MPF stands for maturation-promoting factor (also called M-phase-promoting factor), a protein complex that acts as the master switch driving cells into division. It was first identified in 1971 and has since become one of the most important concepts in cell biology. MPF is made of two parts: a kinase enzyme called CDK1 and its partner protein, cyclin B. Together, they trigger the dramatic physical changes a cell undergoes when it divides.
How MPF Was Discovered
In 1971, scientist Yoshio Masui was working with frog eggs when he noticed something remarkable. Immature frog eggs that hadn’t been exposed to the hormone progesterone could be pushed into maturation simply by injecting them with cytoplasm taken from progesterone-treated eggs. Something in that cytoplasm was triggering the process. Masui called it maturation-promoting factor. Crucially, removing the nucleus from the donor egg didn’t eliminate the effect, meaning whatever this factor was, it lived in the cytoplasm, not the DNA.
It took nearly two more decades before researchers identified the molecular identity of MPF: a complex of the enzyme CDK1 bound to cyclin B. This discovery helped unlock our modern understanding of how all eukaryotic cells, from yeast to human tissue, coordinate division.
The Two Subunits of MPF
MPF is composed of two proteins that depend on each other. CDK1 (cyclin-dependent kinase 1) is the enzymatic half. It’s a kinase, meaning it works by attaching phosphate groups to other proteins, changing their shape and function. On its own, though, CDK1 is inactive. It needs cyclin B to switch on.
Cyclin B is the regulatory half. Its levels rise and fall predictably during the cell cycle, climbing steadily through the growth phases and peaking just as the cell is ready to divide. When enough cyclin B accumulates and binds to CDK1, the complex becomes active. This pairing is what makes MPF a built-in timer: the cell can’t enter division until cyclin B reaches a sufficient concentration.
How MPF Gets Switched On
Even after cyclin B binds CDK1, the complex doesn’t activate immediately. The cell keeps MPF in a “loaded but locked” state through inhibitory chemical tags. Enzymes called Wee1 and Myt1 attach phosphate groups to CDK1 at specific positions (Tyr15 and Thr14), which block its activity. This allows cyclin B to accumulate throughout the G2 phase without prematurely triggering division.
When the cell is ready, a phosphatase enzyme called Cdc25 strips away those inhibitory phosphate groups, unlocking CDK1. This creates a powerful positive feedback loop: once a small amount of MPF becomes active, it further activates Cdc25, which unlocks even more MPF. At the same time, active MPF inhibits Wee1, removing the brakes. The result is a rapid, all-or-nothing switch from the growth phase into mitosis.
Research in fission yeast has shown that MPF activation begins locally, at structures near the cell’s spindle poles, before spreading throughout the entire cell. This initial burst of activity serves as a trigger event that converts the bulk population of inactive MPF to its active state, committing the cell to division.
What MPF Does to the Cell
Once fully active, MPF phosphorylates dozens of target proteins throughout the cell, setting off the visible hallmarks of division. One of its best-studied targets is the nuclear lamina, the mesh-like protein scaffold that gives the nucleus its shape. MPF attaches phosphate groups to lamin proteins, causing the filaments to disassemble. This is what drives nuclear envelope breakdown, the moment when the membrane around the nucleus dissolves so chromosomes become accessible to the division machinery.
MPF also activates condensin complexes, which coil and compact the long strands of DNA into the tight, rod-shaped chromosomes visible under a microscope. It helps reorganize the cell’s internal skeleton to build the mitotic spindle, the structure that will pull chromosome copies apart. In short, nearly every physical change you associate with a dividing cell, from chromosome condensation to spindle formation to the disappearance of the nucleus, traces back to MPF phosphorylating the right proteins at the right time.
How MPF Gets Switched Off
Exiting division requires MPF to be shut down just as decisively as it was activated. The cell accomplishes this by destroying cyclin B. Once all chromosomes are properly attached to the spindle, a large enzyme complex called the APC/C (anaphase-promoting complex) tags cyclin B with a small protein called ubiquitin. This mark sends cyclin B to the cell’s protein-recycling machinery for rapid degradation.
Without its cyclin B partner, CDK1 becomes inactive and MPF ceases to exist as a functional complex. The phosphate groups MPF added to lamins, condensins, and other targets get removed by phosphatases, and the cell reverses all the changes of mitosis: the nuclear envelope reforms, chromosomes decondense, and the cell physically pinches in two. This destruction of cyclin B is what makes the cell cycle a one-way street. Because the activator is physically destroyed rather than simply turned off, the cell cannot slide backward into mitosis. It must build up a fresh supply of cyclin B before dividing again.
MPF in Modern Terminology
In current scientific literature, MPF is used almost interchangeably with “cyclin B-CDK1.” However, some researchers argue this equivalence is slightly imprecise. The original experiments defined MPF as an activity, meaning the entire cascade of events that leads to CDK1 activation, not just the final protein complex itself. A 2015 review in the journal noted that MPF is best regarded as the entire autoregulatory pathway involved in activating cyclin B-CDK1, with the specifics varying depending on the organism being studied. In practice, though, most textbooks and papers treat MPF and cyclin B-CDK1 as synonyms.
Why MPF Matters Beyond the Textbook
Because MPF is the central switch for cell division, any defect in its regulation can have serious consequences. Cancer cells often have disrupted checkpoint signaling, meaning the safety mechanisms that normally prevent a cell from dividing before it’s ready are weakened or absent. CDK1 and its regulatory partners sit at the heart of these checkpoints, making them targets of interest in cancer drug development. Several classes of anticancer drugs work by interfering with cell cycle checkpoints, exploiting the fact that tumor cells with faulty checkpoint controls are more vulnerable to agents that create DNA damage during division.
MPF’s role isn’t limited to cancer biology. It’s also central to understanding fertility, since egg maturation (the original context of Masui’s discovery) depends on MPF activation. Errors in MPF regulation during egg development can contribute to chromosomal abnormalities, connecting this molecular switch to some of the most common causes of miscarriage and developmental disorders.