Dynamic model depicting the mechanism of LEDGF/p75-mediated HIV integration. LEDGF/p75 (green oval) associates with PC4 (red protein) and the RNA polymerase II machinery (yellow ovals) at promoter regions, but steric hindrance may prevent successful recruitment of preintegration complexes (gray oval with viral DNA in red). In this proposed model, LEDGF/p75 remains associated with the RNA pol II transcription elongation complex, potentially interacting with PC4 and menin/MLL complex. While this complex displaces nucleosomes (not depicted) and unwinds DNA to allow RNA polymerization, LEDGF/p75 may recruit HIV PIC and promote integration. This model is consistent with LEDGF/p75-captured DNA sequences and HIV integration sites being present throughout the transcription unit, without specific DNA binding consensus motif.

Overview of the early steps of HIV-1 life cycle. (A) To enter a target cell, HIV-1 gp120 binds to specific cellular receptors, i.e., CD4 and a chemokine coreceptor (CCR5 or CXCR4), triggering the gp41-mediated fusion between the viral and the cellular membrane, and releasing the viral core in the cytoplasm of the host cell. The viral single stranded, positive, RNA genome (black line, flanked by open black squares depicting R-U5 and U3-R in its 5′ and 3′ termini respectively) is reverse transcribed into a linear double stranded cDNA copy (red line, flanked by open red squares representing the LTR = U3-R-U5), which is a component of the preintegration complex (PIC), also containing the viral integrase (IN), as well as other viral and cellular proteins. The PIC is translocated to the nucleus and the viral cDNA is either integrated through the action of IN or remains unintegrated (linear, 1-LTR circles, 2-LTR circles). From this point on, the cellular machinery of the host is recruited to transcribe the viral genome in order to produce all the components required to generate newly infectious particles. (B) The integration process is divided into three major steps: the 3′ processing and the strand transfer reaction, both catalyzed by IN, and the repair of the integrated viral DNA by the DNA repair machinery of the host cell. The PIC-containing viral DNA (red line, with 5′ ends depicted by filled circles) is first processed by the IN-mediated removal of a dinucleotide (GT) at each 3′ end of the viral DNA, leaving a protruding (AC) dinucleotide at the 5′ ends. IN then catalyzes the stable insertion of the processed viral DNA into a target DNA (black line), by simultaneously and asymmetrically breaking the target DNA 5 bp apart (blue bonds) (4 to 6 bp depending on the retrovirus) and joining it to the 3′ recessed ends of the viral DNA, leaving an integration intermediate with unpaired bases at each viral-target DNA junction. The DNA repair machinery of the host cell fills in the five nucleotide gap at each side of the viral DNA and removes the two 5′ overhang nucleotides from the viral DNA, resulting in the duplication of 5 bp of the target DNA at both sides of the proviral DNA. (C and D) Schematic concepts of in vitro integration assays showing half-site integration (C) and concerted or full-site integration (D).