Maturation promoting factor

Maturation-promoting factor (abbreviated MPF, also called mitosis-promoting factor or M-Phase-promoting factor) is a heterodimeric protein composed of cyclin B and cyclin-dependent kinase (CDK1, also known as Cdc2 or p34 kinase) that stimulates the mitotic and meiotic cell cycles. MPF promotes the entrance into mitosis from the G2 phase by phosphorylating multiple proteins needed during mitosis. MPF is activated at the end of G₂ by a phosphatase, which removes an inhibitory phosphate group added earlier.

MPF is made of 2 subunits:

• a subunit that transfers phosphate groups from ATP to specific serine and threonine residues of specific proteins (kinase activity)
• cyclin, a regulatory subunit

Targets of MPF include:

• condensins, which enable chromatin condensation (see prophase)
• various microtubule-associated proteins involved in mitotic spindle formation
• lamins, interaction contributing to degradation of the nuclear envelope
• Histones, H1 and H3
• Golgi matrix, to cause fragmentation

Inhibition of myosin

MPF phosphorylates inhibitory sites on myosin early in mitosis. This prevents cytokinesis. When MPF activity falls at anaphase, the inhibitory sites are dephosphorylated and cytokinesis proceeds.

Activation of MPF

MPF must be activated in order for the cell to transition from G2 to M phase. There are 3 amino acid residues responsible for this G2 to M phase transition. The threonine 161 (Thr-161) on cdk1 must be phosphorylated by a Cyclin Activating Kinase (CAK). CAK only phosphorylates Thr-161 when cyclinB is attached to cdk1.

In addition, two other residues on the cdk1 subunit must be activated. cdc25 removes a phosphate on residues Threonine-14 (Thr-14) and Tyrosine-15 (Tyr-15) and adds a hydroxy group. CyclinB/cdk1 activates cdc25 resulting in a positive feedback loop.

During G1 and S phase, the MPF subunit cdk1 is inactive due to an inhibitory enzyme, Wee1. Wee1 phosphorylizes the Thr-14 and Tyr-15 residues. During the transition of G2 to M phase, MPF inhibits Wee1, again resulting in a positive feedback loop.

Disassembly by APC

MPF is disassembled when anaphase-promoting complex (APC) polyubiquitinates cyclin B, marking it for degradation in a negative feedback loop. As the concentration of CyclinB/cdk1 increases, the heterodimer promotes APC to polyubiquitinate CyclinB/cdk1.

Regulator of Cell Cycle

In 1971, two independent teams of researchers (Yoshio Masui and Clement Markert, as well as Dennis Smith and Robert Ecker) found that frog oocytes arrested in G2 could be induced to enter M phase by microinjection of cytoplasm from oocytes that had been hormonally stimulated. Because the entry of oocytes into meiosis is frequently referred to as oocyte maturation, this cytoplasmic factor was called maturation promoting factor (MPF). Further studies showed, however, that the activity of MPF is not restricted to the entry of oocytes into meiosis. To the contrary, MPF is also present in somatic cells, where it induces entry into M phase of the mitotic cycle .

The MPF is also called the M phase kinase because of its ability to phosphorylate target proteins at a specific point in the cell cycle and thus controlling their ability to function. The MPF has two subunits: The Cyclins and the Cyclin dependent Kinases(CDK) The cyclins are the regulatory subunits that are necessary for the kinase to function with appropriate substrate. The mitotic cyclins can be grouped as cyclins A & B. The CDK is the catalaytic subunit that phosphorylates serine and threonine residues in target proteins.

Role of MPF

The MPF is a heterodimeric protein that act as the B type cyclins. These cyclins have a nine residue sequence in the N- terminal region called as the “destruction box”, which can be recognized by the ubiquitin ligase enzyme and thus destroys the cyclins. In intact cells, cyclin degradation begins shortly after the onset of anaphase (late anaphase), the period of mitosis when sister chromatids are separated and pulled toward opposite spindle poles.

Biochemical studies with Xenopus egg extracts showed that after their synthesis, wild-type mitotic cyclins are modified by addition of ubiquitin, a highly conserved, 76-residue protein. Covalent attachment of chains of ubiquitin, a process called polyubiquitination, marks proteins for rapid degradation in eukaryotic cells by proteasomes, multiprotein cylindrical structures containing numerous proteases

Main Function of MPF

• Triggers the formation of mitotic spindle.
• Promotes mitosis i.e. chromatin condensation.
• Causes nuclear envelope breakdown by phosphorylating the lamins that form an intermediate filament-type network (nuclear lamina) underlying the inner nuclear membrane.
• The three lamins present in the nuclear lamina, lamin A,B & C, are phosphorylated by MPF at serine amino residues. This leads to depolymerisation of nuclear lamina & breakdown of nuclear envelope into small vesicles.