FIGURE 1.
The dimerization interface of HP1a-CSD-CTE has undergone a recent change that reduces its stability. A, multiple sequence alignment of the chromo shadow domains of Drosophila HP1a, HP1b, HP1c, Human/Mouse HP1β, and yeast Swi6. Residues with a blue triangle above them are found in the dimerization interface (9, 12). Residues with a rectangle above or around them were mutated in this study. Brown rectangles: residues important for differences in partner binding observed between HP1a and HP1b. Blue rectangles: mutation impacts chromoshadow domain folding. Red rectangle: mutation has a differential impact on HP2 and PIWI binding in HP1a. Boxed residue: mutation has an impact on CSD dimer stability. Residues that are underlined indicate the last residue of the peptide used for analytical ultracentrifugation (AUC) studies. HP1a sequence from the CSD to the end was used since the last three residues improve its affinity. This length is comparable to that used for HP1b, HP1β, and Swi6. B, top: the crystal structure of Dm HP1a (1s4z) with Arg-188 highlighted together with its pairing amino acid Glu-184. Bottom: the crystal structure of HP1β highlighting the position of Gln-188 (numbering of Dm) and its interacting amino acid Asn-183. C, data collected from equilibrium AUC of HP1a-CSD-CTE-R188Q showing that a monomer/dimer fit to the Lamm equation approximates that data well, as judged by the random distribution of the residuals about the mean.