Question

Protein dimerization is an important mechanism in the activation of many cellular mechanisms. Use the MPF factor regulation of the G2 checkpoint as an example to explain the role of dimerization.

Answer 1

when cell enters G2 , the level of cyclin B rises providing the needed subunit for CDk1 to be active.this oscillation is intrinsic to the cell cycle and drives cells into mitosis once per cycle.

damage to DNA and other external factors are evaluated at the G1 checkpoint,if conditions are inadequate the cell will not be allowed to continue to the S phase of interphase.the G2 checkpoint ensures all of the chromosomes have been replicated and that the replicated DNA is not damaged before cell enters mitosis.

MPF is called maturation promoting factor.this study originates with study of oocytes. oocytes are arrested in G2 phase of cell cycle until hormonal stimulation triggers their entry into M phase of meosis.A cytoplasmic factor presnt in present in hormone treated oocytes was sufficient to trigger the transition from G2 to M in oocytes that had not been exposed to hormone.this cytoplasmic factor is called as maturation promoting factor ( MPF). MPF is also present in somatic cells,where it induces entry into M phase of mitotic cycle.MPF acts as a general regulator of transition from G2 to M.

MPF is a complex of proteins that must be present together before the cell can move from one stage to the next.MPF is composed of cyclin dependent kinase(cdk) and cyclin.basically it is a dimer.

during G1 and S phase the CDK1 subunit of MPF is inactive,due to inhibitory enzyme Wee1.Wee1 phosphorylates Tyr15 residues in humans of cdk1, rendering MPF inactive.during transition of G2 to M phase cdk1 is dephosphorylated by CDC25.the cdk1 subunit is now free and can bind to cyclin B, activate MPF and make the cell enters mitosis.

the active form of dimer called MPF activates mitosis of the cell.MPF is regulating its own concentration in 3 ways,on the one hand it promotes its activity by activating CDC25 and inhibiting Wee1.on the other hand it is indirectly involved in degradation of cyclin and therefore its own destruction.so there are 2 positive and one negative feedback loop playing together to regulate the concentration of MPF and thereby the regulation of mitosis onset.in addition to MPF unreplicated DNA plays a role in regulation of MPF activity.it has opposite effect of MPF on Wee1 and CDC25 , meaning it would stop the cell from dividing.