Spoke: A 120-kD protein associated with a novel filamentous structure on or near kinetochore microtubules in the mitotic spindle

Abstract

We have characterized an antiserum that recognizes a single 120-kD protein in CHO cells which is soluble and cytoplasmically localized in interphase, but which is associated with a novel filamentous structure localized on or near kinetochore microtubules in mid-mitosis. These filaments, one per sister chromatid, run from near the mitotic spindle pole to within ∼0.3 μm of each kinetochore. In metaphase, the staining pattern shows considerable sub-structure at light microscopy resolution, appearing as bright nodes or striations, often with a kinked or helical appearance. This overall localization pattern is retained throughout anaphase, with the filaments shortening as the chromosomes move toward the mitotic spindle poles. Also in anaphase, a separate ring-like structure lacking a tubulin-staining component appears near the spindle poles. As cells exit mitosis, the amount of this antigen in the cell decreases seven- to tenfold. The unusual staining pattern and the specific localization of this antigen on or near kinetochore microtubules in mid-mitosis indicate that the 120-kD protein defines or is associated with an important and previously unrecognized structural element of the mitotic spindle.