STAG3 promotes exit from pluripotency through post-transcriptional mRNA regulation in the cytoplasm.

STAG proteins are key regulators of the cohesin complex and are often linked to alterations in cell identity and disease. Among the mammalian STAG paralogs, STAG3 has been less extensively studied beyond its known roles in meiosis. In this study, we demonstrate that STAG3 is expressed in mouse embryonic stem cells (mESCs) and primordial germ cell-like cells (PGCLCs), where it is required for cell fate decisions. Distinct from the other STAG proteins, STAG3 mediates its effects in the cytoplasm, facilitating the post-transcriptional regulation of gene expression. Furthermore, STAG3 localizes to the centrosome independently of cohesin and interacts with proteins involved in mRNA localization and stability. The knockdown of STAG3 in mESCs using siRNAs results in the destabilization of the centrosome and the key P-body RNA-induced silencing complex (RISC) component TNRC6C, leading to the derepression of P-body localised mRNAs, such as DPPA3. Our results propose a model in which STAG3 collaborates with RNA-binding proteins (RBPs) and specific target mRNAs to control post-transcriptional gene expression and facilitate the transition from pluripotency in mESCs. Given that STAG3 is upregulated in various cancers, our results provide a novel perspective on how STAG proteins might contribute to cell identity and disease. Overall design: Wildtype (WT) E14 mouse embryonic stem cells (mESC) were cultured in naïve (2i/LIF) conditions. Three biological replicates of either untreated (UT) mESCs or mESCs treated for 48hrs with control (Luciferase, siLuc) or Stag3 siRNAs were collected. Total RNA was isolated using NEB Monarch RNA prep kit for transcriptome analysis by bulk Illumina RNA sequencing .