M-Phase promoting factor (MPF) is a complex of p34cdc2 and cyclin B. Results of previous studies in which relative mass amounts of these cell cycle regulators were determined suggested that the accumulation of p34cdc2, rather than cyclin B, could be a limiting factor in the acquisition of meiotic competence in mouse oocytes. Nevertheless, in the absence of measurements of the absolute amount of these components of MPF, it is possible that the molar amount of p34cdc2 is in excess to that of cyclin B, i.e., the accumulation of p34cdc2 is not a limiting factor. We report measurements of the absolute mass of p34cdc2 and cyclin B1, as well as the two proximal regulators of MPF, namely cdc25C and wee1, in meiotically incompetent and competent mouse oocytes. We find that the numbers of molecules of p34cdc2, cyclin B1, cdc25C, and wee1 in meiotically incompetent oocytes are 1.4 × 106, 11.3 × 106, 24.6 × 106, 15.6 × 106, respectively, and in meiotically competent oocytes the numbers are 14.3 × 106, 95.5 × 106, 80.0 × 106, 40.1 × 106, respectively. Thus, the concentration of cyclin B1 is always in excess to that of p34cdc2, and this is consistent with the hypothesis that the accumulation of p34cdc2 plays a role in the acquisition of meiotic competence. Last, the concentration of cdc25C is greater than that of wee1 and the concentration of each is greater than that of p34cdc2 in both meiotically incompetent and competent oocytes.
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