| Protein synthesis in cultured mammalian cells arrested at G2/M phase decreases to about 30% of the rate of interphase cells. To understand the regulatory mechanism of this global reduction, we systematically examined the integrity and modification of translation factors including eIF4GI/II, eIF4E, eIF2α, 4E-BP1, PABP and EF2 during mitosis and interphase. Except a hyper-phosphorylation of 4E-BP1, no significant changes in these factors were observed in mitotic HeLa cells. The hyper-phosphorylation of 4E-BP1 did not affect the efficiency with which cap-binding protein complexes were recruited to mRNA cap structures. This finding may point to novel roles of 4E-BP1 in translational control.; Despite of global reduction of protein synthesis, translation of a few IRES-containing viral and cellular mRNAs had been reported to remain unaffected in mitotic cells. Employing cDNA microarray analysis, we identified that about 2% mRNAs were associated with polysomes, while the majority of mRNAs were occupied by fewer ribosomes in mitotic extracts. Immunoprecipitation assays were used to confirm that the identified mRNAs were indeed translated during mitosis. Intriguingly, seven out of eight tested 5′UTRs of mitotic polysomel-associated mRNA species were found to contain IRES elements. These findings indicate that IRES activities can be modulated in a cell cycle-specific manner. Because IRES-dependent translation prevails during mitosis, it is possible that the spatial organization of the cap-dependent translation machinery is selectively changed, and this change might affect the efficiency of cap-dependent translation.; To address the question whether the dramatic alteration of cellular architecture during mitosis affects the organization of the translational apparatus, we investigated the subcellular distributions of mRNAs, ribosomes and translational factors during interphase and mitosis. We found that ribosomes and cytoplasmic-translated mRNAs were associated with the cytoskeleton during mitosis as well as during interphase. These mRNAs were more efficiently translated when they were associated with the cytoskeleton than free in the cytosol. However, mRNAs that were translated on the rER-bound polysomes during interphase, were relocated onto the cytoskeleton during mitosis. This relocation suggested that, during mitosis, translation of these memebrane-bound mRNAs may be controlled by a mechanism distinct from the mechanism that suppresses the translation of cytoplasm mRNAs. |
