RESUMEN
Integration is a key feature of the retroviral life cycle. This process involves packaging of the viral genome into chromatin, which is often assumed to occur as a post-integration step. In this issue of Cell Host & Microbe, Wang and colleagues (Wang et al., 2016) show that chromatinization occurs before integration, raising new questions about the role of histones in retroviral integration and transcription.
Asunto(s)
Retroviridae/crecimiento & desarrollo , Retroviridae/genética , Ensamble de Virus , Integración Viral/genética , Acetilación , Animales , Proteínas de la Cápside/metabolismo , Línea Celular Tumoral , Cromatina/genética , Cromatina/virología , ADN Viral/genética , ADN Viral/fisiología , Células Madre de Carcinoma Embrionario/virología , Epigenómica , Fibroblastos , Regulación Viral de la Expresión Génica , Histonas/metabolismo , Histonas/fisiología , Humanos , Infecciones/metabolismo , Estadios del Ciclo de Vida , Ratones , Células Madre Embrionarias de Ratones/virología , Proteínas de la Nucleocápside/metabolismo , Infecciones por Retroviridae/terapia , Infecciones por Retroviridae/virología , Transcripción Genética , Integración Viral/fisiologíaRESUMEN
Embryonic stem cells (ESCs) repress the expression of exogenous proviruses and endogenous retroviruses (ERVs). Here, we systematically dissected the cellular factors involved in provirus repression in embryonic carcinomas (ECs) and ESCs by a genome-wide siRNA screen. Histone chaperones (Chaf1a/b), sumoylation factors (Sumo2/Ube2i/Sae1/Uba2/Senp6), and chromatin modifiers (Trim28/Eset/Atf7ip) are key determinants that establish provirus silencing. RNA-seq analysis uncovered the roles of Chaf1a/b and sumoylation modifiers in the repression of ERVs. ChIP-seq analysis demonstrates direct recruitment of Chaf1a and Sumo2 to ERVs. Chaf1a reinforces transcriptional repression via its interaction with members of the NuRD complex (Kdm1a, Hdac1/2) and Eset, while Sumo2 orchestrates the provirus repressive function of the canonical Zfp809/Trim28/Eset machinery by sumoylation of Trim28. Our study reports a genome-wide atlas of functional nodes that mediate proviral silencing in ESCs and illuminates the comprehensive, interconnected, and multi-layered genetic and epigenetic mechanisms by which ESCs repress retroviruses within the genome.
Asunto(s)
Células Madre Embrionarias/virología , Retrovirus Endógenos/genética , Provirus/genética , Animales , Factor 1 de Ensamblaje de la Cromatina/genética , Factor 1 de Ensamblaje de la Cromatina/metabolismo , Células Madre de Carcinoma Embrionario/virología , Epigénesis Genética , Ratones , Proteínas Modificadoras Pequeñas Relacionadas con Ubiquitina/metabolismoRESUMEN
TRIM28 is a transcriptional corepressor which is required for primer binding site (PBS)-dependent restriction of murine leukemia virus (MLV) replication in embryonic stem and embryonic carcinoma (EC) cells. PBS-dependent restriction of MLV leads to transcriptional silencing of the integrated provirus and has been shown to correlate with TRIM28-mediated recruitment of HP1 to the silenced loci. Here we show, using a cell line with a point mutation in the HP1 binding domain of TRIM28, that interaction with HP1 is absolutely required for the PBS-dependent restriction of MLV in the F9 EC cell line.
