Chromatin preferences of the perichromosomal layer constituent pKi-67

The proliferation- associated nuclear protein pKi- 67 relocates from the nucleolus to the chromosome surface during the G2/ M transition of the cell cycle and contributes to the formation of the ' perichromosomal layer'. We investigated the in- vivo binding preferences of pKi- 67 for various chromatin blocks of the mitotic chromosomes from the human and two mouse species, Mus musculus and M. caroli. All chromosomes were decorated with pKi- 67 but displayed a gap of pKi- 67 decoration in the centromere and NOR regions. pKi- 67 distribution in a rearranged mouse chromosome showed that the formation of the centromeric gap was controlled by the specific chromatin in that region. While most chromatin served as a substrate for direct or indirect binding of pKi- 67, we identified three types of chromatin that bound less or no pKi- 67. These were: ( 1) the centromeric heterochromatin defined by the alpha satellite DNA in the human, by the mouse minor satellite in M. musculus and the 60- and 79- bp satellites in M. caroli; ( 2) the pericentromeric heterochromatin in M. musculus defined by the mouse major satellite, and ( 3) NORs in the human and in M. musculus defined by rDNA repeats. In contrast, the conspicuous blocks of pericentromeric heterochromatin in human chromosomes 1, 9 and 16 containing the 5- bp satellite showed intense pKi- 67 decoration. The centromeric gap may have a biological significance for the proper attachment of the chromosomes to the mitotic spindle. In this context, our results suggest a new role for centromeric heterochromatin: the control of the centromeric gap in the perichromosomal layer.