Heterochromatin protein HP1(Hs beta) (p25 beta) and its localization with centromeres in mitosis

Some autoimmune sera containing anticentromere autoantibodies also recognize a doubler of Mr 23 000 (p23) and 25 000 (p25) in addition to CENP (centromere protein)-A (Mr 19 000), -B (Mr 80 000), and -C (Mr 140 000). A p25 antigen (HP1(Hs alpha)) has been shown to be a human homolog of Drosophila HPI (heterochromatin protein I). We have isolated a cDNA clone encoding another form of p25 (HP1(Hs beta) or p25 beta) from a lambda Zap HepG2 library using human autoimmune serum. The deduced amino acid sequence of the clone contained a conserved chromodomain (chromatin modifier domain) in the N-terminal region and a heterochromatin binding domain in the C-terminal region. In immunofluorescence experiments, only affinity purified antibodies reactive with the C-terminal (amino acids 70-185) domain showed nucleoplasmic and heterochromatin staining, whereas N-terminal (amino acids 1-115) specific antibodies were nonreactive. In metaphase chromosome spreads, the C-terminal domain antibody was also localized to the centromeric regions of chromosomes. Association with centromeres was most prominent at anaphase and changed to a more generalized association with whole chromosomes in telophase. The cooccurrence of autoantibodies to centromere proteins and HPI in certain autoimmune diseases might be a reflection of coordinated immune responses to these closely associated sets of proteins.