The conserved HR domain of the Drosophila Suppressor 2 of zeste [Su(z)2] and murine bmi-1 proteins constitutes a locus-specific chromosome binding domain

The related Drosophila Suppressor 2 of zeste [Su(z)2] and Posterior sex combs (Psc) proteins are both locus-specific chromosome binding proteins. They are found at many of the same polytene chromosome loci as other Polycomb-group proteins. The 1,365 amino acid Su(z)2 protein and the 1,603 amino acid Psc protein share a conserved 200 amino acid domain, the homology region (HR). To identify the protein domain responsible for locus-specific chromosome binding, we made a series of Hsp70:cDNA deletion constructs of the Sz(z)2 gene and transformed these into flies. We found that the HR is necessary and sufficient for Su(z)2 locus-specific polytene chromosome binding. The murine Bmi-1 protein also shares the conserved HR domain. When expressed in flies, the Bmi-1 protein showed a locus-specific chromosome binding pattern similar to that of the Su(z)2 and Psc proteins. These results argue that a locus-specific chromosome binding function resides in the HR domain. Other results show that a second, low affinity, non-specific chromosome binding function is localized outside the HR in the Su(z)2 protein, and that the Su(z)2 protein contains at least two nuclear localization signals.