Characterization of the nuclear import pathway for HIV-1 integrase

The karyophilic properties of the human immunodeficiency virus, type I (HIV -1) pre-integration complex (PIC) allow the virus to infect non-dividing ce lls. To better understand the mechanisms responsible for nuclear translocat ion of the PIG, we investigated nuclear import of HIV-1 integrase (IN), a P IG-associated viral enzyme involved in the integration of the viral genome in the host cell DNA. Accumulation of HIV-1 IN into nuclei of digitonin-per meabilized cells does not result from passive diffusion but rather from an active transport that occurs through the nuclear pore complexes. HIV-1 IN i s imported by a saturable mechanism, implying that a limiting cellular fact or is responsible for this process. Although IN has been previously propose d to contain classical basic nuclear localization signals, we found that nu clear accumulation of IN does not involve karyopherins alpha, beta1, and be ta2-mediated pathways. Neither the non-hydrolyzable GTP analog, guanosine 5 '-O-(thiotriphosphate), nor the GTP hydrolysis-deficient Ran mutant, RanQ69 L, significantly affects nuclear import of IN, which depends instead on ATP hydrolysis, Therefore these results support the idea that IN import is not mediated by members of the karyopherin beta family. More generally, in vit ro nuclear import of IN does not require addition of cytosolic factors, sug gesting that cellular factor(s) involved in this active but atypical pathwa y process probably remain associated with the nuclear compartment or the nu clear pore complexes from permeabilized cells.