Chromosome condensation and decondensation factors in the life cycle of eukaryotic cells

In this chapter, we have attempted to review our recent work pertaining to the regulation of the chromosome condensation-decondensation cycle within the life cycle of mammalian cells. The results summarized here strongly suggest that this sequence of events may be regulated by different protein factors. Mitotic factors injected into fully grown X. laevis oocytes induce meiotic maturation, i.e., GVBD and chromosome condensation. These factors, which accumulate slowly in the beginning of G2 and reach a threshold at the G2-mitosis transition, have a great affinity for chromatin and are localized on metaphase chromosomes, as well as in the cytoplasm. They are nondialyzable, heat- and Ca2+-sensitive, Mg2+-dependent nonhistone proteins with an approximate molecular mass of 100,000 Da. At the telophase of mitosis, the mitotic factors are rapidly inactivated by another set of factors, IMF. IMF are also nondialyzable nonhistone proteins, but unlike mitotic factors, are heat-stable. They are also stable over a broad pH range, but are extremely sensitive to low pH. IMF are activated at telophase and remain active throughout the G1 period, thus coinciding with the process of chromosome decondensation. Although evidence implicating IMF in the regulation of chromosome decondensation is still largely circumstantial, data summarized here nevertheless suggest a strong correlation between these two phenomena. The way in which mitotic factors and IMF might bring about the condensation-decondensation of chromosomes has not been established. Our studies on the role of protein phosphorylation and the use of monoclonal antibodies specific for mitotic cells have provided some evidence implicating nonhistone protein phosphorylation-dephosphorylation in the regulation of mitosis. A causal link between these events is suggested, but remains to be established. Characterization of these factors will help us learn about their functions, as well as lead to a better understanding of the events regulating the chromosome condensation-decondensation cycle in eukaryotic cells.