The proliferation-specific human Ki-67 protein is a constituent of compact chromatin

The human nuclear Ki-67 protein (Ki-67p) is expressed in proliferating, but not in quiescent, cells and is therefore widely used as a proliferation marker in histopathological research and practice. However, information regarding its intranuclear location is scarce and controversial. Here we describe the results of cell fractionation and nuclease digestion experiments using nuclei isolated from human HeLa cells in interphase. Ki-67p dissociates at 0.3-0.4 M NaCl from its nuclear binding sites, and gradient centrifugations indicate that the released Ki-67p is most likely a single molecular entity and not complexed to other proteins. In nuclei, prepared under physiological salt conditions, the binding sites are largely resistant against micrococcal nuclease. However, when prepared at very low ionic strengths, chromatin regions with associated Ki-67p become accessible to micococcal-nuclease-producing chromatin fragments that carry bound Ki-67p. We conclude that Ki-67p is a chromatin protein and resides at densely packed regions, probably heterochromatin. Our data provide a useful basis for further biochemical research on this human nuclear protein.