Wee1
Encyclopedia
Wee1 is a nuclear kinase
belonging to the Ser/Thr family
of protein kinases in the fission yeast Schizosaccharomyces pombe
(S. pombe). It has a molecular mass
of 96 kDa
and it is a key regulator of cell cycle
progression.
It influences cell size by inhibiting the entry into mitosis
, through inhibiting Cdc2
. It has homologue
s in many other organisms, including mammals.
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 Cdc2 by phosphorylating it on two different sites, Tyr15 and Thr14. Cdc2 is crucial for the cyclin-dependent passage of the various cell cycle checkpoints.
At least three checkpoints exist for which the inhibition of Cdc2 by Wee1 is important:
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 Cdc2 homologue Cdk1
.
The homologue to Wee1 in budding yeast Saccharomyces cerevisiae
is called Swe1.
Cdc2
and cyclin B
make up the maturation promoting factor
(MPF) which promotes the entry into mitosis. It is inactivated by phosphorylation through Wee1 and activated by the phosphatase Cdc25
. Cdc25 in turn is activated by Polo kinase
and inactivated by Chk1
. Thus in S. pombe Wee1 regulation is mainly under the control of phosphorylation
through the polarity kinase, Pom1
's, pathway including Cdr2 and Cdr1.
At the G2/M transition, Cdc2 is activated by Cdc25 through dephosphorylation of Tyr15. At the same time, Wee1 is inactivated through phosphorylation at its C-terminal catalytic domain by Nim1/Cdr1. Also, the active MPF will promote its own activity by activating Cdc25 and inactivating Wee1, creating a positive feedback loop
, though this is not yet understood in detail.
Higher eukaryotes regulate Wee1 via phosphorylation and degradation
In higher eukaryote
s, Wee1 inactivation occurs both by phosphorylation and degradation
.
The protein complexβ-transducin repeat-containing protein 1/2
(β-TrCP1/2) F-box protein-containing SKP1/Cul1/F-box protein complex SCF
β-TrCP1/2 is an E3 ubiquitin ligase
that functions in Wee1A
ubiquitination. The M-phase
kinases Polo-like kinase (Plk1) and Cdc2 phosphorylate two serine residues in Wee1A which are recognized by SCFβ-TrCP1/2.
S. cerevisiae homologue Swe1
In S. cerevisiae, cyclin-dependent kinase Cdc28 (Cdc2 homologue) is phosphorylated by Swe1 (Wee1 homologue) and dephosphorylated by Mih1 (Cdc25 homologue). Nim1/Cdr1 homologue in S. cerevisiae, Hsl1, together with its related kinases Gin4 and Kcc4 localize Swe1 to the bud-neck
. Bud-neck associating kinases Cla4 and Cdc5 (polo kinase homologue) phosphorylate Swe1 at different stages of the cell cycle. Swe1 is also phosphorylated by Clb2-Cdc28 which serves as a recognition for further phosphorylation by Cdc5.
The S. cerevisiae protein Swe1 is also regulated by degradation. Swe1 is hyperphosphorylated by Clb2-Cdc28 and Cdc5 which may be a signal for ubiquitination and degradation by SCF
E3 ubiquitin ligase complex as in higher eukaryotes.
induced apoptosis
. Negative regulation of MPF by WEE1 causes aberrant mitosis and thus resistance to DNA-damage induced apoptosis. Kruppel-like factor 2
(KLF2) negatively regulates human WEE1, thus increasing sensitivity to DNA-damage induced apoptosis in cancer cells.
inhibitor of mitosis. Thus, the amount of Wee1 protein correlates with the size of the cells:
The fission yeast mutant
wee1, also called wee1-, divides at a significantly smaller cell size than wildtype cells. Since Wee1 inhibits entry into mitosis, its absence will lead to division at a premature stage and sub-normal cell size. Conversely,
when Wee1 expression is increased, mitosis is delayed and cells grow to a large size before dividing.
Protein kinase
A protein kinase is a kinase enzyme that modifies other proteins by chemically adding phosphate groups to them . Phosphorylation usually results in a functional change of the target protein by changing enzyme activity, cellular location, or association with other proteins...
belonging to the Ser/Thr family
Serine/threonine-specific protein kinase
Serine/threonine protein kinases phosphorylate the OH group of serine or threonine .At least 125 of the 500+ human protein kinases are serine/threonine kinases .-Regulation:...
of protein kinases in the fission yeast Schizosaccharomyces pombe
Schizosaccharomyces pombe
Schizosaccharomyces pombe, also called "fission yeast", is a species of yeast. It is used as a model organism in molecular and cell biology. It is a unicellular eukaryote, whose cells are rod-shaped. Cells typically measure 3 to 4 micrometres in diameter and 7 to 14 micrometres in length...
(S. pombe). It has a molecular mass
Molecular mass
The molecular mass of a substance is the mass of one molecule of that substance, in unified atomic mass unit u...
of 96 kDa
Atomic mass unit
The unified atomic mass unit or dalton is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of...
and it is a key regulator of cell cycle
Cell cycle
The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...
progression.
It influences cell size by inhibiting the entry into mitosis
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
, through inhibiting Cdc2
Cdk1
Cyclin dependent kinase 1 also known as Cdk1 or cell division control protein 2 homolog is a highly conserved protein that functions as a serine/threonine 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....
. It has homologue
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
s 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
Kinase
In chemistry and biochemistry, a kinase is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates, a process referred to as phosphorylation. Kinases are part of the larger family of phosphotransferases...
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
Scottish English
Scottish English refers to the varieties of English spoken in Scotland. It may or may not be considered distinct from the Scots language. It is always considered distinct from Scottish Gaelic, a Celtic language....
dialect word wee, meaning small - its discoverer Paul Nurse
Paul Nurse
Sir Paul Maxime Nurse, PRS is a British geneticist and cell biologist. He was awarded the 2001 Nobel Prize in Physiology or Medicine with Leland H. Hartwell and R...
was working at the University of Edinburgh in Scotland at the time of discovery.
Function
At least three checkpoints exist for which the inhibition of Cdc2 by Wee1 is important:
- G2/M checkpoint: Wee1 phosphorylates the amino acids Tyr15 and Thr14 of Cdc2, which keeps the kinase activity of Cdc2 low and prevents entry into mitosisMitosisMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
; in S. pombeSchizosaccharomyces pombeSchizosaccharomyces pombe, also called "fission yeast", is a species of yeast. It is used as a model organism in molecular and cell biology. It is a unicellular eukaryote, whose cells are rod-shaped. Cells typically measure 3 to 4 micrometres in diameter and 7 to 14 micrometres in length...
further cell growth can occur. During mitotic entry the activity of Wee1 is decreased by several regulators and thus Cdc2 activity is increased. In S. pombe, Pom1Pom1Pom1 is a polarity protein kinase in fission yeast, S. pombe, that localizes to cell ends. The gene pom1 codes for a protein 1087 amino acids long with the protein kinase domain likely located at the carboxyl terminus. Pom1 regulates a signaling pathway that includes Cdk1 and ultimately regulates...
, a protein kinase, localizes to the cell poles. This activates a pathway in which Cdr2 inhibits Wee1 through Cdr1. Cdc2 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 cyclinCyclinCyclins are a family of proteins that control the progression of cells through the cell cycle by activating cyclin-dependent kinase enzymes.- Function :...
s and an activating phosphorylation by a cyclin 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 mitois. 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 radiationGamma rayGamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
). The lengthening of the G2 phase depends on Wee1; wee1 mutants have no prolonged G2 phase after gamma irradiation.
Homologues
The WEE1 geneGene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
has two known homologues in humans, WEE1 (also known as WEE1A) and WEE2 (WEE1B). The corresponding proteins are Wee1-like protein kinase
Wee1-like protein kinase
WEE1 homolog , also known as WEE1, is a protein which in humans is encoded by the WEE1 gene.- Function :This gene encodes a nuclear protein, which is a tyrosine kinase belonging to the Ser/Thr family of protein kinases...
and Wee1-like protein kinase 2 which act on the human Cdc2 homologue Cdk1
Cdk1
Cyclin dependent kinase 1 also known as Cdk1 or cell division control protein 2 homolog is a highly conserved protein that functions as a serine/threonine 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....
.
The homologue to Wee1 in budding yeast Saccharomyces cerevisiae
Saccharomyces cerevisiae
Saccharomyces cerevisiae is a species of yeast. It is perhaps the most useful yeast, having been instrumental to baking and brewing since ancient times. It is believed that it was originally isolated from the skin of grapes...
is called Swe1.
Regulation
In S. pombe, Wee1 is phosphorylatedCdc2
Cdk1
Cyclin dependent kinase 1 also known as Cdk1 or cell division control protein 2 homolog is a highly conserved protein that functions as a serine/threonine 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....
and cyclin B
Cyclin B
Cyclin B is a member of the cyclin family.Cyclin B is a mitotic cyclin. The amount of cyclin B and the activity of the cyclin B-Cdk complex rise through the cell cycle until mitosis, where they fall abruptly due to degradation of cyclin B...
make up the maturation promoting factor
Maturation promoting factor
Maturation-promoting factor is a heterodimeric protein composed of cyclin B and cyclin-dependent kinase that stimulates the mitotic and meiotic cell cycles...
(MPF) which promotes the entry into mitosis. It is inactivated by phosphorylation through Wee1 and activated by the phosphatase Cdc25
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 such as Cdc2 and Cdc4, the "cdc" in its name refers to "cell division cycle".Dual-specificity phosphatases are considered a sub-class...
. Cdc25 in turn is activated by Polo kinase
Polo kinase
In enzymology, a polo kinase is a kinase enzyme i.e. one that catalyzes the chemical reactionThus, the two substrates of these enzymes are ATP and protein, whereas their two products are ADP and phosphoprotein....
and inactivated by Chk1
CHEK1
Serine/threonine-protein kinase Chk1 is an enzyme that in humans is encoded by the CHEK1 gene.Chk1 is a kinase that phosphorylates cdc25, an important phosphatase in cell cycle control, particularly for entry into mitosis. Cdc25, when phosphorylated on serine 216 by chk1 becomes bound by an adaptor...
. Thus in S. pombe Wee1 regulation is mainly under the control of phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
through the polarity kinase, Pom1
Pom1
Pom1 is a polarity protein kinase in fission yeast, S. pombe, that localizes to cell ends. The gene pom1 codes for a protein 1087 amino acids long with the protein kinase domain likely located at the carboxyl terminus. Pom1 regulates a signaling pathway that includes Cdk1 and ultimately regulates...
's, pathway including Cdr2 and Cdr1.
At the G2/M transition, Cdc2 is activated by Cdc25 through dephosphorylation of Tyr15. At the same time, Wee1 is inactivated through phosphorylation at its C-terminal catalytic domain by Nim1/Cdr1. Also, the active MPF will promote its own activity by activating Cdc25 and inactivating Wee1, creating a positive feedback loop
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...
, though this is not yet understood in detail.
Higher eukaryotes regulate Wee1 via phosphorylation and degradation
In higher eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s, Wee1 inactivation occurs both by phosphorylation and degradation
Proteolysis
Proteolysis is the directed degradation of proteins by cellular enzymes called proteases or by intramolecular digestion.-Purposes:Proteolysis is used by the cell for several purposes...
.
The protein complexβ-transducin repeat-containing protein 1/2
BTRC (gene)
F-box/WD repeat-containing protein 1A also known as βTrCP1 or Fbxw1 or hsSlimb or pIkappaBalpha-E3 receptor subunit is a protein that in humans is encoded by the BTRC gene....
(β-TrCP1/2) F-box protein-containing SKP1/Cul1/F-box protein complex SCF
SCF complex
Skp, Cullin, F-box containing complex is a multi-protein E3 ubiquitin ligase complex catalyzing the ubiquitination of proteins destined for proteasomal degradation...
β-TrCP1/2 is an E3 ubiquitin ligase
Ubiquitin ligase
A ubiquitin ligase is a protein that in combination with an E2 ubiquitin-conjugating enzyme causes the attachment of ubiquitin to a lysine on a target protein via an isopeptide bond; the E3 ubiquitin ligase targets specific protein substrates for degradation by the proteasome...
that functions in Wee1A
Wee1-like protein kinase
WEE1 homolog , also known as WEE1, is a protein which in humans is encoded by the WEE1 gene.- Function :This gene encodes a nuclear protein, which is a tyrosine kinase belonging to the Ser/Thr family of protein kinases...
ubiquitination. The M-phase
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
kinases Polo-like kinase (Plk1) and Cdc2 phosphorylate two serine residues in Wee1A which are recognized by SCFβ-TrCP1/2.
S. cerevisiae homologue Swe1
In S. cerevisiae, cyclin-dependent kinase Cdc28 (Cdc2 homologue) is phosphorylated by Swe1 (Wee1 homologue) and dephosphorylated by Mih1 (Cdc25 homologue). Nim1/Cdr1 homologue in S. cerevisiae, Hsl1, together with its related kinases Gin4 and Kcc4 localize Swe1 to the bud-neck
Budding
Budding is a form of asexual reproduction in which a new organism grows on another one. The new organism remains attached as it grows, separating from the parent organism only when it is mature. Since the reproduction is asexual, the newly created organism is a clone and is genetically identical...
. Bud-neck associating kinases Cla4 and Cdc5 (polo kinase homologue) phosphorylate Swe1 at different stages of the cell cycle. Swe1 is also phosphorylated by Clb2-Cdc28 which serves as a recognition for further phosphorylation by Cdc5.
The S. cerevisiae protein Swe1 is also regulated by degradation. Swe1 is hyperphosphorylated by Clb2-Cdc28 and Cdc5 which may be a signal for ubiquitination and degradation by SCF
SCF complex
Skp, Cullin, F-box containing complex is a multi-protein E3 ubiquitin ligase complex catalyzing the ubiquitination of proteins destined for proteasomal degradation...
E3 ubiquitin ligase complex as in higher eukaryotes.
Role in cancer
The mitosis promoting factor MPF also regulates DNA-damageDNA repair
DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as UV light and radiation can cause DNA damage, resulting in as many as 1...
induced apoptosis
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
. Negative regulation of MPF by WEE1 causes aberrant mitosis and thus resistance to DNA-damage induced apoptosis. Kruppel-like factor 2
Kruppel-like factors
The Krüppel-like family of transcription factors , so named for their homology to the Drosophila melanogaster Krüppel protein, have been extensively studied for their roles in cell proliferation, differentiation and survival, especially in the context of cancer . All KLF family members are...
(KLF2) negatively regulates human WEE1, thus increasing sensitivity to DNA-damage induced apoptosis in cancer cells.
Mutant phenotype
Wee1 acts as a dosage-dependentGene dosage
Gene dosage is the number of copies of a gene present in a cell or nucleus. An increase in gene dosage can cause higher levels of gene product if the gene is not subject to regulation from elsewhere in the body....
inhibitor of mitosis. Thus, the amount of Wee1 protein correlates with the size of the cells:
The fission yeast mutant
Mutant
In biology and especially genetics, a mutant is an individual, organism, or new genetic character, arising or resulting from an instance of mutation, which is a base-pair sequence change within the DNA of a gene or chromosome of an organism resulting in the creation of a new character or trait not...
wee1, also called wee1-, divides at a significantly smaller cell size than wildtype cells. Since Wee1 inhibits entry into mitosis, its absence will lead to division at a premature stage and sub-normal cell size. Conversely,
when Wee1 expression is increased, mitosis is delayed and cells grow to a large size before dividing.