p21
Cip1 (alternatively p21
Waf1), also known as cyclin-dependent kinase inhibitor 1 or CDK-interacting protein 1, is a
cyclin-dependent kinase inhibitor
A cyclin-dependent kinase inhibitor protein is a protein which inhibits the enzyme cyclin-dependent kinase (CDK). Several function as tumor suppressor proteins. Cell cycle progression is delayed or stopped by cyclin-dependent kinase inhibitors, ...
(CKI) that is capable of inhibiting all cyclin/CDK complexes,
though is primarily associated with inhibition of
CDK2
Cyclin-dependent kinase 2, also known as cell division protein kinase 2, or Cdk2, is an enzyme that in humans is encoded by the ''CDK2'' gene. The protein encoded by this gene is a member of the cyclin-dependent kinase family of Ser/Thr protein ...
.
p21 represents a major target of
p53
p53, also known as Tumor protein P53, cellular tumor antigen p53 (UniProt name), or transformation-related protein 53 (TRP53) is a regulatory protein that is often mutated in human cancers. The p53 proteins (originally thought to be, and often s ...
activity and thus is associated with linking DNA damage to cell cycle arrest.
[Waldman, Todd, Kenneth W. Kinzler, and Bert Vogelstein. "p21 is necessary for the p53-mediated G1 arrest in human cancer cells." Cancer research 55.22 (1995): 5187-5190.] This protein is encoded by the ''CDKN1A''
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
located on
chromosome 6
Chromosome 6 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 6 spans more than 170 million base pairs (the building material of DNA) and represents between 5.5 and 6% of the total ...
(6p21.2) in humans.
Function
CDK inhibition
p21 is a potent
cyclin-dependent kinase inhibitor
A cyclin-dependent kinase inhibitor protein is a protein which inhibits the enzyme cyclin-dependent kinase (CDK). Several function as tumor suppressor proteins. Cell cycle progression is delayed or stopped by cyclin-dependent kinase inhibitors, ...
(CKI). The p21 (CIP1/WAF1) protein binds to and inhibits the activity of
cyclin
Cyclin is a family of proteins that controls the progression of a cell through the cell cycle by activating cyclin-dependent kinase (CDK) enzymes or group of enzymes required for synthesis of cell cycle.
Etymology
Cyclins were originally disco ...
-
CDK2
Cyclin-dependent kinase 2, also known as cell division protein kinase 2, or Cdk2, is an enzyme that in humans is encoded by the ''CDK2'' gene. The protein encoded by this gene is a member of the cyclin-dependent kinase family of Ser/Thr protein ...
, -
CDK1
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 th ...
, and -
CDK4
Cyclin-dependent kinase 4 also known as cell division protein kinase 4 is an enzyme that in humans is encoded by the ''CDK4'' gene. CDK4 is a member of the cyclin-dependent kinase family.
Function
The protein encoded by this gene is a member o ...
/6 complexes, and thus functions as a regulator of
cell cycle
The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and subs ...
progression at
G1 and
S phase
S phase (Synthesis Phase) is the phase of the cell cycle in which DNA is replicated, occurring between G1 phase and G2 phase. Since accurate duplication of the genome is critical to successful cell division, the processes that occur during ...
.
The binding of p21 to CDK complexes occurs through p21's N-terminal domain, which is homologous to the other CIP/KIP CDK inhibitors
p27 and
p57.
Specifically it contains a Cy1 motif in the N-terminal half, and weaker Cy2 motif in the C-terminal domain that allow it to bind CDK in a region that blocks its ability to complex with cyclins and thus prevent CDK activation.
Experiments looking at CDK2 activity within single cells have also shown p21 to be responsible for a bifurcation in CDK2 activity following mitosis, cells with high p21 enter a
G0/quiescent state, whilst those with low p21 continue to proliferate.
Follow up work, found evidence that this bistability is underpinned by double negative feedback between p21 and CDK2, where CDK2 inhibits p21 activity via
ubiquitin ligase
A ubiquitin ligase (also called an E3 ubiquitin ligase) is a protein that recruits an E2 ubiquitin-conjugating enzyme that has been loaded with ubiquitin, recognizes a protein substrate, and assists or directly catalyzes the transfer of ubiquitin ...
activity.
PCNA inhibition
p21 interacts with proliferating cell nuclear antigen (
PCNA
Proliferating cell nuclear antigen (PCNA) is a DNA clamp that acts as a processivity factor for DNA polymerase δ in eukaryotic cells and is essential for replication. PCNA is a homotrimer and achieves its processivity by encircling the DNA, whe ...
), a DNA polymerase accessory factor, and plays a regulatory role in S phase DNA replication and DNA damage repair.
Specifically, p21 has a high affinity for the PIP-box binding region on PCNA, binding of p21 to this region is proposed to block the binding of processivity factors necessary for PCNA dependent S-phase DNA synthesis, but not PCNA dependent
nucleotide excision repair
Nucleotide excision repair is a DNA repair mechanism. DNA damage occurs constantly because of chemicals (e.g. intercalating agents), radiation and other mutagens. Three excision repair pathways exist to repair single stranded DNA damage: Nucle ...
(NER).
As such, p21 acts as an effective inhibitor of S-phase DNA synthesis though permits NER, leading to the proposal that p21 acts to preferentially select polymerase processivity factors depending on the context of DNA synthesis.
Apoptosis inhibition
This protein was reported to be specifically cleaved by
CASP3
Caspase-3 is a caspase protein that interacts with caspase-8 and caspase-9. It is encoded by the ''CASP3'' gene. ''CASP3'' orthologs have been identified in numerous mammals for which complete genome data are available. Unique orthologs are also p ...
-like
caspase
Caspases (cysteine-aspartic proteases, cysteine aspartases or cysteine-dependent aspartate-directed proteases) are a family of protease enzymes playing essential roles in programmed cell death. They are named caspases due to their specific cystei ...
s, which thus leads to a dramatic activation of CDK2, and may be instrumental in the execution of
apoptosis
Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...
following
caspase
Caspases (cysteine-aspartic proteases, cysteine aspartases or cysteine-dependent aspartate-directed proteases) are a family of protease enzymes playing essential roles in programmed cell death. They are named caspases due to their specific cystei ...
activation. However p21 may inhibit apoptosis and does not induce cell death on its own.
The ability of p21 to inhibit apoptosis in response to replication fork stress has also been reported.
Regulation
p53 dependent response
Studies of p53 dependent cell cycle arrest in response to DNA damage identified p21 as the primary mediator of downstream cell cycle arrest. Notably, El-Deiry ''et al.'' identified a protein p21 (WAF1) which was present in cells expressing wild type p53 but not those with mutant p53, moreover constitutive expression of p21 led to cell cycle arrest in a number of cell types.
Dulcic ''et al.'' also found that γ-irradiation of fibroblasts induced a p53 and p21 dependent cell cycle arrest, here p21 was found bound to inactive
cyclin E
Cyclin E is a member of the cyclin family.
Cyclin E binds to G1 phase Cdk2, which is required for the transition from G1 to S phase of the cell cycle that determines initiation of DNA duplication. The Cyclin E/CDK2 complex phosphorylates p27 ...
/
CDK2
Cyclin-dependent kinase 2, also known as cell division protein kinase 2, or Cdk2, is an enzyme that in humans is encoded by the ''CDK2'' gene. The protein encoded by this gene is a member of the cyclin-dependent kinase family of Ser/Thr protein ...
complexes. Working in mouse models, it was also shown that whilst mice lacking p21 were healthy, spontaneous tumours developed and G1 checkpoint control was compromised in cells derived from these mice.
Taken together, these studies thus defined p21 as the primary mediator of p53-dependent cell cycle arrest in response to DNA damage.
Recent work exploring p21 activation in response to DNA damage at a single-cell level have demonstrated that pulsatile p53 activity leads to subsequent pulses of p21, and that the strength of p21 activation is cell cycle phase dependent.
Moreover, studies of p21-levels in populations of cycling cells, not exposed to DNA damaging agents, have shown that DNA damage occurring in mother cell S-phase can induce p21 accumulation over both mother G2 and daughter G1 phases which subsequently induces cell cycle arrest;
this responsible for the bifurcation in CDK2 activity observed in Spencer ''et al.''.
Degradation
p21 is negatively regulated by
ubiquitin ligases
A ubiquitin ligase (also called an E3 ubiquitin ligase) is a protein that recruits an E2 ubiquitin-conjugating enzyme that has been loaded with ubiquitin, recognizes a protein substrate, and assists or directly catalyzes the transfer of ubiquitin ...
both over the course of the cell cycle and in response to DNA damage. Specifically, over the G1/S transition it has been demonstrated that the E3 ubiquitin ligase complex
SCFSkp2
S-phase kinase-associated protein 2 is an enzyme that in humans is encoded by the ''SKP2'' gene.
Structure and function
Skp2 contains 424 residues in total with the ~40 amino acid F-box domain lying closer to the N-terminal region at the 94-14 ...
induces degradation of p21.
Studies have also demonstrated that the E3 ubiquitin ligase complex
CRL4Cdt2 degrades p21 in a PCNA dependent manner over S-phase, necessary to prevent p21 dependent re-replication,
as well as in response to UV irradiation.
Recent work has now found that in human cell lines SCF
Skp2 degrades p21 towards the end of G1 phase, allowing cells to exit a quiescent state, whilst CRL4
Cdt2 acts to degrade p21 at a much higher rate than SCF
Skp2 over the G1/S transition and subsequently maintain low levels of p21 throughout S-phase.
Clinical significance
Cytoplasmic p21 expression can be significantly correlated with lymph node metastasis, distant metastases, advanced
TNM stage (a classification of cancer staging that stands for: tumor size, describing nearby lymph nodes, and distant metastasis), depth of invasion and OS (
overall survival rate
Survival rate is a part of survival analysis. It is the proportion of people in a study or treatment group still alive at a given period of time after diagnosis. It is a method of describing prognosis in certain disease conditions, and can be use ...
). A study on immunohistochemical markers in malignant thymic epithelial tumors shows that p21 expression has a negatively influenced survival and significantly correlated with
WHO
Who or WHO may refer to:
* Who (pronoun), an interrogative or relative pronoun
* Who?, one of the Five Ws in journalism
* World Health Organization
Arts and entertainment Fictional characters
* Who, a creature in the Dr. Seuss book ''Horton Hear ...
(World Health Organization) type B2/B3. When combined with low p27 and high p53,
DFS (Disease-Free Survival) decreases.
p21 mediates the resistance of
hematopoietic cell
A blood cell, also called a hematopoietic cell, hemocyte, or hematocyte, is a cell produced through hematopoiesis and found mainly in the blood. Major types of blood cells include red blood cells (erythrocytes), white blood cells (leukocytes), a ...
s to an infection with
HIV
The human immunodeficiency viruses (HIV) are two species of ''Lentivirus'' (a subgroup of retrovirus) that infect humans. Over time, they cause acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the immune ...
by complexing with the HIV integrase and thereby aborting chromosomal integration of the
provirus A provirus is a virus genome that is integrated into the DNA of a host cell. In the case of bacterial viruses (bacteriophages), proviruses are often referred to as prophages. However, proviruses are distinctly different from prophages and these ter ...
. HIV infected individuals who naturally suppress viral replication have elevated levels of p21 and its associated mRNA. p21 expression affects at least two stages in the HIV life cycle inside CD4 T cells, significantly limiting production of new viruses.
[
*]
Metastatic canine mammary tumors display increased levels of p21 in the primary tumors but also in their metastases, despite increased cell proliferation.
Mice that lack the p21 gene gain the ability to
regenerate lost appendages.
[
*]
Interactions
P21 has been shown to
interact
Advocates for Informed Choice, dba interACT or interACT Advocates for Intersex Youth, is a 501(c)(3) nonprofit organization using innovative strategies to advocate for the legal and human rights of children with intersex traits. The organizati ...
with:
*
Nrf2
Nuclear factor erythroid 2-related factor 2 (NRF2), also known as nuclear factor erythroid-derived 2-like 2, is a transcription factor that in humans is encoded by the ''NFE2L2'' gene. NRF2 is a basic leucine zipper (bZIP) protein that may regul ...
*
BCCIP
BRCA2 and CDKN1A-interacting protein is a protein that in humans is encoded by the ''BCCIP'' gene.
This gene product was isolated on the basis of its interaction with BRCA2 and p21 proteins. It is an evolutionarily conserved nuclear protein with ...
,
[
* CIZ1,]
* CUL4A
Cullin-4A is a protein that in humans is encoded by the ''CUL4A'' gene. CUL4A belongs to the cullin family of ubiquitin ligase proteins and is highly homologous to the CUL4B protein. CUL4A regulates numerous key processes such as DNA repair, chro ...
,
* CCNE1
G1/S-specific cyclin-E1 is a protein that in humans is encoded by the ''CCNE1'' gene.
Function
The protein encoded by this gene belongs to the highly conserved cyclin family, whose members are characterized by a dramatic periodicity in prote ...
,[
* CDK,]
* DDB1
DNA damage-binding protein 1 is a protein that in humans is encoded by the ''DDB1'' gene.
Gene
The gene's position is on chromosome 11q12-q13.
Protein
The DDB1 gene encodes the large subunit of DNA damage-binding protein, a heterodimer composed ...
,[
* DTL,][
* ]GADD45A
Growth arrest and DNA-damage-inducible protein GADD45 alpha is a protein that in humans is encoded by the ''GADD45A'' gene.
Function
This gene is a member of a group of genes, the GADD45 genes, whose transcript levels are increased following st ...
,
* GADD45G
Growth arrest and DNA-damage-inducible protein GADD45 gamma is a protein that in humans is encoded by the ''GADD45G'' gene on chromosome 9. GADD45G is also known as CR6, DDIT2, GRP17, OIG37, and GADD45gamma. GADD45G is involved in several differen ...
,
* HDAC
Histone deacetylases (, HDAC) are a class of enzymes that remove acetyl groups (O=C-CH3) from an ε-N-acetyl lysine amino acid on a histone, allowing the histones to wrap the DNA more tightly. This is important because DNA is wrapped around his ...
,
* PCNA
Proliferating cell nuclear antigen (PCNA) is a DNA clamp that acts as a processivity factor for DNA polymerase δ in eukaryotic cells and is essential for replication. PCNA is a homotrimer and achieves its processivity by encircling the DNA, whe ...
,
* PIM1
Proto-oncogene serine/threonine-protein kinase Pim-1 is an enzyme that in humans is encoded by the ''PIM1'' gene.
Pim-1 is a proto-oncogene which encodes for the serine/threonine kinase of the same name. The pim-1 oncogene was first described in r ...
,
* TK1, and
* TSG101
Tumor susceptibility gene 101, also known as TSG101, is a human gene that encodes for a cellular protein of the same name.
Function
The protein encoded by this gene belongs to a group of apparently inactive homologs of ubiquitin-conjugating enzy ...
.
References
Further reading
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External links
*
''Drosophila'' ''dacapo'' - The Interactive Fly
*
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{{Cell cycle proteins
Cell cycle regulators
Tumor suppressor genes