POST DOCTORAL FELLOW The Centre For Translation Stem Cell Biology Hong Kong, Hong Kong
Abstract: Pioneer transcription factors (PTFs), are a special class of TFs that can bind directly to their target DNA sites even when located on closed chromatin. PTFs bind silent chromatin and initiate regulatory events. However, a disconnect exists between a PTF's cellular function and its biochemical ability to bind nucleosomes. This highlights a key gap in our understanding of what constitutes a PTF and whether the nucleosomal context of its binding site influences its pioneering activity.Oct4 and Brn2 present an interesting pair of homologous TFs with seemingly disparate pioneering activity. Oct4, a well-known PTF essential for stem cell maintenance and pluripotency, contrasts with Brn2, a collaborating TF in neural reprogramming. This provides an opportunity to more clearly dissect the characteristics that define a pioneer factor by contrasting Oct4 with Brn2. Here, we performed binding assays with Widom 601 nucleosomes and free DNA with various POU motifs (Octamer, CpGpal, MORE). We demonstrate that Oct4 exhibits significantly higher affinity and specificity for nucleosomes containing the Octamer motif compared to Brn2. Structural modelling reveals that Oct4's binding mode varies depending on the motif: a monomeric configuration for the Octamer, a potential dimer for CpGpal, and a monomer for MORE. In contrast, Brn2 demonstrates non-specific binding to nucleosomes regardless of the motif. These findings emphasize the critical role of specific DNA sequences in mediating selective nucleosome targeting by PTFs, potentially differentiating them from collaborating factors. Further research into POU factor-nucleosome interactions will clarify their implications for gene regulation and potentially enable the engineering of improved cell fate-driving proteins.
