Micelle-like clusters in phase-separated Nanog condensates: A molecular simulation study

The phase separation model for transcription suggests that transcription factors (TFs), coactivators, and RNA polymerases form biomolecular condensates around active gene loci and regulate transcription. However, the structural details of condensates remain elusive. In this study, for Nanog, a master TF in mammalian embryonic stem cells known to form protein condensates in vitro, we examined protein structures in the condensates using residue-level coarse-grained molecular simulations. Human Nanog formed micelle-like clusters in the condensate. In the micelle-like cluster, the C-terminal disordered domains, including the tryptophan repeat (WR) regions, interacted with each other near the cluster center primarily via hydrophobic interaction. In contrast, hydrophilic disordered N-terminal and DNA-binding domains were exposed on the surface of the clusters. Electrostatic attractions of these surface residues were responsible for bridging multiple micelle-like structures in the condensate. The micelle-like structure and condensate were dynamic and liquid-like. Mutation of tryptophan residues in the WR region which was implicated to be important for a Nanog function resulted in dissolution of the Nanog condensate. Finally, to examine the impact of Nanog cluster to DNA, we added DNA fragments to the Nanog condensate. Nanog DNA-binding domains exposed to the surface of the micelle-like cluster could recruit more than one DNA fragments, making DNA-DNA distance shorter. Author summary In eukaryotic transcription regulation, enhancer elements far from the promoter are known to modulate transcriptional activity, but the molecular mechanism remains elusive. One of these models, the phase separation model, suggests that transcription factors (TFs), coactivators, and transcription machinery form biomolecular condensates that include enhancer and promoter elements, making enhancer-promoter communication possible via the protein network in the condensate. Nanog, one of the core TFs involved in mammalian embryonic stem cells, has been reported to have the ability to form condensates. In this study, we addressed the structural details of Nanog condensates by performing residue-level coarse-grained molecular simulations. We found that Nanog formed micelle-like clusters via primarily hydrophobic interactions between tryptophan repeat regions in the C-terminal disordered domains within the condensate. On the other hand, highly charged N-terminal and DNA-binding domains were exposed to the surface of the micelle and were responsible for bridging many micelles into a condensate. The micelle-like clusters and condensate were dynamic and liquid-like. In addition, Nanog condensates could induce DNA-DNA attraction mediated by micelle-like structures.