IDENTIFICATION AND CHARACTERIZATION OF INTERACTIONS BETWEEN THE VERTEBRATE POLYCOMB-GROUP PROTEIN BMI1 AND HUMAN HOMOLOGS OF POLYHOMEOTIC

In Drosophila melanogaster, the Polycomb-group (PcG) genes have been i dentified as repressors of gene expression, They are part of a cellula r memory system that is responsible for the stable transmission of gen e activity to progeny cells, PcG proteins form a large multimeric, chr omatin-associated protein complex, but the identity of its components is largely unknown, Here, we identify two human proteins, HPH1 and BPH 2, that are associated with the vertebrate PcG protein BMI1, HPH1 and HPH2 coimmunoprecipitate and cofractionate with each other and with BM II, They also colocalize with BMII in interphase nuclei of U-2 OS huma n osteosarcoma and SW480 human colorectal adenocarcinoma cells, HPH1 a nd HPH2 have little sequence homology with each other, except in two h ighly conserved domains, designated homology domains I and II, They sh are these homology domains I and II with the Drosophila PcG protein Po lyhomeotic (Ph), and we, therefore, have named the novel proteins HPH1 and HPH2. HPH1, HPH2, and BMI1 show distinct, although overlapping ex pression patterns in different tissues and cell lines, Two-hybrid anal ysis shows that homology domain II of HPH1 interacts with both homolog y domains I and II of HPH2, In contrast, homology domain I of HPH1 int eracts only with homology domain II of HPH2, but not with homology dom ain I of NPH2. Furthermore, BMI1 does not interact with the individual homology domains, Instead, both intact homology domains I and II need to be present for interactions with BMI1. These data demonstrate the involvement of homology domains I and II in protein-protein interactio ns and indicate that HPH1 and HPH2 are able to heterodimerize.