Sic1
Encyclopedia
Sic1, a protein
, is a stoichiometric inhibitor of Cdk1
-Clb (B-type cyclins) complexes in the budding yeast Saccharomyces cerevisiae
. Because B-type cyclin-Cdk1 complexes are the drivers of S-phase initiation, Sic1 prevents premature S-phase entry. Multisite phosphorylation
of Sic1 is thought to time Sic1 ubiquitination and destruction, and by extension, the timing of S-phase entry.
, the assembly of the prereplicative complex and initiation of bud
formation in yeast.
At the START point in the yeast cell cycle, the G1-cyclin
s Cln3
, Cln1 and Cln 2 activate Cdc28. The activated complex will phosphorylate
Sic1 at multiple sites which leads to its degradation by the SCF complex
. When Sic1 is degraded, the Cdc28-Clb complex is no longer inhibited and the cell can enter the S/M-phase. Thus Sic1 inactivation is essential for transition into S phase (Fig.1).
Cdc28 in complex with B-type cyclin
(Cdc28-Clb) phosphorylates Swi5
, the transcription factor
of Sic1. This promotes the export of Swi5 from the nucleus
to the cytoplasm and avoids further transcription of the cdk inhibitor. Cdc28-Clb also phosphorylates any Sic1 molecules still available and triggers their ubiquitin
-dependent degradation, exactly like Cdc28-Cln. High Cdc28-Clb levels also initiate DNA replication and duplication of the spindle pole bodies (SPBs). Then the metaphase
spindle assembles and chromosome segregation
can occur. The transcription of Sic1 starts during telophase
, mediated by Swi5. Aca2 is another transcription factor of Sic1, but remains inactive until G1. At the end of mitosis, Sic1 is involved in the inactivation of Cdc28-Clb.
In order to be recognised by Cdc4 of the SCF complex
, Sic1 has to be phosphorylated
at least at 6 of the 9 cdk sites (Fig. 2). Sic1 can also be phosphorylated by other kinases, such as , a kinase which becomes essential when Cln1 and Cln2 are absent. Sic1 has also a role in the response to osmostress. The stress-activated protein kinase (SAPK) Hog1 phosphorylates Sic1 at a single residue at the carboxyl terminus. This leads to downregulation of cyclin expression and Sic1 stabilization which arrests the cell cycle.
at multiple sites for ubiquitination-driven degradation (Fig. 2). The multiple phosphorylations are required for Sic1 to be recruited by Cdc4 to the SCF complex. The Cdc4 substrate recognition mechanism includes the interaction with consensus binding motifs on the surface of the folded and phosphorylated Sic1, the so called Cdc4 phospho-degrons (CPD). It has been shown that the optimal consensus sequence for Cdc4 is a phosphorylated serine
or threonine
followed by a proline
and a basic amino acid. However, none of the CPDs on the surface of the Sic1 show such a composition. Therefore, multiple phosphorylation of Sic1 is necessary to get high-affinity binding to Cdc4. Although this mechanism looks inefficient, it provides advantages for a cell because it is possible to measure the environmental Cln/cdc28 concentration. The number of phosphorylated sites corresponds to the concentration of Cln/cdc28 and Sic1 could be considered as a sensor for this protein. In contrast to the many sharp transitions of ultrasensitive kinase cascade feedback loops, this mechanism allows fine tuned regulation. Moreover, because multiple phosphorylations are required, the probability that Sic1 is degraded by random is small. Using multiple phosphorylation of Sic1, the cell evolved a strategy to highly regulate the onset of DNA replication that is absolutely vital to provide genetic stability.
There are several human diseases that are linked to p27Kip1 and other cyclin kinase inhibitors:
Thus, the human Cdk inhibitor p27Kip1 is a potential tumor suppressor protein. If its expression is reduced, the result might be unregulated progression from G1 to S-phase which deregulates cell division and simplifies the formation of tumors.
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...
, is a stoichiometric inhibitor of 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....
-Clb (B-type cyclins) complexes in the 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...
. Because B-type cyclin-Cdk1 complexes are the drivers of S-phase initiation, Sic1 prevents premature S-phase entry. Multisite phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
of Sic1 is thought to time Sic1 ubiquitination and destruction, and by extension, the timing of S-phase entry.
Role of Sic1 in the cell cycle control
In the G1 phase of the cell cycle, Sic1 binds tightly to the Cdc28-Clb complex and inhibits it. Low Cdc28-Clb activity leads to the disassembly of the mitotic spindleMitotic 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...
, the assembly of the prereplicative complex and initiation of bud
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...
formation in yeast.
At the START point in the yeast cell cycle, the G1-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 Cln3
CLN3
Battenin is a protein that in humans is encoded by the CLN3 gene located on chromosome 16.- Function :Battenin is involved in lysosomal function. Many alternatively spliced transcript variants have been found for this gene.-Clinical significance:...
, Cln1 and Cln 2 activate Cdc28. The activated complex will phosphorylate
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
Sic1 at multiple sites which leads to its degradation by the SCF complex
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...
. When Sic1 is degraded, the Cdc28-Clb complex is no longer inhibited and the cell can enter the S/M-phase. Thus Sic1 inactivation is essential for transition into S phase (Fig.1).
Cdc28 in complex with B-type 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 :...
(Cdc28-Clb) phosphorylates Swi5
SWI/SNF
SWI/SNF is a yeast nucleosome remodeling complex composed of several proteins – products of the SWI and SNF genes as well as several other polypeptides...
, the transcription factor
Transcription factor
In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...
of Sic1. This promotes the export of Swi5 from the nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
to the cytoplasm and avoids further transcription of the cdk inhibitor. Cdc28-Clb also phosphorylates any Sic1 molecules still available and triggers their 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...
-dependent degradation, exactly like Cdc28-Cln. High Cdc28-Clb levels also initiate DNA replication and duplication of the spindle pole bodies (SPBs). Then the 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...
spindle assembles and chromosome segregation
Chromosome segregation
Chromosome segregation is a step in cell reproduction or division, where chromosomes pair off with their similar homologous chromosome. In mitosis, a complete copy of each one is made. In meiosis, one chromosome from each pair migrates to opposite ends of the cell and the genes are split to make a...
can occur. The transcription of Sic1 starts during telophase
Telophase
Telophase from the ancient Greek "τελος" and "φασις" , is a stage in both meiosis and mitosis in a eukaryotic cell. During telophase, the effects of prophase and prometaphase events are reversed. Two daughter nuclei form in the cell. The nuclear envelopes of the daughter cells are formed from the...
, mediated by Swi5. Aca2 is another transcription factor of Sic1, but remains inactive until G1. At the end of mitosis, Sic1 is involved in the inactivation of Cdc28-Clb.
Ubiquitin-dependent degradation of Sic1
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...
, Sic1 has to be phosphorylated
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
at least at 6 of the 9 cdk sites (Fig. 2). Sic1 can also be phosphorylated by other kinases, such as , a kinase which becomes essential when Cln1 and Cln2 are absent. Sic1 has also a role in the response to osmostress. The stress-activated protein kinase (SAPK) Hog1 phosphorylates Sic1 at a single residue at the carboxyl terminus. This leads to downregulation of cyclin expression and Sic1 stabilization which arrests the cell cycle.
Multiple phosphorylations allow fine-tuning
Sic1 needs to be phosphorylatedPhosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
at multiple sites for ubiquitination-driven degradation (Fig. 2). The multiple phosphorylations are required for Sic1 to be recruited by Cdc4 to the SCF complex. The Cdc4 substrate recognition mechanism includes the interaction with consensus binding motifs on the surface of the folded and phosphorylated Sic1, the so called Cdc4 phospho-degrons (CPD). It has been shown that the optimal consensus sequence for Cdc4 is a phosphorylated serine
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...
or threonine
Threonine
Threonine is an α-amino acid with the chemical formula HO2CCHCHCH3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar...
followed by a proline
Proline
Proline is an α-amino acid, one of the twenty DNA-encoded amino acids. Its codons are CCU, CCC, CCA, and CCG. It is not an essential amino acid, which means that the human body can synthesize it. It is unique among the 20 protein-forming amino acids in that the α-amino group is secondary...
and a basic amino acid. However, none of the CPDs on the surface of the Sic1 show such a composition. Therefore, multiple phosphorylation of Sic1 is necessary to get high-affinity binding to Cdc4. Although this mechanism looks inefficient, it provides advantages for a cell because it is possible to measure the environmental Cln/cdc28 concentration. The number of phosphorylated sites corresponds to the concentration of Cln/cdc28 and Sic1 could be considered as a sensor for this protein. In contrast to the many sharp transitions of ultrasensitive kinase cascade feedback loops, this mechanism allows fine tuned regulation. Moreover, because multiple phosphorylations are required, the probability that Sic1 is degraded by random is small. Using multiple phosphorylation of Sic1, the cell evolved a strategy to highly regulate the onset of DNA replication that is absolutely vital to provide genetic stability.
Sic1 homologue in human and diseases
The protein p27Kip1 is a human homologue of Sic1, both having a conserved inhibitory domain, but p27Kip1 inhibits G1 cyclins and not cyclin B.There are several human diseases that are linked to p27Kip1 and other cyclin kinase inhibitors:
- All Papillary microcarcinomas (PMCs) of the thyroidThyroidThe thyroid gland or simply, the thyroid , in vertebrate anatomy, is one of the largest endocrine glands. The thyroid gland is found in the neck, below the thyroid cartilage...
have a lower expression of p27Kip1 than normal thyroid tissue. Additionally, the expression of p27Kip1 in more aggressive, metastasising Papillary microcarcinomas is strongly reduced compared to nonmetastasing microcarcinomas. These results suggest that p27Kip1 acts as a tumor suppressorTumor suppressor geneA tumor suppressor gene, or anti-oncogene, is a gene that protects a cell from one step on the path to cancer. When this gene is mutated to cause a loss or reduction in its function, the cell can progress to cancer, usually in combination with other genetic changes.-Two-hit hypothesis:Unlike...
. - Kaposi's sarcomaKaposi's sarcomaKaposi's sarcoma is a tumor caused by Human herpesvirus 8 , also known as Kaposi's sarcoma-associated herpesvirus . It was originally described by Moritz Kaposi , a Hungarian dermatologist practicing at the University of Vienna in 1872. It became more widely known as one of the AIDS defining...
is a type of cancer which appears in combination with AIDSAIDSAcquired immune deficiency syndrome or acquired immunodeficiency syndrome is a disease of the human immune system caused by the human immunodeficiency virus...
and presumably is caused by the human herpesvirus 8 (HHV8)Kaposi's sarcoma-associated herpesvirusKaposi's sarcoma-associated herpesvirus is one of seven currently known human cancer viruses, or oncoviruses. It is also the eighth human herpesvirus; its formal name according to the International Committee on Taxonomy of Viruses is HHV-8. Like other herpesviruses, its informal name is used...
. This virus expresses a viral cyclinCyclinCyclins are a family of proteins that control the progression of cells through the cell cycle by activating cyclin-dependent kinase enzymes.- Function :...
which builds a complex with Cdk6. This KSHV-cyclin-Cdk6 complex phosporylates and destabilises p27Kip1 which results in a low level of p27Kip1. This suggests that the degradation of p27Kip1 is associated with the development of the tumors. - Patients with a gastric adenocarcinomaAdenocarcinomaAdenocarcinoma is a cancer of an epithelium that originates in glandular tissue. Epithelial tissue includes, but is not limited to, the surface layer of skin, glands and a variety of other tissue that lines the cavities and organs of the body. Epithelium can be derived embryologically from...
(stomach cancerStomach cancerGastric cancer, commonly referred to as stomach cancer, can develop in any part of the stomach and may spread throughout the stomach and to other organs; particularly the esophagus, lungs, lymph nodes, and the liver...
) have a higher chance of survival if the tumor has high p27Kip1 expression. Low p27Kip1 expression can lead to tumor de-differentiation, increased penetration through the gastric wall, lymph node metastasis and advanced tumor stage.
Thus, the human Cdk inhibitor p27Kip1 is a potential tumor suppressor protein. If its expression is reduced, the result might be unregulated progression from G1 to S-phase which deregulates cell division and simplifies the formation of tumors.