Integration of the human immunodeficiency virus (HIV-1) cDNA into the human genome is catalysed by integrase. Several studies have shown the importance of the interaction of cellular cofactors with integrase for viral integration and infectivity. In this study, we produced a stable and functional complex between the wild-type full-length integrase (IN) and the cellular cofactor LEDGF/p75 that shows enhanced in vitro integration activity compared with the integrase alone. Mass spectrometry analysis and the fitting of known atomic structures in cryo negatively stain electron microscopy (EM) maps revealed that the functional unit comprises two asymmetric integrase dimers and two LEDGF/p75 molecules. In the presence of DNA, EM revealed the DNA-binding sites and indicated that, in each asymmetric dimer, one integrase molecule performs the catalytic reaction, whereas the other one positions the viral DNA in the active site of the opposite dimer. The positions of the target and viral DNAs for the 3 0 processing and integration reaction shed light on the integration mechanism, a process with wide implications for the understanding of viral-induced pathologies.

Integration of a DNA copy of the human immunodeficiency virus (HIV)-1 RNA genome into the human genome is an essential step in the viral replication cycle. This process is catalysed by a viral protein, integrase (IN). The first key reaction in the overall integration process is the cleavage of a dinucleotide from each 3¢-end of the viral DNA (substrate DNA), a reaction termed 3¢-end processing. In the second step, DNA strand transfer, a pair of processed DNA ends of the same viral DNA is inserted into the host cellular DNA (target DNA). The 3¢-end processing reaction requires a conserved nucleotide sequence at the viral DNA ends, but the second reaction does not absolutely require specific sequences within the host DNA. The 3¢-processing of viral DNA extremities is the first step in the integration process catalysed by human immunodeficiency virus (HIV)-1 integrase (IN). This reaction is relatively inefficient and processed DNAs are usually detected in vitro under conditions of excess enzyme. Despite such experimental conditions, steady-state Michaelis-Menten formalism is often applied to calculate characteristic equilibrium ⁄ kinetic constants of IN. We found that the amount of processed product was not significantly affected under conditions of excess DNA substrate, indicating that IN has a limited turnover for DNA cleavage. Therefore, IN works principally in a singleturnover mode and is intrinsically very slow (single-turnover rate constant ¼ 0.004 min )1 ), suggesting that IN activity is mainly limited at the chemistry step or at a stage that precedes chemistry. Moreover, fluorescence experiments showed that IN-DNA product complexes were very stable over the time-course of the reaction. Binding isotherms of IN to DNA substrate and product also indicate tight binding of IN to the reaction product. Therefore, the slow cleavage rate and limited product release prevent or greatly reduce subsequent turnover. Nevertheless, the time-course of product formation approximates to a straight line for 90 min (apparent initial velocity), but we show that this linear phase is due to the slow single-turnover rate constant and does not indicate steady-state multiple turnover. Finally, our data ruled out the possibility that there were large amounts of inactive proteins or dead-end complexes in the assay. Most of complexes initially formed were active although dramatically slow.Abbreviations HIV, human immunodeficiency virus; IN, integrase; LTR, long terminal repeat; PIC, preintegration complex; r, anisotropy; RSV, Rous sarcoma virus.

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