A mammalian homologue of Drosophila heterochromatin protein 1 (HP1) is a component of constitutive heterochromatin

The Drosophila HP1 gene contains a highly conserved sequence, the chromobox, which can be used to isolate HP1-like genes from both mouse (M31 and M32) and man (HSM1) (Singh et al., 1991). Here we report that a monoclonal antibody (MoAb) raised against the M31 protein recognises a 26-kDa protein in murine and human nuclear extracts and localises to large masses of condensed chromatin within murine interphase nuclei, some of which are associated with the nucleoli. At metaphase, the MoAb binds to the centromeres of both human and murine chromosomes. The evolutionary conservation of this chromosomal localisation indicates that the M31 protein is likely to be important in the packaging of mammalian chromosomal DNA into constitutive heterochromatin.