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

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 ma rker in histopathological research and practice. However, information regar ding its intranuclear location is scarce and controversial. Here we describ e the results of cell fractionation and nuclease digestion experiments usin g nuclei isolated from human HeLa cells in interphase. Ki-67p dissociates a t 0.3-0.4 M NaCl from its nuclear binding sites, and gradient centrifugatio ns indicate that the released Ki-67p is most likely a single molecular enti ty and not complexed to other proteins. In nuclei, prepared under physiolog ical salt conditions, the binding sites are largely resistant against micro coccal nuclease. However, when prepared at very low ionic strengths, chroma tin 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 r esearch on this human nuclear protein. (C) 2000 Academic Press.