Phosphoproteins are components of mitotic microtubule organizing centers.

Abstract

Protein phosphorylation was suggested as an important control mechanism for the events leading toward the initiation and completion of mitosis. Using a monoclonal antibody recognizing a class of phosphoproteins abundant in mitotic [PTK1] cells, the localization of a subset of these phosphoproteins to several discrete mitotic structures was demonstrated. Patchy immunofluorescence was present in the interphase nuclei, but a significant increase in nuclear immunofluorescence was apparent at prophase. Subsequent mitotic stages demonstrated that immunoreactive material was particularly apparent at microtubule organizing centers, i.e., centrosomes, kinetochores and midbodies. Intense centrosomal localization occurred at the prophase-prometaphase transition and persisted until the reformation of the nuclear membrane in early G1. The cytoplasm of mitotic cells also contained immunoreactive material in sharp contrast to interphase cells that exhibited no cytoplasmic fluorescent staining. Much of the diffuse immunofluorescent cytoplasmic material was removed by a brief lysis of the cells with 0.15% Triton X-100 prior to fixation. The localization of the remaining immunoreactive material afer detergent lysis to mitotic microtubule organizing centers suggests that they contain phosphoprotein structural components important, perhaps, in the mitotic phase-interphase transition.