Immunolocalization of the 100 KDa Nucleolar Protein During the Mitotic Cycle in CHO Cells

The localization of a major nucleolar protein with a molecular weight of 100,000 has been followed during mitosis in Chinese hamster ovary CHO cells using specific antibodies to this protein and immunocytochemical techniques. The 100 kDa protein was visualized at discrete sites on metaphase chromosomes, corresponding to nucleolus organizer regions, and in large, immunostained nucleolar remnants that are discarded in the cytoplasm after nucleolar disintegration. After mitosis, the 100 kDa protein was shown to play an early role in nucleolar reformation. It was first detected in small deposits around the anaphase chromosomes. In telophase, the protein accumulated simultaneously in prenucleolar bodies and in the reforming nucleoli. The early presence of the 100 kDa protein in the telophase nucleus suggests that it is essential for the reestablishment of nucleolar function after mitosis. Thus this protein is present throughout the CHO cell cycle, an observation which supports the hypothesis that it plays a fundamental role in cell organization.