Cell proliferation-associated nuclear antigen defined by antibody Ki-67: a new kind of cell cycle-maintaining proteins.

A decade of studies on the human nuclear antigen defined by monoclonal antibody Ki-67 (the "Ki-67 protein") has made it abundantly clear that this structure is strictly associated with human cell proliferation and that the expression of this protein can be used to assess the growth fraction of a given cell population. Until recently the Ki-67 protein was described as a nonhistone protein that is highly susceptible to protease treatment. We have isolated and sequenced cDNAs encoding for this antigen and found two isoforms of the full length cDNA of 11.5 and 12.5 kb, respectively, sequence and structure of which are thus far unique. The gene encoding the Ki-67 protein is organized in 15 exons and is localized on chromosome 10. The center of this gene is formed by an extraordinary 6845 bp exon containing 16 successively repeated homologous segments of 366 bp ("Ki-67 repeats"), each containing a highly conserved new motif of 66 bp ("Ki-67 motif"). The deduced peptide sequence of this central exon possess 10 ProGluSerThr (PEST) motifs which are associated with high turnover proteins such as other cell cycle-related proteins, oncogenes and transcription factors, etc. Like the latter proteins the Ki-67 antigen plays a pivotal role in maintaining cell proliferation because Ki-67 protein antisense oligonucleotides significantly inhibit 3H-thymidine incorporation in permanent human tumor cell lines in a dose-dependent manner.