RAP1 regulates TIP60 function during fate transition between 2 cell-like and pluripotent states

In mammals, the conserved telomere binding protein RAP1 serves a diverse set of non- telomeric functions including activation of the NF-kB signaling pathway, maintenance of metabolic function in vivo, and transcriptional regulation. Here, we uncover the mechanism by which RAP1 modulates gene expression. Using a separation-of-function allele, we show that RAP1 transcriptional regulation is independent of TRF2-mediated binding to telomeres and does not involve direct binding to genomic loci. Instead, RAP1 interacts with the TIP60/p400 complex and modulates its histone acetyltransferase activity. Notably, we show that deletion of RAP1 in mouse embryonic stem cells increases the fraction of 2-cell-like cells. Specifically, RAP1 enhances the repressive activity of Tip60/p400 across a subset of 2-cell-stage genes, including Zscan4 and the endogenous retrovirus MERVL. Preferential upregulation of genes proximal to MERVL elements in Rap1 deficient settings implicate these endogenous retroviral elements in the de- repression of proximal genes. Altogether, our study reveals an unprecedented link between RAP1 and TIP60/p400 complex in the regulation of totipotency.