Chromatin-association of the Polycomb group protein BMI1 is cell cycle-regulated and correlates with its phosphorylation status

The human proto-oncogene Bmi1 is a member of the mammalian Polycomb Group ( Pc-G) genes. The subnuclear distribution of the BMI1 protein was studied in several primary human and tumor-derived cell lines using immunohistochemic al and biochemical methods. In primary and tumor cells, nuclear BMI1 shows a fine-grain distribution over chromatin, usually dense in interphase nucle i and significantly weaker along mitotic chromosomes. In addition, BMI1 pre ferentially associates with several distinct heterochromatic domains in tum or cell lines. In both primary and tumor cell lines a marked cell cycle-reg ulation of Pc-G-chromatin interaction is observed: nuclear BMI1-staining di ssipates in late S phase and is reestablished early in G(1)-phase, Chromati n-association of BMI1 inversely correlates with its phosphorylation status in a cell cycle-dependent fashion: at G(1)/S, hypophosphorylated BMI1 is sp ecifically retained in the chromatin-associated nuclear protein fraction, w hereas during G(2)/M, phosphorylated BMI1 is not chromatin-bound. Our findi ngs indicate a strict cell cycle-controlled regulation of Pc-G complex-chro matin association and provide molecular tools for improving our understandi ng of Pc-G complex regulation and function in mammalian cells.