore Tnp3 binds the viral capsid proteins and tRNAs incorporated into viral particles. Interaction between Tnp3, capsid and tRNAs is stronger in the presence of RanGTP, consistent with the possibility that Tnp3 is an export factor for these substrates. In agreement with this interpretation, we found that Tnp3 exports from the nuclei viral tRNAs in a RanGTP-dependent way. Tnp3 also binds and exports from the nuclei some species of cellular tRNAs with a defective 3′CCA end. Depletion of Tnp3 results in a re-distribution of HIV-1 capsid proteins between nucleus and cytoplasm however HIV-1 bearing the N74D mutation in capsid, which is insensitive to Tnp3 depletion, does not show nucleocytoplasmic redistribution of capsid proteins. We propose that Tnp3 promotes HIV-1 infection by displacing any capsid and tRNA that remain bound to the pre-integration complex after nuclear entry to facilitate integration. The results also provide evidence for a novel tRNA nucleocytoplasmic trafficking pathway in human cells.

Author Summary

HIV-1, the causative agent of AIDS, is a virus that enters the nucleus of infected cells and must integrate its genome into the host cell DNA. Here we show that efficient HIV-1 integration depends on a host cell factor called Transportin 3. We also show that Transportin 3 can export out of the nucleus the viral capsid proteins and viral transfer RNAs (tRNAs) and that this activity is important for HIV-1 integration. We propose that Transportin 3 facilitates a maturation process inside the nucleus by removing remaining capsid proteins and tRNAs still bound to the virus. The mature viral complex, free of any bulky component, can then more easily integrate. Transportin 3 also export certain cellular tRNAs out of the nucleus, presumably as a way to control cell metabolism. By feeding into this tRNA shuttling pathway, HIV-1 can complete its life cycle. Our work sheds new light into the biology of HIV-1 and points to the existence of a new pathway in human cells to shuttle certain tRNAs between nucleus and cytoplasm.