Cellular co-factors of HIV-1 integrationIntroductionCandidate host proteins involved in retroviral integrationThe barrier-to-autointegration factor prevents suicidal integration in vitroThe high mobility group chromosomal protein A1 might affect transcriptionIntegrase interactor 1: the first integrase-binding partner identifiedLEDGF/p75 tethers HIV-1 integrase to the chromosomesRole of LEDGF/p75 during lentiviral integrationConcluding remarks and perspectives: validated co-factors as new antiviral targetsAcknowledgementsReferencesCellular co-factors of HIV-1 integrationBe´ne´dicte Van Maele, Katrien Busschots, Linos Vandekerckhove, Frauke Christand Zeger DebyserMolecular Virology and Gene Therapy, Molecular Medicine, KULAK and K.U. Leuven, Kapucijnenvoer 33 3000 Leuven, Flanders,BelgiumTo achieve productive infection, the reverse transcribedcDNA of human immunodeficiency virus type 1 (HIV-1) isinserted in the host cell genome. The main proteinresponsible for this reaction is the viral integrase.However, studies indicate that the virus is assisted bycellular proteins, or co-factors, to achieve integrationinto the infected cell. The barrier-to-autointegrationfactor (BAF) might prevent autointegration. Its abilityto bridge DNA and the finding that the nuclear lamina-associated polypeptide-2a interacts with BAF suggest arole in nuclear structure organization. Integrase inter-actor 1 was found to directly interact with HIV-1integrase and to activate its DNA-joining activity, andthe high mobility group chromosomal protein A1 mightapproximate both long terminal repeat (LTR) ends andfacilitate integrase binding by unwinding the LTRtermini. Furthermore, the lens-epithelium-derivedgrowth factor (LEDGF; also known as p75) seems totether HIV-1 integrase to the chromosomes. Although adirect role in integration has only been demonstrated forLEDGF/p75, to date, each validated cellular co-factor forHIV-1 integration could constitute a promising newtarget for antiviral therapy.IntroductionHuman immunodeficiency virus type 1 (HIV-1) has only alimited genetic make up, although its replication in thehuman cell requires multiple and distinct activities.Consequently, the virus takes advantage of cellularproteins and cellular pathways to complete the differentsteps in its life cycle. The integration of reversetranscribed viral cDNA into a host chromosome is one ofthe essential steps in the HIV-1 life cycle. The key proteinfor retroviral integration is the 32-kDa viral integrasepresent in the virion. Integrase is encoded by the pol geneof the virus, translated as part of a large Gag-Polpolyprotein and processed into its mature form by theviral protease. Integrase consists of an N-terminal domain(residues 1–50) containing a zinc-binding motif, a catalyticcore domain (residues 50–212) carrying the catalytic Asp-Asp35-Glu motif and a less-conserved C-terminal domain(residues 212–288). Retroviral integration occurs intwo well-characterized catalytic steps, referred to as30-processing and strand transfer (Figure 1). During30-processing, integrase removes a pGT dinucleotide atthe 30end of each viral long terminal repeat (LTR) that isadjacent to a highly conserved CA dinucleotide [1]. Thisreaction takes place in the cytoplasm within the pre-integration complex (PIC) [2]. This nucleoprotein complexcontains linear viral DNA and several viral proteinsincluding matrix, reverse transcriptase, integrase andnucleocapsid. Cellular proteins have also been identifiedin functional PICs, some of which will be described in moredetail later. Following the nuclear import of the PIC, thestrand transfer occurs in the nucleus. There integrasemediates a concerted nucleophilic attack by the30-hydroxyl residues of the viral DNA on phosphodiesterbridges located on either side of the major groove in thetarget DNA [1]. In a single transesterification reaction,the processed CA-30-OH viral DNA ends are ligated to the50-O-phosphate ends of the target DNA. Because the 30ends of the target DNA are not joined after strandtransfer, the product is a gapped intermediate product inwhich the 50-phosphate ends of the viral DNA are notattached to the 30-OH ends of the host DNA. Theintegration process is completed by cleavage of theunpaired dinucleotides from the 50ends of the viral DNAand repair of the single-stranded gaps created betweenthe viral and target DNA. This repair is probablyaccomplished by host-cell DNA-repair enzymes [3].Integration is, thus, a multi-step process that takesplace in different compartments of the cell.Although purified recombinant integrase is necessaryand sufficient to carry out 30processing and strand transferin the test tube, a variety of viral and cellular proteins havebeen put forward as important partners in establishing theintegrated provirus in the infected cell. We refer to thesecellular proteins that aid viral replication as co-factors.Here, we first give a brief overview of the previouslyreported cellular co-factors of retroviral integration beforediscussing in detail lens-epithelium-derived growth factor(LEDGF; also known as p75), a newly described bindingpartner of lentiviral integrases [4].Cellular co-factors of integration have been identified indifferent ways: (i) by in vitro reconstitution of enzymaticactivity of salt-stripped PICs [5,6]; (ii) by using the yeast-two-hybrid assay [7,8]; and (iii) by co-immunoprecipitation[4,9]. The discovery of new candidate co-factors wasoriginally based on the in vitro reconstitution of enzymaticactivity of salt-stripped PICs. Purified PICs from infectedcells manifest two properties that are not reproduced inassays using recombinant integrase and synthetic DNAsubstrates: (i) PICs efficiently insert both viral DNA endsinto a target DNA in a concerted manner [2,10], whereasCorresponding author: Debyser, Z. ([email protected]).Available online 5 January 2006Review TRENDS in Biochemical Sciences Vol.31 No.2 February 2006www.sciencedirect.com 0968-0004/$ - see front matter Q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tibs.2005.12.002reactions with recombinant integrase mainly result in theinsertion of a single viral DNA end in a single strand of theduplex target DNA [11,12]; (ii) PICs preferentially inte-grate the viral DNA into a target DNA intermolecularly,thereby avoiding suicidal intramolecular autointegration[2].Therefore,in vitro reactions using purified PICsresemble the in vivo situation more. High salt treatmentof these PICs gives