MOS ONCOGENE PRODUCT ASSOCIATES WITH KINETOCHORES IN MAMMALIAN SOMATIC-CELLS AND DISRUPTS MITOTIC PROGRESSION

The mos protooncogene has opposing effects on cell cycle progression. It is required for reinitiation of meiotic maturation and for meiotic progression through metaphase II, yet it is an active component of cyt ostatic factor. mos is a potent oncogene in fibroblasts, but high leve ls of expression are lethal. The lethality of mos gene expression in m ammalian cells could be a consequence of a blockage induced by its cyt ostatic factor-related activity, which may appear at high dosage in mi totic cells. We have directly tested whether expression of the Mos pro tein can block mitosis in mammalian cells by microinjecting a fusion p rotein between Escherichia coli maltose-binding protein and Xenopus c- Mos into PtK1 epithelial cells and analyzing the cells by video time-l apse and immunofluorescence microscopy. Time-course analyses showed th at Mos blocked mitosis by preventing progression to a normal metaphase . Chromosomes frequently failed to attain a bipolar orientation and we re found near one pole. Injection of a kinase-deficient mutant Mos had no effect on mitosis, indicating that the blockage of mitotic progres sion required Mos kinase activity. Antitubulin immunostaining of cells blocked by Mos showed that microtubules were present but that spindle morphology was abnormal. Immunostaining for the Mos fusion protein sh owed that both wild-type and kinase mutant proteins Idealized at the k inetochores. Our results suggest that mitotic blockage by Mos may resu lt from an action of the Mos kinase on the kinetochores, thus increasi ng chromosome instability and preventing normal congression.