Background:  Trafficking of HIV-1 pre-integration complexes (PIC) into and within nuclei is poorly understood. Karyophilic properties of PIC components have therefore drawn much interest. The transcriptional co-activator LEDGF/p75 interacts with HIV-1 integrase and determines nuclear localization of GFP-HIV-1 integrase fusions. It is not known if LEDGF/p75 is a component of PIC, if it interacts with integrases of other retroviruses, or what the functional implications are.

Methods:  HIV-1, FIV, and MoMLV integrases were stably expressed to determine their intracellular trafficking. Stable anti-LEDGF/p75 siRNA expression was used. PIC were purified from cytoplasm of infected cells and analyzed in integration assays with and without prior immunoprecipitation for LEDGF/p75. Viral infection was analyzed and 2 LTR circles were quantified.

Results:  The lentiviral integrases localized to cell nuclei, while the oncoretroviral integrase was pan-cellular. Both lentiviral integrases lacked transferable nuclear localization signals. Intracellular trafficking of each was determined instead by LEDGF/p75, which was required for nuclear localization. Stable siRNA expression eliminated detectable LEDGF/p75 expression but preserved expression of splice variant LEDGF/p52, and caused dramatic redistribution to the cytoplasm of each stably-expressed lentiviral integrase. In contrast, the distribution of MoMLV integrase was unaffected. In addition, endogenous LEDGF/p75 co-immunoprecipitated specifically with each lentiviral integrase. In vitro integration assays with PIC isolated from infected cell cytoplasm then showed that endogenous LEDGF/p75 is a component of functional HIV-1 and FIV PIC. LEDGF/p75-deficient cells, whether dividing or growth-arrested, nevertheless supported HIV-1 and FIV infection normally, and 2-LTR circle DNA accumulation in nondividing cell nuclei (as well as internally-promoted reporter expression from unintegrated DNA) were equivalent to that of LEDGF/p75-wild type cells. Virions produced in LEDGF/p75-deficient cells were normally infectious.

Conclusions:  LEDGF/p75 interaction accounts fully for the cellular trafficking of diverse lentiviral but not oncoretroviral integrases. This protein is also a component of functional HIV-1 and FIV PIC. Despite entirely explaining nuclear targeting of integrase, this lentiviral-specific interaction does not provide a non-redundant mechanism for PIC nuclear import.

Keywords: integrase; LEDGF/p75; pre-integration complex