In most species, including Schizosaccharomyces cerevisiae, Cdc2/cyclin B mitosis-inducing kinase remains inhibited during interphase due to phosphorylation of tyrosine residues in the Cdc2 ATP-binding region. This site is phosphorylated by Wee1 kinase and dephosphorylated by Cdc25 phosphatase. In fission yeast, another element of the G2/M-controlled Nim1/Cdr1 kinase has been identified as an effective mitotic inducer. These studies suggest that Nim1 works by inhibiting Wee1, perhaps by direct phosphorylation. Consistent with this model, we report here that Wee1 is hyperphosphorylated in cells that overproduce Nim1. Similarly, Wem1 phosphorylation was reduced in nim1 cells. The highly purified Nim1 kinase phosphorylates Wee1 in vitro, thereby strongly inhibiting Wee1 kinase. These observations indicate that Nim1 promotes the onset of mitosis by inhibiting Wee1.

Introductions

The G2-M phase transition in eukaryotes is regulated by the synergistic and opposite activities of a series of unique protein kinases and phosphatases. This cascade converges on Cdc2, a serine / threonine protein kinase required to enter mitosis. In Schizosaccharomyces pombe, the inactivation of the Cdc2/cyclin B complex is achieved by phosphorylation of tyrosine 15 by Wee1. The role of Wee1 kinase is opposite to that of Cdc25 phosphatase, which dephosphorylates Cdc2 phosphate on tyrosine 15 and activates the Cdc2/cyclin B complex. Little is known about the regulatory signals upstream of cdc25 and wee1. Genetics suggest that mitosis inducer nim1/cdr1 acts upstream of wee1 and may be a negative regulator of wee1

Wee1

Wee1 is a nuclear kinase belonging to the protein kinase Ser/Thr family in S. pombe. With a molecular weight of 96 kDa, Wee1 is a key regulator of cell cycle progression. It inhibits cells from entering mitosis by inhibiting Cdk1, which affects cell size. Wee1 has homologs in many other organisms, including mammals. Regulation of cell size is essential to ensure cell function. In addition to environmental factors such as nutrition, growth factors, and functional load, cell size is also controlled by the cell size checkpoint. Wee1 is part of this checkpoint. It is a kinase that determines the point in time when it enters mitosis, which affects the size of daughter cells. Because cell division occurs prematurely, loss of Wee1 function will produce cells that are smaller than normal daughter cells. Its name comes from the Scottish dialect word wee, meaning small-its discoverer, Paul Nurse, was working at the University of Edinburgh, Scotland at the time of discovery.

Figure 1. Protein structure of Wee1.

Functions

G2/M checkpoint: Wee1 phosphorylates the amino acids Tyr15 and Thr14 of Cdk1, thereby reducing the kinase activity of Cdk1 and preventing it from entering mitosis; in Schizosaccharomyces pombe, further cell growth may occur. Wee1-mediated Cdk1 inactivation has been shown to be the result of substrate competition. During mitotic entry, Wee1 activity is reduced by several regulators, so Cdk1 activity is increased. In Schizosaccharomyces pombe, a protein kinase, Pom1, is located at the cell pole. This activates the pathway by which Cdr2 inhibits Wee1 through Cdr1. Cdk1 itself negatively regulates Wee1 through phosphorylation, leading to a positive feedback loop. Only reduced Wee1 activity is not enough to enter mitosis: cyclin synthesis and Cdk-activated kinase (CAK) activated phosphorylation are also required.

Cell size checkpoints: There is evidence of cell size checkpoints that prevent small cells from entering mitosis. Wee1 plays a role in this checkpoint by coordinating cell size and cell cycle progression.

DNA damage checkpoint: This checkpoint also controls the G2/M transition. In Schizosaccharomyces pombe, this checkpoint delays the mitotic entry of cells with DNA damage, such as caused by gamma rays. The elongation of the G2 phase depends on Wee1; the wee1 mutant does not have an extended G2 phase after γ-ray irradiation.

References:

1. Parker, L. L; et al. Phosphorylation and inactivation of the mitotic inhibitor Weel by the nim1/cdr1 kinase. Nature, 363(6431), 736–738.