Heterochromatin formation in mammalian cells: Interaction between histonesand HP1 proteins

Members of the heterochromatin protein 1 (HP1) family are silencing nonhist one proteins. Here, we show that in P-19 embryonal carcinoma (EC) nuclei, H P1 alpha, beta, and gamma form homo- and heteromers associated with nucleos omal core histones. In vitro, all three HP1s bind to tailed and tailless nu cleosomes and specifically interact with the histone-fold of histone H3. Fu rthermore, HP1 alpha interacts with the linker histone H1. HP1 alpha binds to H3 and H1 through its chromodomain (CD) and hinge region, respectively. Interestingly, the Polycomb (Pc1/M33) CD also interacts with H3, and HP1 al pha and Pc1/M33 binding to H3 is severely impaired by CD mutations known to abrogate HP1 and Polycomb silencing in Drosophila. These results define a novel function for the conserved CD and suggest that HP1 self-association a nd histone binding may play a crucial role in HP1-mediated heterochromatin assembly.