Post-mitotic BET-induced reshaping of integrase quaternary structure supports wild-type MLV integration

The Moloney murine leukemia virus (MLV) is a prototype gammaretrovirus requiring nuclear disassembly before DNA integration. In the nucleus, integration site selection towards promoter/enhancer elements is mediated by the host factor bromo- and extraterminal domain (BET) proteins (bromodomain (Brd) proteins 2, 3 and 4). MLV-based retroviral vectors are used in gene therapy trials. In some trials leukemia occurred through integration of the MLV vector in close proximity to cellular oncogenes. BET-mediated integration is poorly understood and the nature of integrase oligomers heavily debated. Here, we created wild-type infectious MLV vectors natively incorporating fluorescent labeled IN and performed single-molecule intensity and Förster resonance energy transfer experiments. The nuclear localization of the MLV pre-integration complex neither altered the IN content, nor its quaternary structure. Instead, BET-mediated interaction of the MLV intasome with chromatin in the post-mitotic nucleus reshaped its quaternary structure.