Final answer:
MPF is composed of cyclin and cdk, with cyclin's levels fluctuating and peaking during G2 and early mitosis. At the end of mitosis, phosphatase enzymes degrade cyclin, leading to the inactivation of MPF and thereby halting its kinase activity. This self-regulation of MPF activity is pivotal for proper cell cycle progress.
Step-by-step explanation:
How MPF Switches Itself Off
MPF, or Maturation Promoting Factor, is crucial for the regulation of the cell cycle, specifically for its role in initiating mitosis. MPF is composed of two subunits: cyclin and cyclin-dependent kinase (cdk). The MPF activity is closely tied to the concentrations of cyclin, which vary throughout the cell cycle. The levels of cyclin increase and peak during the G2 phase and early mitosis, and then degrade towards the end of mitosis. This degradation of cyclin is the key to how MPF turns itself off.
At the end of mitosis, proteins that were phosphorylated by MPF initiate the breakdown of cyclin. This decrease in cyclin concentration leads to the inactivation of MPF as the cyclin subunit is necessary for MPF's kinase activity. Without cyclin, cdk loses its kinase activity, effectively shutting down MPF. This self-regulation ensures that MPF's activity is limited to the appropriate stage of the cell cycle, thus preventing premature or inappropriate entry into mitosis. The discovery of this mechanism contributed to the awarding of the Nobel Prize in Physiology or Medicine in 2001 to Leland H. Hartwell, R. Timothy Hunt, and Paul M. Nurse.