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Concept
Wee1
Summary
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.
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.
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.
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Cyclin-dependent kinase 1
Cyclin-dependent kinase 1 also known as CDK1 or cell division cycle protein 2 homolog is a highly conserved protein that functions as a serine/threonine protein kinase, and is a key player in cell cycle regulation. It has been highly studied in the budding yeast S. cerevisiae, and the fission yeast S. pombe, where it is encoded by genes cdc28 and cdc2, respectively. With its cyclin partners, Cdk1 forms complexes that phosphorylate a variety of target substrates (over 75 have been identified in budding yeast); phosphorylation of these proteins leads to cell cycle progression.
Cdc25
Cdc25 is a dual-specificity phosphatase first isolated from the yeast Schizosaccharomyces pombe as a cell cycle defective mutant. As with other cell cycle proteins or genes such as Cdc2 and Cdc4, the "cdc" in its name refers to "cell division control". Dual-specificity phosphatases are considered a sub-class of protein tyrosine phosphatases. By removing inhibitory phosphate residues from target cyclin-dependent kinases (Cdks), Cdc25 proteins control entry into and progression through various phases of the cell cycle, including mitosis and S ("Synthesis") phase.
Cell cycle checkpoint
Cell cycle checkpoints are control mechanisms in the eukaryotic cell cycle which ensure its proper progression. Each checkpoint serves as a potential termination point along the cell cycle, during which the conditions of the cell are assessed, with progression through the various phases of the cell cycle occurring only when favorable conditions are met. There are many checkpoints in the cell cycle, but the three major ones are: the G1 checkpoint, also known as the Start or restriction checkpoint or Major Checkpoint; the G2/M checkpoint; and the metaphase-to-anaphase transition, also known as the spindle checkpoint.