Abstract
Retroviral nucleocapsid proteins harbor nucleic acid chaperoning activities that mostly rely on the N-terminal basic residues and the CCHC zinc finger motif. Such chaperoning is essential for virus replication, notably for genomic RNA selection and packaging in virions, and for reverse transcription of genomic RNA into DNA. Recent data revealed that HIV-1 nucleocapsid restricts reverse transcription during virus assembly – a process called late reverse transcription – suggesting a regulation between RNA packaging and late reverse transcription. Indeed, mutating the HIV-1 nucleocapsid basic residues or the two zinc fingers caused a reduction in RNA incorporated and an increase in newly made viral DNA in the mutant virions. MoMuLV nucleocapsid has an N-terminal basic region similar to HIV-1 nucleocapsid but a unique zinc finger. This prompted us to investigate whether the N-terminal basic residues and the zinc finger of MoMuLV and HIV-1 nucleocapsids play a similar role in genomic RNA packaging and late reverse transcription. To this end, we analyzed the genomic RNA and viral DNA contents of virions produced by cells transfected with MoMuLV molecular clones where the zinc finger was mutated or completely deleted or with a deletion of the N-terminal basic residues of nucleocapsid. All mutant virions showed a strong defect in genomic RNA content indicating that the basic residues and zinc finger are important for genomic RNA packaging. In contrast to HIV-1 nucleocapsid-mutants, the level of viral DNA in mutant MoMuLV virions was only slightly increased. These results confirm that the N-terminal basic residues and zinc finger of MoMuLV nucleocapsid are critical for genomic RNA packaging but, in contrast to HIV-1 nucleocapsid, they most probably do not play a role in the control of late reverse transcription. In addition, these results suggest that virus formation and late reverse transcription proceed according to distinct mechanisms for MuLV and HIV-1.
Highlights
The retroviral nucleocapsid (NC) corresponds to the C-terminal domain of the Gag polyprotein precursor and found as mature protein upon Gag processing by the viral protease (PR) during virus formation and budding
Our results show that the basic residues and the unique zinc finger (ZF) play a major role in genomic RNA (gRNA) packaging, and the basic residues are important determinant for virus release, underlying the similarity between Murine Leukemia Virus (MuLV) and HIV NC’s
NC is involved in the RTion reation with at least two key partners, the RT enzyme and the genomic RNA template
Summary
The retroviral nucleocapsid (NC) corresponds to the C-terminal domain of the Gag polyprotein precursor and found as mature protein upon Gag processing by the viral protease (PR) during virus formation and budding. While NC of betaretroviruses (i.e. Mason-Pfizer Monkey Virus, MPMV), alpharetroviruses (i.e. Rous Sarcoma Virus, RSV) and lentiviruses (i.e. Human Immunodeficiency Virus; HIV) have two ZFs, gammaretroviruses, such as the prototypic Murine Leukemia Virus (MuLV), have only one NC ZF This unique ZF and the basic residues on its N-terminal side are required for MuLV infectivity [5,6,7,8]. Dimerization of the gRNA induces a structural RNA switch that exposes conserved UCUG elements that bind NC with high affinity [14,15,16] Such genome recognition by NC promotes the specific packaging of the gRNA in a dimeric form into newly made viral particles [17,18]
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