Nuclear compartments play diverse roles in regulating gene expression, yet the molecular forces and components driving compartment formation are not well understood. Studying how the lncRNA Xist establishes the inactive-X-chromosome (Xi)-compartment, we found that the Xist RNA-binding-proteins PTBP1, MATR3, TDP43, and CELF1 form a condensate to create an Xidomain that can be sustained in the absence of Xist. The E-repeat-sequence of Xist serves a multivalent binding-platform for these proteins. Without the E-repeat, Xist initially coats the Xchromosome during XCI onset but subsequently disperses across the nucleus with loss of gene silencing. Recruitment of PTBP1, MATR3, TDP-43 or CELF1 to DE-Xist rescues these phenotypes, and requires both self-association of MATR3 and TDP-43 and a heterotypic PTBP1-MATR3-interaction. Together, our data reveal that Xist sequesters itself within the Xi-territory and perpetuates gene silencing by seeding a protein-condensate. Our findings uncover an unanticipated mechanism for epigenetic memory and elucidate the interplay between RNA and RNA-bindingproteins in creating compartments for gene regulation.
Main textThe function of long non-coding RNAs (lncRNAs) and the mechanisms by which they act remain largely unknown. One of the best studied lncRNAs is Xist, which orchestrates X-chromosome inactivation (XCI) in placental female mammals 1-7 . By spreading across one X-chromosome and mediating chromosome-wide gene silencing, Xist equalizes X-linked gene expression with that of males 8-12 . XCI initiates when Xist is induced on one of the two X-chromosomes in pluripotent cells of the implanting blastocyst, or upon induction of differentiation in embryonic stem cells (ESCs) 4,13 , the latter providing a powerful model for the mechanistic dissection of XCI-initiation.Intriguingly, Xist shapes nuclear organization during XCI-initiation. Xist establishes a transcriptionally silent, intra-chromosomal domain (or compartment) by specifically localizing to the X-chromosome from which it is transcribed and inducing the compaction of the forming inactive X-chromosome (Xi), the enrichment of heterochromatin proteins, the repositioning of silenced genes into the center of the Xi, and the exclusion of active transcriptional regulators, such as RNA polymerase II 1,2,14-20 . Yet, the mechanisms that drive and maintain the Xist RNA within a spatially confined region to establish this Xi-domain remain unclear.6 the E-repeat occurred on the 129 allele, which also harbors 11 copies of an MS2-RNA tag within Xist 15 , yielding the X129 Xist DE, MS2 XCas Xist WT genotype (referred to as DE ESCs below) ( Fig. 2a and Extended Data Fig. 5a-d). We ensured that DE ESCs maintained two X-chromosomes, and differentiated normally, as judged by morphological changes and loss of NANOG expression (Extended Data Fig. 5e-g). When transcribed from the X129 Xist DE, MS2 allele, Xist exon 6 was spliced to a cryptic site within exon 7 to generate an RNA missing specifically the E-repeat (Extended Data Fig. 6).RNA FISH over ...