IDENTIFICATION OF A 102-KDA PROTEIN (CYTOCENTRIN) IMMUNOLOGICALLY RELATED TO KERATIN-19, WHICH IS A CYTOPLASMICALLY DERIVED COMPONENT OF THE MITOTIC SPINDLE POLE

The mAb RK7, previously shown to recognize keratin 19, was also found to cross-react with a biologically unrelated 102 kDa protein, which be comes associated with the poles of the mitotic apparatus. This newly i dentified protein, called cytocentrin, is a stable cellular component, may be at least in part phosphorylated, and displays a cell cycle-dep endent cellular localization. In interphase cells, it is diffusely dis tributed in the cytosol and shows no affinity for cytoplasmic microtub ules. It becomes localized to the centrosome in early prophase, prior to nuclear envelope breakdown, separation of replicated centrosomes, a nd nucleation of mitotic apparatus microtubules. During metaphase, cyt ocentrin is located predominately at the mitotic poles, often appearin g as an aggregate of small globular sub-components; it also associates with some polar microtubules. In late anaphase/early telophase cytoce ntrin dissociates entirely from the mitotic apparatus and becomes temp orarily localized with microtubules in the midbody, from which it disa ppears by late telophase. In taxol-treated cells cytocentrin was assoc iated with the center of the miniasters but also showed affinity for s ome cytoplasmic microtubules. Studies employing G2-synchronized cells and nocodazole demonstrated that cytocentrin can become associated wit h mitotic centrosomes independently of tubulin polymerization and that microtubules regrow from antigen-containing foci. We interpret these results to suggest that cytocentrin is a cytoplasmic protein that beco mes specifically activated or modified at the onset of mitosis so that it can affiliate with the mitotic poles where it may provide a link b etween the pericentriolar material and other components of the mitotic apparatus.