The Ki-67 protein: fascinating forms and an unknown function.

The Ki-67 protein is a nuclear and nucleolar protein, which is tightly associated with somatic cell proliferation. Antibodies raised against the human Ki-67 protein paved the way for the immunohistological assessment of cell proliferation, particularly useful in numerous studies on the prognostic value of cell growth in clinical samples of human neoplasms. The primary structure revealed potential phosphorylation site for a range of essential kinases, PEST sequences, and a forkhead-associated domain, which are features present in a variety of cell-cycle-regulating proteins, but information about the position of the Ki-67 protein within the protein network that drives the cell cycle remained scarce. There is now evidence that posttranslational modifications based on phosphorylation by cdc2 kinase and PKC accompany the remarkable redistribution of the Ki-67 protein from the interior of the nucleus to the perichromosomal layer during mitosis and vice versa. The discovery of Ki-67 equivalents in other species is advantageous for a precise and cross-species investigation of the structural requirements for its yet unknown function. The recently published data add new pieces to the challenging puzzle of this multifaceted protein, which are waiting to be put together.