Bromo- and Extraterminal Domain Chromatin Regulators Serve as Cofactors for Murine Leukemia Virus Integration

Abstract

Retroviruses depend on the virally encoded IN proteins to facilitate stable insertion of their reverse-transcribed genomes into host cell chromosomes. INs recognize the attachment (att) sites at the ends of long terminal repeats (LTRs) in viral DNA to carry out two sequential enzymatic reactions. In the first reaction, referred to as 3= processing, IN removes dinucleotides from the 3= ends of viral DNA to expose the 3= OH groups attached to the invariant CA dinucleotides. In the second reaction, DNA strand transfer, IN inserts the processed 3= termini into opposing strands of the host chromosomal DNA via a transesterification mechanism (1, 2). Host cell enzymes complete the process by repairing the single-stranded gaps on both sides of integrated viral DNA. Consequently, the resulting provirus is flanked by short duplications of the target DNA sequences. The duplication size appears to be retroviral genus specific, being 5 bp for human immunodeficiency virus type 1 (HIV-1) and 4 bp for murine leukemia virus (MLV) (3-5). The terminal cleavage and strand transfer steps can be observed in vitro with purified recombinant retroviral IN and DNA substrates, demonstrating that IN alone is sufficient to carry out these reactions (3, 6).Retroviral IN consists of three structural domains (reviewed in reference 7). The N-terminal domain (NTD) contains the zinc binding HHCC motif, and a highly conserved catalytic core domain (CCD) contains the essential active site Asp, Asp, and Glu (D, D-35-E motif) residues, which are directly involved in the catalytic activities of IN. The C-terminal domain (CTD) is least conserved (8-11). Mounting evidence suggests that IN functions as a tetramer (12-15). Recent crystal structures of the prototype foamy virus (PFV) IN bound to its viral and host DNA substrates revealed that all three IN domains participate in tetramerization and interactions with viral DNA (16,17).Retroviral integration into cellular DNA does not occur in a random manner with respect to various genomic features (reviewed in reference 18). HIV-1 and other lentiviruses show a remarkable preference for integration within active transcription units (19). In contrast, MLV, a gammaretrovirus, preferentially integrates near transcription start sites and CpG islands, features that are largely avoided by HIV-1 (20, 21). The remaining retroviral genera show other, albeit far less contrasting, integration patterns (22). Integration site selection of HIV-1 and other lentiviruses was shown to depend on the cellular protein lens epithelium-derived growth factor (LEDGF) (reviewed in reference 23). The IN binding domain (IBD) located within the C-terminal region of LEDGF mediates its interactions with HIV-1 and other lentiviral . LEDGF associates with chromatin via its N-terminal PWWP domain, which selectively binds to nucleosomes containing H3 trimethylated on Lys36 (27, 28), an epigenetic mark associated with bodies of transcription units (29). In cells depleted of LEDGF/p75, HIV-1 integration and replication were significantly affect...

Single-Nucleotide-Specific Targeting of the Tf1 Retrotransposon Promoted by the DNA-Binding Protein Sap1 of Schizosaccharomyces pombe

“…Retroviruses such as human immunodeficiency virus 1 (HIV-1) and murine leukemia virus (MLV), which share significant similarities with LTR retrotransposons in their genetic structures and mechanisms of propagation (Levin and Moran 2011), depend on host factors for targeting integration. HIV-1 insertion is directed to the body of RNA polemerase II-transcribed genes by host factor LEDGF, while MLV IN interacts with BET proteins to direct its integration into enhancer sequences of RNA polemerase II-transcribed genes (Ciuffi et al 2005;Llano et al 2006;Shun et al 2007;Gupta et al 2013;Sharma et al 2013). The mechanism by which Tf1 accomplishes targeting appears to be significantly different from the preceding examples except perhaps for MLV, which does integrate into promoter sequences.…”

Le rôle des protéines BET dans l’intégration des γ-rétrovirus

“…Pour comprendre comment IN de MLV sélectionne les sites d'intégration, trois équipes ont cherché les protéines cellulaires capables d'interagir avec elle. Les partenaires identifiés sont trois protéines apparentées, BRD2 (bromodomaincontaining protein 2), BRD3 et BRD4, qui présentent deux bromodomaines préférentiellement dans le corps des gènes transcrits ( Figure 1B) [2][3][4][6][7][8][9][10][11][12]. Les LV sont moins oncogéniques que les RV [10].…”

“…Le tropisme distinct des RV et des LV a permis d'identifier les protéines virales le risque de dérégulation de gènes de l'hôte [2][3][4][5][6][7][8][9][10][11][12]. Les essais de théra-pie génique témoignent de ces deux aspects.…”

“…L'ARN géno-mique viral simple brin est rétrotranscrit en un ADN double brin qui s'intègre dans le génome de l'hôte pour former le provirus. Ces deux étapes sont respectivement catalysées par la rétrotrans-criptase (RT) et l'intégrase (IN), deux enzymes codées par le gène viral pol [1][2][3][4] (➜). Cette propriété d'intégration pré-sentée par les rétrovirus a suscité le développement d'outils de transfert de gènes, les rétrovecteurs 1 .…”

“…Leurs dérivés artificiels, défectifs et intégratifs à quelques exceptions près, seront appelés rétrovecteurs (dérivés des rétrovirus, quels qu'ils soient), -rétrovecteurs (RV, très souvent dérivés de gamma-rétrovirus murins) et lentivecteurs (LV, dérivés des lentivirus, généralement le VIH, virus de l'immunodéficience humaine). Les résultats présen-tés ici ont été principalement obtenus avec des RV défectifs dérivés du -rétrovirus murin MLV mais sont probablement valables pour tous les -rétrovirus et tous les RV ( Figure 1A) [3,8,11,12].…”