The biology of retroviruses is really all about the processes of reverse transcription, so it’s frustrating that nearly 50 years after their discovery, the steps from cytoplasmic entry until integration of the reverse transcribed genome are still mysterious. They occur in ill-defined reverse-transcription- and pre-integration-complexes (RTC, PIC) with various host and viral proteins implicated.
A new paper in eLife describes quantitative detection of functional RTC/PIC by labeling nascent DNA combined with detection of viral integrase. Interestingly, the virus capsid (CA) protein remains associated with cytoplasmic RTC/PIC in primary human macrophages, but is lost from nuclear PIC in a HeLa-derived cell line. The capsid-targeted inhibitor PF74 exhibits a bimodal mechanism, blocking RTC/PIC association with the host factor CPSF6 and nuclear entry at low, and abrogating reverse transcription at high concentrations. This newly developed system is ideally suited for studying retroviral post-entry events and the roles of host factors including DNA sensors and signaling molecules.