Distinct Gag interaction properties of HIV-1 RNA 5' leader conformers reveal a mechanism for dimeric genome selection.

During HIV-1 assembly, two copies of viral genomic RNAs (gRNA) are selectively packaged into new viral particles. This process is mediated by specific interactions between HIV-1 Gag and the packaging signals at 5' RNA leader (5'L). 5'L is able to adopt different conformations, which promotes either gRNA dimerization and packaging or Gag translation. Dimerization and packaging are coupled. Whether the selective packaging of gRNA dimer is due to favorable interactions between Gag and 5'L in packaging conformation is not known. Here, using RNAs mimicking the two 5'L conformers, we show that 5'L conformation dramatically affects Gag-RNA interactions. Compare to the RNA in translation conformation (5'L T ), the one in packaging conformation (5'L P ) is able to bind more Gag molecules. Gag associates with 5'L P faster than it binds to 5'L T ; Gag dissociates from 5'L P much slower. The Gag-5'L P complex is more stable at high salt. The NC-SP2-p6 region of Gag likely accounts for the faster association and slower dissociation kinetics for the Gag-5'L P interaction and for the higher stability.In summary, our data suggests that conformational changes play an important role in the selection of dimeric genomes, probably by affecting the dissociation kinetics and stability of the Gag-5'L complex.