A Heterologous, High-Affinity RNA Ligand for Human Immunodeficiency Virus Gag Protein Has RNA Packaging Activity

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Abstract
Retroviral RNA encapsidation depends on the specific binding of Gag proteins to packaging (ψ) signals in genomic RNA. We investigated whether an in vitro-selected, high-affinity RNA ligand for the nucleocapsid (NC) portion of the Gag protein from human immunodeficiency virus type 1 (HIV-1) could mediate packaging into HIV-1 virions. We find that this ligand can functionally substitute for one of the Gag-binding elements (termed SL3) in the HIV-1 ψ locus to support packaging and viral infectivity in cis . By contrast, this ligand, which fails to dimerize spontaneously in vitro, is unable to replace a different ψ element (termed SL1) which is required for both Gag binding and dimerization of the HIV-1 genome. A single point mutation within the ligand that eliminates high-affinity in vitro Gag binding also abolishes its packaging activity at the SL3 position. These results demonstrate that specific binding of Gag or NC protein is a critical determinant of genomic RNA packaging.