HIV-1 Gag co-localizes with euchromatin histone marks at the nuclear periphery

The retroviral Gag protein of human immunodeficiencyvirus type 1 (HIV-1) plays a central role in the selection of unspliced viral genomic RNA (gRNA) for packaging into new virions. Previously, we demonstrated that full-length HIV-1 Gag undergoes nuclear trafficking,where it associates with unspliced viral RNA (USvRNA) at transcription sites. To further examine the kinetics of HIV-1 Gag nuclear localization, we used biochemical and imaging techniques to determine the timing of HIV-1 entry into the nucleus. We also aimed to determine more precisely Gag's subnuclear distribution to test the hypothesis that Gag associates with euchromatin, the transcriptionally active region of the nucleus. We observed that HIV-1 Gag localized to the nucleus at low expression levels shortly after its synthesis in the cytoplasm, suggesting that nuclear traffickingwas not strictly concentration dependent. Furthermore, we found that HIV-1 Gag preferentially localized to the transcriptionally active euchromatin fraction compared to the heterochromatin-rich region in a latently infected T-cell line (J-Lat 10.6) treated with latency-reversal agents. Interestingly, HIV-1 Gag was more closely co-localized with euchromatin-associated histone marks near the nuclear periphery, the preferred location of HIV-1 proviral integration. Although the precise function of Gag's association with histones in transcriptionally active chromatin regions remains uncertain, together with previous reports, this findingis consistent with a potential role for euchromatin-associated Gag molecules to initiate the selection of newly transcribed USvRNA in the nucleus for incorporation into virions.