M33, a mammalian homologue of Drosophila Polycomb localises to euchromatin within interphase nuclei but is enriched within the centromeric heterochromatin of metaphase chromosomes

We have raised a monoclonal antibody (MoAb), MAC 402, to the mouse M33 protein, a mammalian homologue of the Drosophila Polycomb protein. MAC 402 recognises a band of 59.7 kDa by Western blot analysis and gives rise to two types of staining pattern within interphase nuclei. Surprisingly, at metaphase, M33 protein is enriched within centromeric heterochromatin. This pattern is also seen for the product of the bmi-1 gene, which is a homologue of the Drosophila Polycomb-group (Pc-G) gene Posterior sex combs. We speculate as to the reasons for this relocalisation of mammalian Pc-G gene products to the centromere at metaphase and its implication for the cell-to-cell inheritance of the states of gene repression that are conferred by this highly conserved group of genes.