UC San Diego

HP1(Heterochromatin Protein 1) is a major component of heterochromatin, which is a highly condensed form of DNA playing an important role in multiple cellular activities including gene silencing. New research proposes that HP1 proteins could compartmentalize DNA into compacted chromatin by phase separation, which could be promoted by diverse HP1-binding proteins. STAT is a promising candidate. Besides its canonical role in JAK-STAT signaling pathway, previous research in our lab indicated that STAT contains HP1-binding PXVXI motif. Unphosphorylated STAT could bind to HP1 and maintain the stability of heterochromatin while phosphorylated STAT disperses from HP1, resulting in heterochromatin disruption. Thus, I hypothesized that phosphorylation induced conformational change on STAT, switching it from an HP1-binding state to a DNA-binding state. In this paper, I constructed computational models among HP1α, STAT3, and DNA to examine the influence of phosphorylation on STAT3’s binding affinity to both HP1α and DNA. During the preparation stage, I modified and constructed biomolecular structures for protein docking by Pymol and SWISS-MODEL. I imported the prepared biomolecular structures into HADDOCK and the web server provided potential binding complexes as output. I used the PRODIGY program to measure the Gibbs free energy and equilibrium constant of binding among unphosphorylated STAT3, HP1α and DNA as well as phosphorylated STAT3, HP1α and DNA. Compared to phosphorylated STAT3 homodimers, unphosphorylated STATA3 homodimers have higher binding affinity to HP1α and lower binding affinity to DNA. Although computational model has limitations and needs confirmation by further experiments in vitro, my results support the conclusion that phosphorylation drives STAT from HP1-binding to DNA-binding.