Nuclear responses to MPF activation and inactivation in Xenopus oocytes and early embryos

The cell cycle of most organisms is highlighted by characteristic changes i n the appearance and activity of the nucleus. Structural changes in the nuc leus are particularly evident when a cell begins to divide. At this time, t he nuclear envelope is disassembled, the chromatin condenses into metaphase chromosomes, and the chromosomes associate with a newly formed spindle. Up on completion of cell division the nuclear envelope reassembles around the chromosomes as they form telophase nuclei, and subsequently interphase nucl ei, in the daughter cells. The cytoplasmic control of nuclear behavior has been the theme of Yoshio Masui's research for much of his career. His pione ering demonstration that the cytoplasm of maturing amphibian oocytes causes the resumption of the meiotic cell cycle when it is injected into an immat ure oocyte provided unequivocal evidence that a cytoplasmic factor could in itiate the transition from interphase to metaphase (M-phase) in intact cell s. As described in several reviews in this and the previous issue of Biolog y of the Cell (see Beckhelling and Ford; Duesbery and Vande Woude; Mailer), Masui initially called this activity maturation promoting factor (MPF), bu t when it was realized that it was a ubiquitous regulator of both mitotic a nd meiotic cell cycles, MPF came to stand for M-phase promoting factor. Bio chemical evidence indicates that MPF activity is composed of a mitotic B-ty pe cyclins and cyclin-dependent kinase 1. The increase in the protein kinas e activity of cdkl initiates the changes in the nucleus associated with ooc yte maturation and with the entry into mitosis. This article will attempt t o provide a brief summary of the responses of the nucleus to the activation of MPF In addition, the effect of MPF inactivation on nuclear envelope ass embly at the end of mitosis will be discussed. This article is written as a tribute to Yoshio Masui on his retirement from the University of Toronto, and as an expression of gratitude for his guidance while I was a student in his laboratory. I have felt very privileged to have known him as a mentor and a friend.((C) Elsevier, Paris).