Crosstalk of the Hippo pathway effector YAP and the Myb-MuvB complex in transcriptional regulation of cell cycle genes

• Applicant: Professor Dr. Stefan Gaubatz
• Subject Area: Cell Biology ; Biochemistry
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 445069133

In our previous work that formed the basis for the project, we identified a novel link between YAP, the downstream effector of the Hippo signaling pathway and the MMB protein complex which plays a central role in transcriptional regulation of cell cycle genes. We found that YAP activates MMB-target genes by binding to distant enhancers that interact with the promoters of MMB-regulated cell cycle genes through chromatin looping. Mechanistically, YAP directly interacts with the B-MYB subunit of MMB and increases its chromatin binding, leading to increased expression of mitotic genes. ATAC-seq, ChIP-seq and CRISPRi (CRISPR-interference) experiments lead to a model in which YAP-mediated enhancer activation promotes phosphorylation of RNA Pol II at Ser5 at MMB-target genes, thus facilitating the early steps after transcription imitation, and extending previous studies that YAP primarily stimulates transcriptional elongation. Additionally, we observed increased histone acetylation by YAP at the promoters of MMB target genes, supporting that the MMB-complex functions as a platform to recruit co-activators to the TSS of cell cycle genes. Through two complementary approaches - proximity proteomics and a siRNA screen - we identified several chromatin proteins as potential co-factors of MMB-dependent transcription, including subunits of various histone acetyltransferase complexes, such as the adaptor proteins TRRAP and WDR5. Finally, MY-COMP, a fragment of B-MYB that, when overexpressed interferes with the endogenous YAP-B-MYB interaction, highlighted the functional importance of the YAP-B-MYB interaction in cell cycle regulation and tumorigenesis. To better understand the molecular mechanisms through which YAP activates MMB target genes from distant enhancers, we will first focus on the novel co-factors of YAP-MMB-dependent transcription and investigate how they modulate transcription of cell cycle genes using a combination of biochemical assays, rescue assays and genome-wide studies. Secondly, we will investigate the relationship between YAP-mediated activation of MMB-target genes, promoter acetylation by HAT-complexes and transcription by RNA Pol II. To do so, we will profile the nascent transcriptome by TTchem-seq and integrate the data with ChIP-seq data for histone modifications and RNA Pol II. Finally, we will investigate the functional importance of co-factor recruitment for YAP-mediated transformation and cell cycle regulation in an in vivo liver cancer model driven by oncogenic YAP and in cellular models. A better understanding of the molecular mechanisms underlying cell cycle regulation by YAP and B-MYB could in the future have significant implications for both basic science and potential clinical applications.

• DFG Programme: Research Grants