ABSTRACT
To infect nondividing cells, HIV-1 needs to cross the nuclear membrane. The importin transportin-SR2 (TRN-SR2 or transportin-3) has been proposed to mediate HIV-1 nuclear import, but the detailed mechanism remains unresolved. The direct interaction of TRN-SR2 with HIV-1 integrase (IN) has been proposed to drive HIV-1 nuclear import. Alternatively, TRN-SR2 may play an indirect role by mediating nuclear import of cleavage and polyadenylation specificity factor 6 (CPSF6). To unravel the role of TRN-SR2, we designed CRISPR/Cas9 guide RNAs targeting different exons of TNPO3. Although this approach failed to generate full knockouts, monoallelic knockout clones were generated with indel mutations. HIV-1 replication was hampered in those clones at the level of HIV-1 nuclear import without an effect on the cellular distribution of the TRN-SR2 cargoes CPSF6 or alternative splicing factor1/pre-mRNA splicing factor SF2 (ASF/SF2). Recombinant ΔV105 TRN-SR2 expressed in clone 15.15 was 2-fold impaired for interaction with HIV-1 IN and classified as an interaction mutant. Our data support a model whereby TRN-SR2 acts as a cofactor of HIV-1 nuclear import without compromising the nuclear import of cellular cargoes. CRISPR/Cas9-induced mutagenesis can be used as a method to generate interface mutants to characterize host factors of human pathogens. IMPORTANCE Combination antiretroviral therapy (cART) effectively controls HIV-1 by reducing viral loads, but it does not cure the infection. Lifelong treatment with cART is a prerequisite for sustained viral suppression. The rapid emergence of drug-resistant viral strains drives the necessity to discover new therapeutic targets. The nuclear import of HIV-1 is crucial in the HIV-1 replication cycle, but the detailed mechanism remains incompletely understood. This study provides evidence that TRN-SR2 directly mediates HIV-1 nuclear import via the interaction with HIV-1 integrase. The interaction between those proteins is therefore a promising target toward a rational drug design which could lead to new therapeutic strategies due to the bottleneck nature of HIV-1 nuclear import.
Subject(s)
Cell Nucleus/virology , HIV-1/metabolism , beta Karyopherins/metabolism , Active Transport, Cell Nucleus , CRISPR-Cas Systems , Cell Nucleus/metabolism , HIV Infections/genetics , HIV Infections/virology , HIV Integrase/genetics , HIV Integrase/metabolism , HIV-1/genetics , Humans , Protein Binding , Serine-Arginine Splicing Factors/genetics , Serine-Arginine Splicing Factors/metabolism , beta Karyopherins/genetics , mRNA Cleavage and Polyadenylation Factors/genetics , mRNA Cleavage and Polyadenylation Factors/metabolismABSTRACT
The HIV-1 capsid protein performs multiple roles in virus replication both during assembly and particle release and during virus trafficking into the nucleus. In order to decipher the roles of capsid protein during early replication, a reliable method to follow its intracellular distribution is required. To complement existing approaches to track HIV-1 capsid during early infection, we developed an HIV-1 imaging strategy, relying on viruses incorporating enhanced green fluorescent protein (eGFP)-tagged capsid (CA-eGFP) protein and mCherry-tagged integrase (IN-mCherry). Wild-type infectivity and sensitivity to inhibition by PF74 point to the functionality of CA-eGFP-containing complexes. Low numbers of CA-eGFP molecules were located inside the viral core and imported into the nucleus without significant loss in intensity. Less than 5% of particles carrying both CA-eGFP and IN-mCherry retained both labelled proteins after nuclear entry, implying a major uncoating event at the nuclear envelope dissociating IN and CA. Still, 20% of all CA-eGFP-containing complexes were detected in the nucleus. Unlike for IN-mCherry complexes, addition of the integrase inhibitor raltegravir had no effect on CA-eGFP-containing complexes, suggesting that these may be not (yet) competent for integration. Our imaging strategy offers alternative visualization of viral capsid trafficking and helps clarify its potential role during integration.IMPORTANCE HIV-1 capsid protein (CA) builds a conical shell protecting viral genomic RNA inside the virus particles. Upon entry into host cells, this shell disassembles in a process of uncoating, which is coordinated with reverse transcription of viral RNA into DNA. After uncoating, a portion of CA remains associated with the viral DNA and mediates its nuclear import and, potentially, integration into host DNA. In this study, we tagged CA with eGFP to follow its trafficking in host cells and address potential CA roles in the nucleus. We found that while functional viruses import the tagged CA into the nucleus, this capsid protein is not part of integration-competent complexes. The roles of nuclear CA thus remain to be established.
