Genomic RNA Binding Promotes Retroviral Gag Protein Interactions in an Assembly-Competent Conformation Leading to Selective Genome Packaging

In HIV-1 infected cells, full-length viral genomic RNA (gRNA) is selectively packaged by the HIV-1 Gag protein despite a vast excess of spliced viral and host RNAs. The mechanism of this selective packaging is incompletely understood but a region of gRNA known as “Psi” mediates specific Gag interaction. HIV-1 Gag binds Psi and non-Psi RNA with similar affinity under physiological salt concentration (∼150 mM NaCl), but the salt dependence of these two binding events differs dramatically (Webb, JA, et al, RNA 2013). HIV-1 Gag binds Psi RNA with a strong non-electrostatic binding component and a small effective charge (Zeff ∼ 5). In contrast, HIV-1 Gag binds non-Psi RNA with a very weak non-electrostatic binding component and a larger effective charge (Zeff ∼ 9). In this work, we use salt-titration binding assays to study the effect of various Gag and Psi RNA mutations on the Psi and non-Psi binding interactions of HIV-1 and Rous sarcoma virus Gag proteins. Our findings are consistent with a model in which Gag binding to cognate Psi RNA shifts the equilibrium from a non-assembling to an assembling Gag state, providing an advantage for nucleation of assembly only on gRNA.