Heterochromatin protein 1 (HP1) is a nonhistone chromosomal protein, first identified in Drosophila, that plays a dose-dependent role in gene silencing. Three orthologs, HP1α, HP1β, and HP1γ, have been characterized in mammals. While HP1α and HP1β have been unambiguously localized in heterochromatin by immunocytochemical methods, HP1γ has been found either exclusively associated with euchromatin or present in both euchromatin and heterochromatin. Here, using an antibody directed against a peptide epitope at the carboxyl-terminal end of the molecule, we localize HP1γ in both euchromatin and heterochromatin compartments of interphase nuclei, as well as in the pericentromeric chromatin and arms of mitotic chromosomes of 3T3 cells. This dual location was also observed in nuclei expressing HP1γ as a fusion protein with green fluorescent protein. In contrast, when the distribution of HP1γ was analyzed with antibodies directed against an amino-terminal epitope, the protein was detectable in euchromatin and not in heterochromatin, except for transient heterochromatin staining during the late S phase, when the heterochromatin undergoes replication. These data suggest that the controversial immunolocalization of HP1γ in chromatin is due to the use of antibodies directed against topologically distinct epitopes, those present at the amino-terminal end of the molecule being selectively masked in nonreplicative heterochromatin.
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