CDK1-cyclin B1 is a universal cell cycle kinase required for mitotic/meiotic cell cycle entry and its activity needs to decline for mitotic/meiotic exit. During their maturation, mouse oocytes proceed through meiosis I and arrest at second meiotic metaphase with high CDK1-cyclin B1 activity. Meiotic arrest is achieved by the action of a cytostatic factor (CSF), which reduces cyclin B1 degradation. Meiotic arrest is broken by a Ca²⁺ signal from the sperm that accelerates it. Here we visualised degradation of cyclin ∷ GFP in oocytes and found that its degradation rate was the same for both meiotic divisions. Ca²⁺ was the necessary and sufficient trigger for cyclin B1 destruction during meiosis II; but it played no role during meiosis I and furthermore could not accelerate cyclin B1 destruction during this time. The ability of Ca²⁺ to trigger cyclin B1 destruction developed in oocytes following a restabilisation of cyclin B1 levels at about 12 h of culture. This was independent of actual first polar body extrusion. Thus, in metaphase I arrested oocytes, Ca²⁺ would induce cyclin B1 destruction and the first polar body would be extruded. In contrast to some reports in lower species, we found no evidence that oocyte activation was associated with an increase in 26S proteasome activity. We therefore conclude that Ca²⁺ mediates cyclin B1 degradation by increasing the activity of an E3 ubiquitin ligase. However, this stimulation occurs only in the presence of the ubiquitin ligase inhibitor CSF. We propose a model in which Ca²⁺ directly stimulates destruction of CSF during mammalian fertilisation.
Developmental Biology Vol. 269, Issue 1, p. 206-219