Expression and functional analysis of three isoforms of human heterochromatin-associated protein HP1 in Drosophila

Heterochromatin-associated protein 1 (HP1) is a nonhistone chromosomal protein associated with pericentromeric heterochromatin in Drosophila. HP1-like proteins have also been found associated with heterochromatin in human cells. The goal of this study was to determine whether proteins of the structurally conserved human HP1 family exhibit conserved heterochromatin targeting and silencing properties in Drosophila. We established transgenic lines of Drosophila melanogaster expressing each of the three human HP1 proteins, HP1Hsalpha, HP1HSbeta, and HP1Hsgamma, under the Hsp70 heat shock promoter. We show that all three isoforms of human HP1 are stably expressed in Drosophila and are associated with heterochromatin in Drosophila chromosomes. Like Drosophila HP1, all three human HP1 proteins are delocalized by an HP1-POLYCOMB chimeric protein, implying that both human HP1 and Drosophila HP1 interact in a common protein complex, and that at least some aspects of heterochromatin structure are highly conserved throughout the evolution of eukaryotes. Ectopic expression of two of the three human HP1 family proteins significantly enhances heterochromatic silencing in Drosophila.