Characterization of a Novel Heterochromatin Protein 1 Homolog "HP1c" in the Silkworm, Bombyx mori

Simple Summary Heterochromatin protein 1 (HP1) plays a major role in the formation and maintenance of heterochromatin and in the regulation of gene expression. Five HP1 genes have been found in Drosophila melanogaster and three HP1 genes in Homo sapiens, while in Bombyx mori, two HP1 genes (BmHP1a and BmHP1b) have been reported. In the present study, we analyzed the function of the novel Bombyx mori HP1 gene (BmHP1c), the third HP1 gene in silkworm. BmHP1c has different characteristics from BmHP1a and BmHP1b in terms of transcriptional repression activity, dimer formation, subcellular localization, and effects of RNAi on cell cycle progression. These findings indicate that BmHP1c plays a different role than BmHP1a and BmHP1b. Heterochromatin protein 1 plays an important role in chromatin structure and gene expression regulation. Three HP1 genes have been found in Homo sapiens, and five HP1 genes have been reported in Drosophila melanogaster. On the other hand, in Bombyx mori, only two HP1 genes, BmHP1a and BmHP1b, were reported. In this research, we have reported the molecular and functional characterization of a novel Bombyx mori HP1 gene (BmHP1c), which had stronger transcriptional repression activity than BmHP1a. BmHP1a and BmHP1b is reported to form homo- and heterodimers, but in co-immunoprecipitation experiments, no homo- or hetero-dimer formation of BmHP1c with the other silkworm HP1s is detected. The intracellular localization of BmHP1c is not only in the nucleus but also in the cytoplasm like mammalian HP1 gamma. In contrast to human HP1a and b, all three BmHP1s were localized preferentially in the regions poorly stained with DAPI. Interestingly, the double knockdown of BmHP1a and b, but not BmHP1c with a or b, arrested the cell cycle at the G2/M phase. These results suggest that BmHP1c is not essential for cell progression and plays a different role than BmHP1a and BmHP1b.