Anaphase-promoting complex
Encyclopedia
Anaphase-Promoting Complex, also called cyclosome (APC/C), is an E3 ubiquitin ligase
that marks target cell cycle proteins for degradation by the 26S proteasome
. The APC/C is a large complex of 11–13 subunit protein
s, including a cullin (Apc2) and RING (Apc11) subunit much like SCF
. Other parts of the APC/C still have unknown functions, but are highly conserved.
It was the discovery of the APC/C (and SCF
) and the key role that they have in eukaryotic cell reproduction that established once and for all the importance of ubiquitin
-mediated proteolysis in eukaryotic cell biology. Once perceived as a system exclusively involved in removing damaged protein from the cell, ubiquitination and subsequent protein degradation by the proteasome
is now perceived as a universal regulatory mechanism for signal transduction
whose importance approaches that of protein phosphorylation
.
to anaphase
by tagging specific proteins for degradation. The two proteins of most importance that get degraded in this process as substrates of the APC/C are securin
and S and M cyclin
s. Securin
releases separase
, a protease, after being degraded which in turn triggers the cleavage of cohesin
, the protein complex that binds sister chromatid
s together. During metaphase
, sister chromatids are linked by intact cohesin complexes. When securin undergoes ubiquitination by the APC/C and releases separin, which degrades cohesin, sister chromatids become free to move to opposite poles for anaphase. The APC/C also targets the mitotic cyclins for degradation, resulting in the inactivation of M-CdK (mitotic cyclin-dependent kinase
) complexes, promoting exit from mitosis
and cytokinesis
.
Unlike the SCF, activator subunits control the APC/C. Cdc20
and Cdh1
are the two activators of particular importance to the cell cycle. These proteins target the APC/C to specific sets of substrates at different times in the cell cycle, thus driving it forward. The APC/C also plays an integral role in maintenance of chromatin metabolism, particularly in G1 and G0, and plays a key role in phosphorylation of H3 through destruction of the aurora A kinase.
APC/C substrates have recognition amino acid sequences that enable the APC/C to identify them. The most common sequence is known as the destruction box or D-box. APC/C brings together an E2 ubiquitin-conjugating enzyme
and the D-box rather than being an intermediate covalent carrier. The D-box should have a version of the following amino acid
sequence: RXXLXXXXN, where R is arginine
, X is any amino acid, L is Leucine
, and N is asparagine
. The Ken-box is another motif of importance. Its sequence should resemble the one that follows: KENXXXN, where K is lysine
and E is glutamate. The last amino acid position in the Ken-box is highly variable. The APC/C probably has many substrate interaction sites, and while most of the APC/C's targets have at least one, if not both, of the aforementioned recognition sequences, they are not sufficient to instigate APC/C ubiquitination alone. Though it has been shown that mutations in the sequences do inhibit destruction of the proteins "in vivo", there is still much to learn about how proteins are targeted by the APC/C.
inhibits the APC/C until all sister-kinetochores are attached to opposite poles of the mitotic spindle
, a process known as chromosome biorientation. When all kinetochores are properly attached, the spindle checkpoint
is silenced and the APC/C can become active. M-Cdks phosphorylate subunits on the APC/C that promote binding to Cdc20. Securin and M Cyclins (cyclin A and cyclin B) are then targeted by APC/CCdc20 for degradation. Once degraded, separin is released, cohesin is degraded and sister chromatids are prepared to move to their respective poles for anaphase.
It is likely that, in animal cells, at least some of the activation of APC/CCdc20 occurs early in the cell cycle (prophase or prometaphase) based on the timing of the degradation of its substrates. Cyclin A
is degraded early in mitosis, supporting the theory, but cyclin B
and securin are not degraded until metaphase. The molecular basis of the delay is unknown, but is believed to involve the key to the correct timing of anaphase initiation. In animal cells the spindle checkpoint system contributes to the delay if it needs to correct the bi-orientation of chromosomes. Though how the spindle checkpoint system inhibits cyclin B and securin destruction while allowing cyclin A to be degraded is unknown. The delay may also be explained by unknown interactions with regulators, localization and phosphorylation changes.
This initiates a negative feedback
loop. While activation of APC/CCdc20 requires M-Cdk, the complex is also responsible for breaking the cyclin to deactivate M-CdK. This means that APC/CCdc20 fosters its own deactivation. It is possible that this negative feedback is the backbone of Cdk activity controlled by M and S cyclin concentration oscillations.
, to grow and produce factors necessary for the next cell cycle. Entry into another round of mitosis is prevented by inhibiting Cdk activity. While different processes are responsible for this inhibition, an important one is activation of the APC/C by Cdh1. This continued activation prevents the accumulation of cyclin that would trigger another round of mitosis and instead drives exit from mitosis.
In the beginning of the cell cycle Cdh1 is phosphorylated by M-Cdk, preventing it from attaching to APC/C. APC/C is then free to attach to Cdc20 and usher the transition from metaphase to anaphase. As M-Cdk gets degraded later in mitosis, Cdc20 gets released and Cdh1 can bind to APC/C, keeping it activated through the M/G1 transition. Cdc20 is also a target of APC/CCdh1, ensuring that APC/CCdc20 is shut down. APC/CCdh1 then continues working in G1 to tag S and M cyclins for destruction. However, G1/S cyclins are not substrates of APC/CCdh1 and therefore accumulate throughout this phase and phosphorylate Cdh1. By late G1, enough of the G1/S cyclins have accumulated and phosphorylated Cdh1 to inactivate the APC/C until the next metaphase.
(Review)
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 marks target cell cycle proteins for degradation by the 26S proteasome
Proteasome
Proteasomes are very large protein complexes inside all eukaryotes and archaea, and in some bacteria. In eukaryotes, they are located in the nucleus and the cytoplasm. The main function of the proteasome is to degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks...
. The APC/C is a large complex of 11–13 subunit protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s, including a cullin (Apc2) and RING (Apc11) subunit much like 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...
. Other parts of the APC/C still have unknown functions, but are highly conserved.
It was the discovery of the APC/C (and 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...
) and the key role that they have in eukaryotic cell reproduction that established once and for all the importance of ubiquitin
Ubiquitin
Ubiquitin is a small regulatory protein that has been found in almost all tissues of eukaryotic organisms. Among other functions, it directs protein recycling.Ubiquitin can be attached to proteins and label them for destruction...
-mediated proteolysis in eukaryotic cell biology. Once perceived as a system exclusively involved in removing damaged protein from the cell, ubiquitination and subsequent protein degradation by the proteasome
Proteasome
Proteasomes are very large protein complexes inside all eukaryotes and archaea, and in some bacteria. In eukaryotes, they are located in the nucleus and the cytoplasm. The main function of the proteasome is to degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks...
is now perceived as a universal regulatory mechanism for signal transduction
Signal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
whose importance approaches that of protein phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
.
Function
The APC/C's main function is to trigger the transition from metaphaseMetaphase
Metaphase, from the ancient Greek μετά and φάσις , is a stage of mitosis in the eukaryotic cell cycle in which condensed & highly coiled chromosomes, carrying genetic information, align in the middle of the cell before being separated into each of the two daughter cells...
to 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....
by tagging specific proteins for degradation. The two proteins of most importance that get degraded in this process as substrates of the APC/C are 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...
and S and M cyclin
Cyclin
Cyclins are a family of proteins that control the progression of cells through the cell cycle by activating cyclin-dependent kinase enzymes.- Function :...
s. 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...
releases 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 :...
, a protease, after being degraded which in turn triggers the cleavage of cohesin
Cohesin
Cohesin is a protein complex that regulates the separation of sister chromatids during cell division, either mitosis or meiosis.- Structure :...
, the protein complex that binds sister chromatid
Chromatid
A chromatid is one of the two identical copies of DNA making up a duplicated chromosome, which are joined at their centromeres, for the process of cell division . They are called sister chromatids so long as they are joined by the centromeres...
s together. During metaphase
Metaphase
Metaphase, from the ancient Greek μετά and φάσις , is a stage of mitosis in the eukaryotic cell cycle in which condensed & highly coiled chromosomes, carrying genetic information, align in the middle of the cell before being separated into each of the two daughter cells...
, sister chromatids are linked by intact cohesin complexes. When securin undergoes ubiquitination by the APC/C and releases separin, which degrades cohesin, sister chromatids become free to move to opposite poles for anaphase. The APC/C also targets the mitotic cyclins for degradation, resulting in the inactivation of M-CdK (mitotic 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]]...
) complexes, promoting 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...
and cytokinesis
Cytokinesis
Cytokinesis is the process in which the cytoplasm of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation...
.
Unlike the SCF, activator subunits control the APC/C. Cdc20
CDC20
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 , a large 11-13 subunit complex that initiates chromatid separation...
and Cdh1
CDH1 (gene)
Cadherin-1 also known as CAM 120/80 or epithelial cadherin or uvomorulin is a protein that in humans is encoded by the CDH1 gene. CDH1 has also been designated as CD324 . It is a tumor suppressor gene.- Function :Cadherin-1 is a classical member of the cadherin superfamily...
are the two activators of particular importance to the cell cycle. These proteins target the APC/C to specific sets of substrates at different times in the cell cycle, thus driving it forward. The APC/C also plays an integral role in maintenance of chromatin metabolism, particularly in G1 and G0, and plays a key role in phosphorylation of H3 through destruction of the aurora A kinase.
APC/C substrates have recognition amino acid sequences that enable the APC/C to identify them. The most common sequence is known as the destruction box or D-box. APC/C brings together an E2 ubiquitin-conjugating enzyme
Ubiquitin-conjugating enzyme
Ubiquitin-conjugating enzymes, also known as E2 enzymes and more rarely as ubiquitin-carrier enzymes, perform the second step in the ubiquitination reaction that targets a protein for degradation via the proteasome.The ubiquitination process covalently attaches ubiquitin, a short protein of 76...
and the D-box rather than being an intermediate covalent carrier. The D-box should have a version of the following amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
sequence: RXXLXXXXN, where R is arginine
Arginine
Arginine is an α-amino acid. The L-form is one of the 20 most common natural amino acids. At the level of molecular genetics, in the structure of the messenger ribonucleic acid mRNA, CGU, CGC, CGA, CGG, AGA, and AGG, are the triplets of nucleotide bases or codons that codify for arginine during...
, X is any amino acid, L is Leucine
Leucine
Leucine is a branched-chain α-amino acid with the chemical formula HO2CCHCH2CH2. Leucine is classified as a hydrophobic amino acid due to its aliphatic isobutyl side chain. It is encoded by six codons and is a major component of the subunits in ferritin, astacin and other 'buffer' proteins...
, and N is asparagine
Asparagine
Asparagine is one of the 20 most common natural amino acids on Earth. It has carboxamide as the side-chain's functional group. It is not an essential amino acid...
. The Ken-box is another motif of importance. Its sequence should resemble the one that follows: KENXXXN, where K is lysine
Lysine
Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG....
and E is glutamate. The last amino acid position in the Ken-box is highly variable. The APC/C probably has many substrate interaction sites, and while most of the APC/C's targets have at least one, if not both, of the aforementioned recognition sequences, they are not sufficient to instigate APC/C ubiquitination alone. Though it has been shown that mutations in the sequences do inhibit destruction of the proteins "in vivo", there is still much to learn about how proteins are targeted by the APC/C.
Metaphase to Anaphase Transition
As metaphase begins, the spindle checkpointSpindle 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...
inhibits the APC/C until all sister-kinetochores are attached to opposite poles of the mitotic spindle
Mitotic spindle
In cell biology, the spindle fibers are the structure that separates the chromosomes into the daughter cells during cell division. It is part of the cytoskeleton in eukaryotic cells...
, a process known as chromosome biorientation. When all kinetochores are properly attached, the 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...
is silenced and the APC/C can become active. M-Cdks phosphorylate subunits on the APC/C that promote binding to Cdc20. Securin and M Cyclins (cyclin A and cyclin B) are then targeted by APC/CCdc20 for degradation. Once degraded, separin is released, cohesin is degraded and sister chromatids are prepared to move to their respective poles for anaphase.
It is likely that, in animal cells, at least some of the activation of APC/CCdc20 occurs early in the cell cycle (prophase or prometaphase) based on the timing of the degradation of its substrates. Cyclin A
Cyclin A
Cyclin A is a member of the cyclin family.Cyclin A binds to S phase Cdk2 and is required for the cell to progress through the S phase. Cyclin A/ Cdk2 is inhibited by the complex p21CIP.-External links:*...
is degraded early in mitosis, supporting the theory, but 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...
and securin are not degraded until metaphase. The molecular basis of the delay is unknown, but is believed to involve the key to the correct timing of anaphase initiation. In animal cells the spindle checkpoint system contributes to the delay if it needs to correct the bi-orientation of chromosomes. Though how the spindle checkpoint system inhibits cyclin B and securin destruction while allowing cyclin A to be degraded is unknown. The delay may also be explained by unknown interactions with regulators, localization and phosphorylation changes.
This initiates a negative feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
loop. While activation of APC/CCdc20 requires M-Cdk, the complex is also responsible for breaking the cyclin to deactivate M-CdK. This means that APC/CCdc20 fosters its own deactivation. It is possible that this negative feedback is the backbone of Cdk activity controlled by M and S cyclin concentration oscillations.
M to G1 Transition
Upon completion of mitosis, it is important that cells (except for embryonic ones) go through a growth period, known as G1 phaseG1 phase
The G1 phase is a period in the cell cycle during interphase, before the S phase. For many cells, this phase is the major period of cell growth during its lifespan. During this stage new organelles are being synthesized, so the cell requires both structural proteins and enzymes, resulting in great...
, to grow and produce factors necessary for the next cell cycle. Entry into another round of mitosis is prevented by inhibiting Cdk activity. While different processes are responsible for this inhibition, an important one is activation of the APC/C by Cdh1. This continued activation prevents the accumulation of cyclin that would trigger another round of mitosis and instead drives exit from mitosis.
In the beginning of the cell cycle Cdh1 is phosphorylated by M-Cdk, preventing it from attaching to APC/C. APC/C is then free to attach to Cdc20 and usher the transition from metaphase to anaphase. As M-Cdk gets degraded later in mitosis, Cdc20 gets released and Cdh1 can bind to APC/C, keeping it activated through the M/G1 transition. Cdc20 is also a target of APC/CCdh1, ensuring that APC/CCdc20 is shut down. APC/CCdh1 then continues working in G1 to tag S and M cyclins for destruction. However, G1/S cyclins are not substrates of APC/CCdh1 and therefore accumulate throughout this phase and phosphorylate Cdh1. By late G1, enough of the G1/S cyclins have accumulated and phosphorylated Cdh1 to inactivate the APC/C until the next metaphase.
Further reading
(Review)(Review)