External Biophysical mechanisms of heterochromatin formation Seminar details August 29, 2018, 12:00 pm @ Murray Seminar Room Dr. Geeta Narlikar, UCSF, San Francisco Host: Tom Owen-Hughes Abstract The gene silencing function of heterochromatin is proposed to arise in part from the ability of HP1 proteins to promote chromatin compaction. We have recently found that the heterochromatin protein HP1 promotes chromatin self-association and liquid droplet formation. To understand how HP1 action at the nucleosome level regulates higher order chromatin organization we are using hydrogen-deuterium exchange, NMR, and crosslinking mass-spectrometry to assess changes in nucleosome dynamics and conformation. We find that the S. pombe HP1 protein, Swi6, substantially increases the accessibility and dynamics of buried core histones. Restraining these dynamics impairs Swi6 mediated segregation of chromatin into phase-separated droplets. We propose that exposed nucleosomal regions participate in multi-valent interactions to drive the higher-order chromatin organization of heterochromatin. Our results raise the possibility that histone core dynamics more generally regulate higher-order chromatin assemblies.