Membraneless compartments within cells that concentrate specific molecules, known as biomolecular condensates, are increasingly being recognized in controlling cancer-related processes, such as oncogenic transcription. Lyons et al. set out to explore the rules of specificity and function for condensates formed by oncogenic fusion proteins.
Next, to find proteins that might be selectively compartmentalized in TFE3 fusion-mediated condensates, the authors centrifuged reconstituted condensates to fractionate out proteins present in condensates in nuclear extracts containing purified PRCC–TFE3 or ASPL–TFE3. This revealed the presence of two of the largest subunits of RNA polymerase II (RNA Pol II), which do not typically interact directly with DNA-binding transcription factors, suggesting this might be a means to circumvent the normal regulation of RNA Pol II recruitment, which takes place via transcriptional coactivators. Further experiments demonstrated that it was the C-terminal domain (CTD) of RNA Pol II that was specifically responsible for partitioning it into TFE3 fusion-mediated condensates. Chromatin immunoprecipitation followed by sequencing (ChIP–seq) for TFE3, RNA Pol II and acetylated histone H3 lysine 27 (H3K27ac) in a patient-derived tRCC cell line with PRCC–TFE3 showed that more RNA Pol II was recruited to genomic regions bound by PRCC–TFE3 compared to WT TFE3 in a ccRCC cell line, and that the neighbouring chromatin was marked with more H3K27ac, indicative of transcriptional activity.
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