Transcriptional Regulation in Embryonic Stem Cells
Embryonic stem cells (ESCs), which are derived from the inner cell mass of the mammalian blastocyst, exhibit the unique properties of self-renewal and pluripotency. As a function of these properties, ESCs are valuable tools for understanding early development, performing drug screening, and developing regenerative medicine technologies. Central to the maintenance of ESC pluripotency is the gene regulatory network governed by OCT4, SOX2, and NANOG. In addition to these core factors, the mammalian XPC DNA repair complex functions in a dual role as a transcriptional coactivator of OCT4/SOX2. To investigate XPC's remarkable functional versatility, I combined structural and biochemical approaches to solve the first cryo-EM structures of XPC in its native and DNA-bound states. Intriguingly, we found that mammalian but not yeast XPC complexes are capable of activating OCT4/SOX2 transcription, in contrast to DNA repair function, which is conserved across all metazoans. By synthesizing biochemical information on XPC's OCT4/SOX2 interaction interfaces with these structural models, I identified the structural basis for the separation of these two functional modalities of XPC: conserved DNA repair functionality vs. non-conserved transcriptional coactivation capacity.
In investigating the basis of the XPC:SOX2 interaction, toward the end of my PhD I made an astonishing discovery that the XPC:SOX2 interaction is entirely RNA-mediated. Intriguingly, this interaction is not RNA sequence specific, thus representing a promiscuous mode of interaction and functionality. Furthermore, this novel RNA-mediated SOX2:XPC interaction is required for full transcriptional activity, thus leading to an updated model in which RNA functions as a second coactivator in the OCT4/SOX2/XPC gene regulatory network. More broadly, this work suggests that RNA may be a general mediator of many transcription factor:coactivator interaction networks and that transcription output itself as RNA molecules can further amplify nearby hubs of transcriptional activity.
