HP1 controls genomic targeting of four novel heterochromatin proteins in Drosophila

Heterochromatin is important for the maintenance of genome stability and regulation of gene expression; yet our knowledge of heterochromatin structure and function is incomplete. We identified four novel Drosophila heterochromatin proteins (HPs). Three of these proteins (HP3, HP4 and HP5) interact directly with HP1, whereas HP6 in turn binds to each of these three proteins. Immunofluorescence microscopy and genome-wide mapping of in vivo binding sites shows that all four proteins are components of heterochromatin. Depletion of HP1 causes redistribution of all four proteins, indicating that HP1 is essential for their heterochromatic targeting. Finally, mutants of HP4 and HP5 are dominant suppressors of position effect variegation, demonstrating their importance in heterochromatic gene silencing. These results indicate that HP1 acts as a docking platform for several mediator proteins that contribute to heterochromatin function.