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Residual HIV-1 DNA Flap-independent nuclear import of cPPT/CTS double mutant viruses does not support spreading infection

Candela Iglesias12, Mathieu Ringeard13, Francesca Di Nunzio1, Juliette Fernandez1, Raphael Gaudin14, Philippe Souque1, Pierre Charneau1* and Nathalie Arhel1*

Author Affiliations

1 Molecular Virology and Vaccinology Unit, Department of Virology, Institut Pasteur, Paris, France

2 Centro de Investigacion en Enfermedades Infecciosas, Mexico Tlalpan, Mexico

3 Molecular Virology Laboratory, Institut de Génétique Humaine, Montpellier, France

4 Immunity and Cancer Unit, Institut Curie, Paris, France

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Retrovirology 2011, 8:92  doi:10.1186/1742-4690-8-92

Published: 10 November 2011



The human immunodeficiency virus type 1 (HIV-1) central DNA Flap is generated during reverse transcription as a result of (+) strand initiation at the central polypurine tract (cPPT) and termination after a ca. 100 bp strand displacement at the central termination sequence (CTS). The central DNA Flap is a determinant of HIV-1 nuclear import, however, neither cPPT nor CTS mutations entirely abolish nuclear import and infection. Therefore, to determine whether or not the DNA Flap is essential for HIV-1 nuclear import, we generated double mutant (DM) viruses, combining cPPT and CTS mutations to abolish DNA Flap formation.


The combination of cPPT and CTS mutations reduced the proportion of viruses forming the central DNA Flap at the end of reverse transcription and further decreased virus infectivity in one-cycle titration assays. The most affected DM viruses were unable to establish a spreading infection in the highly permissive MT4 cell line, nor in human primary peripheral blood mononuclear cells (PBMCs), indicating that the DNA Flap is required for virus replication. Surprisingly, we found that DM viruses still maintained residual nuclear import levels, amounting to 5-15% of wild-type virus, as assessed by viral DNA circle quantification. Alu-PCR quantification of integrated viral genome also indicated 5-10% residual integration levels compared to wild-type virus.


This work establishes that the central DNA Flap is required for HIV-1 spreading infection but points to a residual DNA Flap independent nuclear import, whose functional significance remains unclear since it is not sufficient to support viral replication.