Identification of the LEDGF/p75 HIV-1 integrase-interaction domain and NLS reveals NLS-independent chromatin tethering

To investigate the basis for the LEDGF/p75 dependence of HIV-1 integrase (IN) nuclear localization and chromatin association, we used cell lines made stably deficient in endogenous LEDGF/p75 by RNAi to analyze determinants of its location in cells and its ability to interact with IN. Deletion of C-terminal LEDGF/p75 residues 340-417 preserved nuclear and chromatin localization but abolished the interaction with IN and the tethering of IN to chromatin. Transfer of this IN-binding domain (IBD) was sufficient to confer HIV-1 IN interaction to GFP. HRP-2, the only other human protein with an identifiable IBD domain, was found to translocate IN to the nucleus of LEDGF/p75(–) cells. However, in contrast to LEDGF/p75, HRP-2 is not chromatin bound and does not tether IN to chromatin. A single classical nuclear localization signal (NLS) in the LEDGF/p75 N-terminal region (146RRGRKRKAEKQ156) was found by deletion mapping and was shown to be transferable to pyruvate kinase. Four central basic residues in the NLS are critical for its activity. Strikingly, however, stable expression studies with NLS(+/–) and IBD(+/–) mutants revealed that the NLS, although responsible for LEDGF/p75 nuclear import, is dispensable for stable, constitutive nuclear association of LEDGF/p75 and IN. Both wild-type LEDGF/p75 and NLS-mutant LEDGF/p75 remain entirely chromatin associated throughout the cell cycle, and each tethers IN to chromatin. Thus, these experiments reveal stable nuclear sequestration of a transcriptional regulator by chromatin during the nuclear-cytosolic mixing of cell division, which additionally enables stable tethering of IN to chromatin. LEDGF/p75 is a multidomain adaptor protein that interacts with the nuclear import apparatus, lentiviral IN proteins and chromatin by means of an NLS, an IBD and additional chromatin-interacting domains.