Dynamics of the HP1 Hinge Region with DNA Measured by Site-Directed Spin Labeling-EPR Spectroscopy

An epigenetic reader, heterochromatin protein 1 (HP1), possesses two conserved domains, the chromoshadow domain (CSD) and chromodomain (CD). The CD and CSD are connected by the hinge region (HR). N, C-tails and HR are estimated to be disordered. In 50% glycerol, EPR spectra from side-chain spin labels of the disordered regions were resolved into two motional fractions or nanoseconds to slower timescales; however, the fraction of slower dynamics was labeled to be site-specific. The slow fraction of the HR (middle region) was abolished by monomer mutation and reduced by N-tail truncation in the dimer state. Within the HR, by truncation of the C-tail, the slow fraction of dynamics of the C-terminal region in the HR were reduced. In addition, the slow fraction of dynamics of the basal region in the C-tail, but not the C-tail end, was significantly reduced by truncating the N-tail-CD-HR. Together, the middle region of the HR was loosely organized by direct or indirect interaction with the N-tail, exerted from one monomer to the other, and the C-terminal region in the HR contacted the C-tail within a monomer in an autoinhibited state, as presumably proposed. Surprisingly, DNA did not affect the spectra, irrespective of glycerol addition, while it clearly restricted the CD and the CSD in HP1α, one of three paralogs. In contrast, DNA did not show a significant effect on the dynamics of all regions examined in another paralog HP1γ. We propose that HP1α undergoes very rapid diffusion due to sliding as a fuzzy complex of the HR and that CD and CSD are tethered with similar dynamics around DNA, which is in agreement with reported molecular dynamics simulations [Watanabe et al. , Biophys. J. 114 : 2336–2352, 2018].