Biology:SKI protein

Short description: Protein-coding gene in the species Homo sapiens

Generic protein structure example

The SKI protein is a nuclear proto-oncogene that is associated with tumors at high cellular concentrations. SKI has been shown to interfere with normal cellular functioning by both directly impeding expression of certain genes inside the nucleus of the cell as well as disrupting signaling proteins that activate genes.

SKI negatively regulates transforming growth factor-beta (TGF-beta) by directly interacting with Smads and repressing the transcription of TGF-beta responsive genes. This has been associated with cancer due to the large number of roles that peptide growth factors, of which TGF-beta are a subfamily, play in regulating cellular functions such as cell proliferation, apoptosis, specification, and developmental fate.

The name SKI comes from the Sloan-Kettering Institute where the protein was initially discovered.

Structure

Gene

The SKI proto-oncogene is located at a region close to the p73 tumor suppressor gene at the locus 1p36.3 locus of a gene, suggesting a similar function to the p73 gene.

Protein

Crystal structure of the Dachshund-homology domain of human SKI.

The SKI protein has a 728 amino acid sequence, with multiple domains. It is expressed both inside and outside of the nucleus. It is in the same family as the SnoN protein. The different domains have different functions, with the primary domains interacting with Smad proteins. The protein has a helix-turn-helix motif, a cysteine and histidine rich area which gives rise to the zinc finger motif, a basic amino acid region, and leucine zipper. All these domains, including a proline rich region, are consistent with the fact that the protein must have domains that allow it to interact with other proteins. The protein also has hydrophobic regions which come into contact with Smad proteins rich in leucine and phenylalanine amino acid regions. Recent studies have suggested a domain similar to the Dachshund protein. The SKI-Dachshund homology domain (SKI-DHD) contains the helix turn helix domains of the protein and the beta-alpha-beta turn motifs.

Function

The SKI oncogene is present in all cells, and is commonly active during development. Specifically, avian fibroblasts depend on the SKI protein as a transcription co-regulator inducing transformation. The aforementioned DHD region is specifically employed for protein-protein interactions, while the 191 amino acid C terminus mediates oligomerization. Recent research shows that the SKI protein in cancerous cells acts as a suppressor, inhibiting transforming growth factor β (TGF- β) signaling. TGF- β is a protein which regulates cell growth. Signaling is regulated by a family of proteins called the Smad proteins. SKI is present in all adult and embryonic cells at low levels, however an over expression of the protein is characteristic of tumor cells. It is thought that high levels of SKI protein inactivate tumor suppression by displacement of other proteins and interference with the signaling pathway of TGF- β. The SKI protein and the CPB protein compete for binding with the Smad proteins, specifically competing with the Smad-3 and CReB-binding protein interactions. SKI also directly interacts with the R-Smad ∙ Smad-4 complex, which directly represses normal transcription of the TGF-β responsive genes, inactivating the cell's ability to stop growth and division, creating cancerous cells.

SKI has been linked to various cancers including human melanomas, esophageal squamous cell carcinoma, cervical cancer and the process of tumor progression. The link of SKI with human melanoma has been the most studied area of the protein's link to cancer. Currently it is thought that the SKI protein prevents response to TFG- β levels, causing tumor formation.

Related research

Other research has identified proteins similar to Ski. The SnoN protein was identified as a similar protein and is often discussed in conjugation with the Ski protein in publications. Recent research suggests that the role of SnoN could be somewhat different, and could potentially even play an antagonistic role.

Other recent studies have determined Fussel-15 and Fussel-18 to be homologous to the Ski/Sno family of proteins. Fussel-15 has been found to play much the same role as the Ski/Sno proteins, however its expression is not as ubiquitous as the Ski/Sno proteins. Fussel-18 has been found to have an inhibitory role in the TGF-beta signaling.

Dachshund and SKIDA1 are also in the Ski/Sno/Dac family (InterPro: IPR003380, IPR023216.

Interactions

SKI protein has been shown to interact with:

• HIPK2,
• MECP2,
• Mothers against decapentaplegic homolog 1 and
• Mothers against decapentaplegic homolog 2,
• Mothers against decapentaplegic homolog 3,
• NFIX,
• Promyelocytic leukemia protein,
• SKIL, and
• SNW1.

Further reading

• "Repression of TGF-beta signaling by the oncogenic protein SKI in human melanomas: consequences for proliferation, survival, and metastasis.". Oncogene 22 (20): 3123–9. 2003. doi:10.1038/sj.onc.1206452. PMID 12793438. 
• "Isolation of human cDNA clones of ski and the ski-related gene, sno.". Nucleic Acids Res. 17 (14): 5489–500. 1989. doi:10.1093/nar/17.14.5489. PMID 2762147. 
• "The cellular homologue of the transforming gene of SKV avian retrovirus maps to human chromosome region 1q22----q24.". Cytogenet. Cell Genet. 43 (3–4): 181–6. 1987. doi:10.1159/000132318. PMID 3026737. 
• "SnoI, a novel alternatively spliced isoform of the ski protooncogene homolog, sno.". Nucleic Acids Res. 21 (19): 4632–8. 1993. doi:10.1093/nar/21.19.4632. PMID 8233802. 
• "Complex formation between proteins encoded by the ski gene family.". J. Biol. Chem. 268 (18): 13710–6. 1993. doi:10.1016/S0021-9258(18)86915-0. PMID 8514802. 
• "DNA binding and transcriptional activation by the Ski oncoprotein mediated by interaction with NFI.". Nucleic Acids Res. 25 (19): 3895–903. 1997. doi:10.1093/nar/25.19.3895. PMID 9380514. 
• "The Ski oncoprotein interacts with Skip, the human homolog of Drosophila Bx42.". Oncogene 16 (12): 1579–86. 1998. doi:10.1038/sj.onc.1201687. PMID 9569025. 
• "Heterodimers of the SnoN and Ski oncoproteins form preferentially over homodimers and are more potent transforming agents.". Nucleic Acids Res. 27 (4): 1006–14. 1999. doi:10.1093/nar/27.4.1006. PMID 9927733. 
• "The Ski oncoprotein interacts with the Smad proteins to repress TGFbeta signaling.". Genes Dev. 13 (17): 2196–206. 1999. doi:10.1101/gad.13.17.2196. PMID 10485843. 
• "Interaction of the Ski oncoprotein with Smad3 regulates TGF-beta signaling.". Mol. Cell 4 (4): 499–509. 1999. doi:10.1016/S1097-2765(00)80201-4. PMID 10549282. 
• "c-Ski acts as a transcriptional co-repressor in transforming growth factor-beta signaling through interaction with smads.". J. Biol. Chem. 274 (49): 35269–77. 2000. doi:10.1074/jbc.274.49.35269. PMID 10575014. 
• "The Huntington's disease protein interacts with p53 and CREB-binding protein and represses transcription.". Proc. Natl. Acad. Sci. U.S.A. 97 (12): 6763–8. 2000. doi:10.1073/pnas.100110097. PMID 10823891. Bibcode: 2000PNAS...97.6763S. 
• "Role of PML and PML-RARalpha in Mad-mediated transcriptional repression.". Mol. Cell 7 (6): 1233–43. 2001. doi:10.1016/S1097-2765(01)00257-X. PMID 11430826. 
• "The Ski protein family is required for MeCP2-mediated transcriptional repression.". J. Biol. Chem. 276 (36): 34115–21. 2001. doi:10.1074/jbc.M105747200. PMID 11441023. 
• "Ski interacts with the evolutionarily conserved SNW domain of Skip.". Nucleic Acids Res. 29 (17): 3469–76. 2001. doi:10.1093/nar/29.17.3469. PMID 11522815. 
• "Cytoplasmic localization of the oncogenic protein Ski in human cutaneous melanomas in vivo: functional implications for transforming growth factor beta signaling.". Cancer Res. 61 (22): 8074–8. 2001. PMID 11719430. 
• "c-Jun associates with the oncoprotein Ski and suppresses Smad2 transcriptional activity.". J. Biol. Chem. 277 (32): 29094–100. 2002. doi:10.1074/jbc.M202831200. PMID 12034730. 
• "Structural mechanism of Smad4 recognition by the nuclear oncoprotein Ski: insights on Ski-mediated repression of TGF-beta signaling.". Cell 111 (3): 357–67. 2002. doi:10.1016/S0092-8674(02)01006-1. PMID 12419246. 
• "Ski is involved in transcriptional regulation by the repressor and full-length forms of Gli3.". Genes Dev. 16 (22): 2843–8. 2002. doi:10.1101/gad.1017302. PMID 12435627. 
• "The transforming activity of Ski and SnoN is dependent on their ability to repress the activity of Smad proteins.". J. Biol. Chem. 278 (33): 30540–7. 2003. doi:10.1074/jbc.M304016200. PMID 12764135. https://digital.library.unt.edu/ark:/67531/metadc733634/.