CDC20
Encyclopedia
The cell-division cycle protein 20 is an essential regulator of cell division that is encoded by the CDC20 gene
in humans. To the best of current knowledge its most important function is to activate the anaphase promoting complex (APC), a large 11-13 subunit complex that initiates chromatid separation and entrance into anaphase
. The APCCdc20 protein complex has two main downstream targets. Firstly, it targets securin
for destruction, enabling the eventual destruction of cohesin and thus sister chromatid separation. It also targets S and M-phase (S/M) cyclins for destruction, which inactivates S/M cyclin-dependent kinase
s (Cdks) and allows the cell to exit from mitosis
. A closely related protein, Cdc20homologue-1 (Cdh1) plays a complementary role in the cell cycle.
CDC20 appears to act as a regulatory protein interacting with many other proteins at multiple points in the cell cycle. It is required for two microtubule-dependent processes: nuclear movement prior to anaphase, and chromosome separation.
and colleagues made cell-division cycle mutants that failed to complete major events in the cell cycle in the yeast strain S. cerevisiae. Hartwell found mutants that did not enter anaphase and thus could not complete mitosis; this phenotype could be traced back to the CDC20 gene. However, even after the biochemistry of the protein was eventually elucidated, the molecular role of Cdc20 remained elusive until the discovery of the APC in 1995.
s. Near its C-terminus it contains seven WD40 repeat
s, which are multiple short, structural motifs of around 40 amino acids that often play a role in binding with larger protein complexes. In the case of Cdc20, they arrange into a seven-bladed beta propeller. The human Cdc20 is about 499 amino acids long, and contains at least four phosphorylation sites near the N-terminus. In between these phosphorylation sites, which play regulatory roles, are the C-box, the KEN-box, the Mad2-interacting motif, and the Cry box. The KEN-box, as well as the Cry box, are important recognition and degradation sequences for the APCCdh1 complex (see below).
with:
However, the most important interaction of Cdc20 is with the Anaphase Promoting Complex. The APC is a large E3 ubiquitin ligase, which triggers the metaphase to anaphase transition by marking select proteins for degradation. The two main targets of the APC are the S/M cyclins and the protein securin. S/M cyclins activate cyclin-dependent kinases (Cdks), which have a vast array of downstream effects that work to guide the cell through mitosis. They must be degraded for cells to exit mitosis. Securin is a protein that inhibits separase
, which in turn inhibits cohesin, a protein that holds sister chromatids together. Therefore, in order for anaphase to progress, securin must be inhibited so that cohesin can be cleaved by separase. These processes are dependent on both the APC and Cdc20: When Cdks phosphorylate the APC, Cdc20 can bind and activate it, allowing both the degradation of Cdks and the cleavage of cohesin. APC activity is dependent on Cdc20 (and Cdh1), because Cdc20 often binds the APC substrates directly. In fact, it is thought that Cdc20 and Cdh1 (see below) are receptors for the KEN-box and D-box motifs on substrates. However, these sequences are normally not sufficient for ubiquitination and degradation; much remains to be learned about how Cdc20 binds its substrate.
How can S/M cyclins reappear to shepherd the cell into mitosis? The APCCdc20 does not recognize G1/S cyclins. Their concentration rises during G1, activativing G1/S Cdks, which in turn phosphorylate Cdh1 and gradually relieve the inhibition on S/M cyclins.
and Mad3, when bound to Cdc20, trigger its autoubiquitiniation. ( For more information see Spindle Checkpoint
.)
Gene
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...
in humans. To the best of current knowledge its most important function is to activate the anaphase promoting complex (APC), a large 11-13 subunit complex that initiates chromatid separation and entrance into anaphase
Anaphase
Anaphase, from the ancient Greek ἀνά and φάσις , is the stage of mitosis or meiosis when chromosomes move to opposite poles of the cell....
. The APCCdc20 protein complex has two main downstream targets. Firstly, it targets securin
Securin
Securin is a protein involved in control of the metaphase-anaphase transition and anaphase onset. Following bi-orientation of chromosome pairs and inactivation of the spindle checkpoint system, the underlying regulatory system, which includes securin, produces an abrupt stimulus that induces highly...
for destruction, enabling the eventual destruction of cohesin and thus sister chromatid separation. It also targets S and M-phase (S/M) cyclins for destruction, which inactivates S/M cyclin-dependent kinase
Cyclin-dependent kinase
thumb|350px|Schematic of the cell cycle. outer ring: I=[[Interphase]], M=[[Mitosis]]; inner ring: M=Mitosis; G1=[[G1 phase|Gap phase 1]]; S=[[S phase|Synthesis]]; G2=[[G2 phase|Gap phase 2]]...
s (Cdks) and allows the cell to exit from 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...
. A closely related protein, Cdc20homologue-1 (Cdh1) plays a complementary role in the cell cycle.
CDC20 appears to act as a regulatory protein interacting with many other proteins at multiple points in the cell cycle. It is required for two microtubule-dependent processes: nuclear movement prior to anaphase, and chromosome separation.
History
Cdc20, along with a handful of other Cdc proteins, was discovered in the early 1970s when HartwellLeland H. Hartwell
Leland Harrison Hartwell is former president and director of the Fred Hutchinson Cancer Research Center in Seattle, Washington. He shared the 2001 Nobel Prize in Physiology or Medicine with Paul Nurse and R...
and colleagues made cell-division cycle mutants that failed to complete major events in the cell cycle in the yeast strain S. cerevisiae. Hartwell found mutants that did not enter anaphase and thus could not complete mitosis; this phenotype could be traced back to the CDC20 gene. However, even after the biochemistry of the protein was eventually elucidated, the molecular role of Cdc20 remained elusive until the discovery of the APC in 1995.
Biochemistry
Cdc20 is a protein related to the beta subunit of heterotrimeric G proteinHeterotrimeric G protein
"G protein" usually refers to the membrane-associated heterotrimeric G proteins, sometimes referred to as the "large" G proteins. These proteins are activated by G protein-coupled receptors and are made up of alpha , beta and gamma subunits, the latter two referred to as the beta-gamma...
s. Near its C-terminus it contains seven WD40 repeat
WD40 repeat
The WD40 repeat is a short structural motif of approximately 40 amino acids , often terminating in a tryptophan-aspartic acid dipeptide...
s, which are multiple short, structural motifs of around 40 amino acids that often play a role in binding with larger protein complexes. In the case of Cdc20, they arrange into a seven-bladed beta propeller. The human Cdc20 is about 499 amino acids long, and contains at least four phosphorylation sites near the N-terminus. In between these phosphorylation sites, which play regulatory roles, are the C-box, the KEN-box, the Mad2-interacting motif, and the Cry box. The KEN-box, as well as the Cry box, are important recognition and degradation sequences for the APCCdh1 complex (see below).
Interactions
CDC20 has been shown to interactProtein-protein interaction
Protein–protein interactions occur when two or more proteins bind together, often to carry out their biological function. Many of the most important molecular processes in the cell such as DNA replication are carried out by large molecular machines that are built from a large number of protein...
with:
- ANAPC7ANAPC7Anaphase-promoting complex subunit 7 is an enzyme that in humans is encoded by the ANAPC7 gene.-Interactions:ANAPC7 has been shown to interact with ANAPC1, ANAPC4, CDC27 and CDC20.-Further reading:...
- BUB1BBUB1BMitotic checkpoint serine/threonine-protein kinase BUB1 beta is an enzyme that in humans is encoded by the BUB1B gene.-Interactions:BUB1B has been shown to interact with AP2B1, HDAC1, BUB3, MAD2L1, Gamma-synuclein, BRCA2 and CDC20.-Further reading:...
, - CDC16CDC16Cell division cycle protein 16 homolog is a protein that in humans is encoded by the CDC16 gene.-Interactions:CDC16 has been shown to interact with CDC27 and CDC20.-Further reading:...
, - CDC27CDC27Cell division cycle protein 27 homolog is a protein that in humans is encoded by the CDC27 gene.-Interactions:CDC27 has been shown to interact with ANAPC1, ANAPC4, ANAPC5, ANAPC11, FZR1, CDC20, CDH1, CDC23, PIN1, CDC16, ANAPC7, MAD2L1 and ANAPC10....
, - Cyclin A1,
- FBXO5FBXO5F-box only protein 5 is a protein that in humans is encoded by the FBXO5 gene.-Interactions:FBXO5 has been shown to interact with SKP1A, FZR1 and CDC20.-Further reading:...
, - HDAC1HDAC1Histone deacetylase 1 is an enzyme that in humans is encoded by the HDAC1 gene.-Interactions:HDAC1 has been shown to interact with RAD9A, DNMT3L, MTA1, FKBP3, CDC20, CDH1, Host cell factor C1, BUB3, Nuclear receptor co-repressor 2, IKZF1, Prohibitin, EVI1, DDX5, DNMT3A, Mothers against...
, - HDAC2Histone deacetylase 2Histone deacetylase 2 is an enzyme that in humans is encoded by the HDAC2 gene.-Interactions:Histone deacetylase 2 has been shown to interact with CHD3, MTA1, FKBP3, CHD4, RCOR1, Retinoblastoma protein, CDC20, RBBP4, CDH1, EED, GTF2I, Host cell factor C1, PHF21A, BUB3, MXD1, HDAC10, RELA, Sp3...
, and - MAD2L1MAD2L1Mitotic spindle assembly checkpoint protein MAD2A is a protein that in humans is encoded by the MAD2L1 gene.-Interactions:MAD2L1 has been shown to interact with MAD2L2, CDC20, BUB1B, Estrogen receptor beta, ADAM17, UBD, CDC27 and Mad1....
.
However, the most important interaction of Cdc20 is with the Anaphase Promoting Complex. The APC is a large E3 ubiquitin ligase, which triggers the metaphase to anaphase transition by marking select proteins for degradation. The two main targets of the APC are the S/M cyclins and the protein securin. S/M cyclins activate cyclin-dependent kinases (Cdks), which have a vast array of downstream effects that work to guide the cell through mitosis. They must be degraded for cells to exit mitosis. Securin is a protein that inhibits separase
Separase
Separase is a cysteine protease responsible for triggering anaphase by hydrolysing cohesin which is the protein responsible for binding sister chromatids during metaphase. In humans, separase is encoded by the ESPL1 gene.- Discovery :...
, which in turn inhibits cohesin, a protein that holds sister chromatids together. Therefore, in order for anaphase to progress, securin must be inhibited so that cohesin can be cleaved by separase. These processes are dependent on both the APC and Cdc20: When Cdks phosphorylate the APC, Cdc20 can bind and activate it, allowing both the degradation of Cdks and the cleavage of cohesin. APC activity is dependent on Cdc20 (and Cdh1), because Cdc20 often binds the APC substrates directly. In fact, it is thought that Cdc20 and Cdh1 (see below) are receptors for the KEN-box and D-box motifs on substrates. However, these sequences are normally not sufficient for ubiquitination and degradation; much remains to be learned about how Cdc20 binds its substrate.
Regulation
The APCCdc20 complex regulates itself so that it is present during the appropriate times of the cell cycle. In order for Cdc20 to bind the APC, specific APC subunits must be phosphorylated by Cdk1 (among other Cdks). Therefore, when cdk activity is high in mitosis, and the cell must prepare to enter anaphase and exit mitosis, the APCCdc20 complex is activated. Once active, APCCdc20 promotes the degradation of Cdks by inactivating S/M cyclins. Cdk degradation brings about lower rates of APC phosphorylation and thus lower rates of Cdc20 binding. In this way, the APCCdc20 complex inactivates itself by the end of mitosis. However, because the cell does not immediately enter the cell cycle, Cdks can not immediately be reactivated. Multiple different mechanisms inhibit Cdks in G1: Cdk inhibitor proteins are expressed, and cyclin gene expression is down-regulated. Importantly, cyclin accumulation is also prevented by Cdh1.Cdc20 and Cdh1
Cdc20-homologue 1 (Cdh1) plays a complimentary role to Cdc20 in cell cycle progression. During the time of APCCdc20 activity, Cdh1 is phosphorylated and cannot bind to the APC. After metaphase, however, S/M-Cdks are inactivated by APCCdc20, and Cdh1 can exist in a non-phosphorylated state and bind the APC. This enables the APC to continue to degrade S/M cyclins (and thus S/M Cdks) until they are needed again in the next S-phase.How can S/M cyclins reappear to shepherd the cell into mitosis? The APCCdc20 does not recognize G1/S cyclins. Their concentration rises during G1, activativing G1/S Cdks, which in turn phosphorylate Cdh1 and gradually relieve the inhibition on S/M cyclins.
Spindle Assembly Checkpoint
Cdc20 is also a part of, and regulated by, the Spindle Assembly Checkpoint (SAC). This checkpoint ensures that anaphase proceeds only when the centromeres of all sister chromatids lined up on the metaphase plate are properly attached to microtubules. The checkpoint is held active by any unattached centromere; only when all centromeres are attached will anaphase commence. The APCCdc20 is an important target of the SAC, which consists of several different proteins, including Mad2, Mad3(BubR1), and Bub3. In fact, these three proteins, together with Cdc20, likely form the mitotic checkpoint complex (MCC), which inhibits APCCdc20 so that anaphase cannot begin prematurely. Moreover, Bub1 phosphorylates and thus inhibits Cdc20 directly, while in yeast Mad2MAD2
MAD2 is an essential spindle checkpoint protein. The spindle checkpoint system is a regulatory system that restrains progression through the metaphase-to-anaphase transition. The Mad2 gene was first identified in the yeast S. cerevisiae in a screen for genes which when mutated would confer...
and Mad3, when bound to Cdc20, trigger its autoubiquitiniation. ( For more information see Spindle Checkpoint
Spindle checkpoint
In order to preserve one cell's identity and its proper functioning, it is necessary to maintain constant the appropriate number of chromosomes after each cell division...
.)