Fig. 5: HP1α oligomerizes through HP1α_CSD.

a Overlayed ¹H,¹⁵N HSQC spectra of ¹⁵N-labeled FL HP1α collected in the presence of increasing amounts of KAP1_Hbox peptide. Spectra are color coded according to the protein:peptide molar ratio. HP1α_CSD signals in FL HP1α are broadened beyond detection, indicating that the size of this part of HP1α becomes too large, i.e. HP1α_CSD multimerization (higher order than detectable dimerization) reduces tumbling rate. b Superimposed ¹H,¹⁵N HSQC spectra of ¹⁵N-labeled FL HP1α (black) and HP1α_CSD (pink). c Molecular mass distribution histograms of FL WT and L139E/L150E HP1α at indicated concentrations in mass photometry assay. Maxima of the fits are labeled (kDa). d Overlayed ¹H,¹⁵N HSQC spectra of ¹⁵N-labeled I165E HP1α_CSD collected in the presence of increasing amounts of unlabeled WT HP1α_CSD. Spectra are color coded according to the molar ratio of CSDs. e The crystal structure of the apo-state of HP1α_CSD is depicted in a ribbon diagram (two dimers form an asymmetric unit, see Supplementary Fig. 9a). Two protomers of one HP1α_CSD dimer (CSD1 and CSD2) are colored wheat and grey, and both protomers of the neighboring dimer are colored yellow. The dimers interact through their β-sheets. f A close view of the β-sheet binding interface, with the residues involved in the contact shown as sticks and labeled. Dashed lines represent hydrogen bonds. g Bar plot of resonance changes in ¹H,¹⁵N HSQC spectra of ¹⁵N-labeled HP1α_CSD’ induced by the 20-fold molar excess of unlabeled HP1α_CSD’. h Most perturbed in (g) residues are mapped on the structure of the apo-HP1α_CSD, colored blue and labeled. i Superimposed ¹H,¹⁵N HSQC spectra of ¹⁵N-labeled FL L139E/L150E HP1α (black) and L139E/L150E HP1α_CSD (light green). j Molecular mass distribution histograms of FL WT and L139E/L150E HP1α in the presence of KAP1_Hbox at indicated concentrations in mass photometry assays. Maxima of the fits are labeled (kDa). k Overlayed ¹H,¹⁵N HSQC spectra of ¹⁵N-labeled L139E/L150E HP1α_CSD recorded in the presence of increasing amounts of KAP1_Hbox peptide. Spectra are color coded according to the protein:peptide molar ratio.