Wee1 is a nuclear kinase belonging to the Ser/Thr family of protein kinases in the fission yeast Schizosaccharomyces pombe (S. pombe). Wee1 has a molecular mass of 96 kDa and is a key regulator of cell cycle progression. It influences cell size by inhibiting the entry into mitosis, through inhibiting Cdk1. Wee1 has homologues in many other organisms, including mammals.
Introduction The regulation of cell size is critical to ensure functionality of a cell. Besides environmental factors such as nutrients, growth factors and functional load, cell size is also controlled by a cellular cell size checkpoint. Wee1 is a component of this checkpoint. It is a kinase determining the timepoint of entry into mitosis, thus influencing the size of the daughter cells. Loss of Wee1 function will produce smaller than normal daughter cell, because cell division occurs prematurely. Its name is derived from the Scottish dialect word wee, meaning small - its discoverer Paul Nurse was working at the University of Edinburgh in Scotland at the time of discovery.
Discovery / History The discovery of the Wee1 gene is accredited to Paul Nurse, who first identified it in fission yeast (Schizosaccharomyces pombe) in 1978. In his initial experiments, Nurse demonstrated Wee1 to be a negative regulator of mitosis, such that Wee1+ activity was critical in preventing premature mitosis in Cdc25+ (a mitotic inducer) yeast cells and increased Wee1+ expression could further delay cell cycle progression until cells grew to be larger in size. Following this data, Nurse, together with Pierre Thuriaux, analyzed fifty two Wee mutants undergoing mitosis – those lacking in Wee1 were comparatively smaller; their analysis led them to a model (later demonstrated to be true) where Wee1 is a dosage-dependent inhibitor of Cdc2, whose activity is required for a cell's entry into M phase. As a result of these discoveries and its contributions to our understanding of cell cycle control, Nurse went on to win the 2001 Nobel Prize in Medicine or Physiology (shared also with Lee Hartwell and Tim Hunt). Wee1 is part of a family of three serine/threonine protein kinases, consisting of Wee1 (also known as WEE1A), PKMYT1, and Wee2 (WEE1B). These three kinases have similar sequences in their respective kinase domains, but exhibit differences in their localization, regulation, and activation patterns. PKMYT1 is the only one of the three that is not typically found in the nucleus, but rather, is associated with the membranes of the endoplasmic reticulum and golgi apparatus. And while Wee1 and PKMYT1 both play critical roles in regulating entry into mitosis (i.e. working together to inhibit Cdk1 as it moves in / out of the cell nucleus), WEE1B was first discovered in Xenopus oocytes and is most active at the metaphase II exit point of meiosis prior to fertilization. Additionally, WEE1B is not as well-researched in the field of oncology as compared to its family members (both of which have better understood roles in cancer development), as there does not seem to be much evidence supporting WEE1B's role in tumorigenesis.
Function
Wee1 inhibits Cdk1 by phosphorylating it on two different sites, Tyr15 and Thr14. Cdk1 is crucial for the cyclin-dependent passage of the various cell cycle checkpoints. At least three checkpoints exist for which the inhibition of Cdk1 by Wee1 is important:
G2/M checkpoint: Wee1 phosphorylates the amino acids Tyr15 and Thr14 of Cdk1, which keeps the kinase activity of Cdk1 low and prevents entry into mitosis; in S. pombe further cell growth can occur. Wee1 mediated inactivation of Cdk1 has been shown to be ultrasensitive as a result of substrate competition. During mitotic entry the activity of Wee1 is decreased by several regulators and thus Cdk1 activity is increased. In S. pombe, Pom1, a protein kinase, localizes to the cell poles. This activates a pathway in which Cdr2 inhibits Wee1 through Cdr1. Cdk1 itself negatively regulates Wee1 by phosphorylation, which leads to a positive feedback loop. The decreased Wee1 activity alone is not sufficient for mitotic entry: Synthesis of cyclins and an activating phosphorylation by a Cdk activating kinase (CAK) are also required. Cell size checkpoint: There is evidence for the existence of a cell size checkpoint, which prevents 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 S. pombe this checkpoint delays the mitosis entry of cells with DNA damage (for example induced by gamma radiation). The lengthening of the G2 phase depends on Wee1; wee1 mutants have no prolonged G2 phase after gamma irradiation. Epigenetic function of Wee1 kinase has also been reported. Wee1 was shown to phosphorylate histone H2B at tyrosine 37 residue which regulated global expression of histones.
Homologues
The WEE1 gene has two known homologues in humans, WEE1 (also known as WEE1A) and WEE2 (WEE1B). The corresponding proteins are Wee1-like protein kinase and Wee1-like protein kinase 2 which act on the human Cdk1 homologue Cdk1. The homologue to Wee1 in budding yeast Saccharomyces cerevisiae is called Swe1.
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