pp39mos is associated with p34cdc2 kinase in c-mosxe-transformed NIH 3T3 cells

Evidence of an interaction between Mos and Hsp70: a role of the Mos residue serine 3 in mediating Hsp70 association

c-Mos is a germ cell-specific MAP kinase kinase kinase (MAPKKK) that plays an essential role during meiotic divisions of oocytes. c-Mos is a key component of an activity, cytostatic factor, required for metaphase II arrest of unfertilized eggs in vertebrates. To understand the regulation of c-Mos, we are investigating c-Mos-interacting proteins. We provide evidence that mouse c-Mos binds to Hsp70, a molecular chaperone. Hsp70 was found to associate with Mos ectopically expressed in COS-1 cells. Mos-Hsp70 complexes could be immunoprecipitated with both Mos and Hsp70 antibodies. Despite a low-abundance of Mos, the Hsp70 antibody immunoprecipitated Mos as the major protein. Of importance, the Mos protein present in anti-Hsp70 immunoprecipitates functioned as an active MAPKKK indicating that it is not grossly misfolded. It is known that c-Mos protein kinase activity in cell extracts of transfected COS-1 or NIH3T3 cells is labile. We found that the inclusion of adenosine triphosphate (ATP) in cell extracts protected against the loss of Mos kinase activity. In the absence of ATP from cell extracts, protein kinase activity of Mos was lost within 6 h on ice even though the Mos protein was not degraded and remained bound to Hsp70. Based on our identification of c-Mos-Hsp70 interaction, one of the roles of ATP may be to assist the regulation of c-Mos via ATP involvement in the protein-folding function of Hsp70 and possibly other molecular chaperones. We also detected by coimmunoprecipitation a physical association between endogenous c-Mos and Hsp70 in Xenopus eggs. To provide further evidence for the functional significance of Hsp70 interaction to Mos function, we show that the residue serine 3 in Mos, which is important for the regulation of protein kinase activity of Mos is also important for Hsp70 association.

The N-terminal domain of c-Myc associates with alpha-tubulin and microtubules in vivo and in vitro

The polymerization of alpha- and beta-tubulin into microtubules results in a complex network of microfibrils that have important structural and functional roles in all eukaryotic cells. In addition, microtubules can interact with a diverse family of polypeptides which are believed to directly promote the assembly of microtubules and to modulate their functional activity. We have demonstrated that the c-Myc oncoprotein interacts in vivo and in vitro with alpha-tubulin and with polymerized microtubules and have defined the binding site to the N-terminal region within the transactivation domain of c-Myc. In addition, we have shown that c-Myc colocalizes with microtubules and remains tightly bound to the microtubule network after detergent extraction of intact cells. These findings suggest a potential role for Myc-tubulin interaction in vivo.

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 cytostatic factor. mos is a potent oncogene in fibroblasts, but high levels of expression are lethal. The lethality of mos gene expression in mammalian cells could be a consequence of a blockage induced by its cytostatic factor-related activity, which may appear at high dosage in mitotic cells. We have directly tested whether expression of the Mos protein can block mitosis in mammalian cells by microinjecting a fusion protein between Escherichia coli maltose-binding protein and Xenopus c-Mos into PtK1 epithelial cells and analyzing the cells by video time-lapse and immunofluorescence microscopy. Time-course analyses showed that Mos blocked mitosis by preventing progression to a normal metaphase. Chromosomes frequently failed to attain a bipolar orientation and were found near one pole. Injection of a kinase-deficient mutant Mos had no effect on mitosis, indicating that the blockage of mitotic progression 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 showed that both wild-type and kinase mutant proteins localized at the kinetochores. Our results suggest that mitotic blockage by Mos may result from an action of the Mos kinase on the kinetochores, thus increasing chromosome instability and preventing normal congression.