HP1 is a heterochromatin-associated protein, which was first described in Drosophila. It plays an important role in the silencing of genes located next to heterochromatin. HP1 interacts with important architectural proteins of the nucleus like the lamin B receptor (LBR), the origin recognition complex protein (
orc2), the inner centromere protein (INCENP) and the SET domain protein Su(var)3-9. We are interested in analysing the cytological function of HP1 in C. elegans , an organism that has either no or only small amounts of heterochromatin. Two HP1 homologues can be identified in C. elegans : HP1.1 (K08H2.6) and HP1.2 (K01G5.2a). We created integrated transgenic lines which express either HP1.1::GFP or a mixture of HP1.1::YFP and histone H1.1::CFP. The latter serves as a living color stain of chromosomes and chromatin. Cell cycles of embryonic cells were recorded as time series of confocal laser scanning images. During interphase, HP1.1 is prominently concentrated in a number of distinct spot-like structures (mostly 6) in the nuclear periphery. During mitosis this static localization of HP1 disappears and HP1.1 relocates to chromosomal sub-structures in a very dynamic way. In metaphase a fraction of HP1.1 is concentrated at the telomeres of the chromosomes, and another fraction is co-localized with H1.1::CFP. During anaphase HP1.1 separates from the chromatin and forms a thin layer between the chromosomes and the mitotic spindle. This corresponds to the outer part of the holocentric centromeres. Beginning with the 60-cell stage of embryogenesis, HP1.1::GFP is expressed in most tissues. dsRNA interference with HP1.1 expression showed multiple and variable defects including embryonic death, slow growth, dumpy-like animals, larval arrest, and sterility. Interestingly, HP1.1 is not involved in the prominent chromatin silencing in the germ line of C. elegans as shown by HP1.1 RNAi in LET-858::GFP reporter animals. The molecular mechanisms of HP1.1 localization to the spot-like structures in interphase were analyzed by RNAi with established HP1 interacting proteins in the HP1.1::GFP reporter strain. RNAi with the set domain proteins C41G7.4 and C15H11.5 relocated HP1.1::GFP to the cytoplasm. RNAi with the lamin B receptor (B0250.7) led to a diffuse intranuclear distribution of HP1.1::GFP in the nuclei. RNAi with
orc2 (F59E10.1) expression resulted in misregulation of the quantity of HP1.1 expression and a modified intranuclear distribution. We conclude that the distinct spot-like localization of HP1.1 in the interphase nucleus is dependent on all four potential HP1 interacting proteins.