FIG. 4.

HP1γ is phosphorylated on serine 93 in cells approaching senescence. (A) Control or HP1γ immunoprecipitates (IP) from growing (control-infected) or senescent (activated Ras-infected) WI38 cells were treated with or without λ-phosphatase, separated by SDS-PAGE, and Western blotted. (B) Alignment of regions of HP1α, -β, and -γ. Residues shaded gray are nonidentical but conserved; residues shaded black are identical. The numbers across the top indicate the residue numbers of candidate phosphoacceptor sites in HP1γ which are not conserved in both HP1α and -β. Residue numbers are taken from sequences in the Swiss-Prot database. (C) HA-tagged wild-type HP1γ and its mutants were ectopically expressed in WI38 cells, together with an activated Ras oncogene, and then Western blotted with anti-HA antibody.