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{{Short description|Mammalian protein found in Homo sapiens}}
{{infobox protein
| Name = scleraxis homolog A (mouse)
| caption =
| image =
| width =
| HGNCid = 24312
| Symbol = SCXA
| AltSymbols =
| EntrezGene = 333927
| OMIM = 609067
| RefSeq =
| UniProt = Q7RTU7
| PDB =
| ECnumber =
| Chromosome = 8
| Arm = q
| Band = 24.3
| LocusSupplementaryData =
}}
{{infobox protein
| Name = scleraxis homolog B (mouse)
| caption =
| image =
| width =
| HGNCid = 32322
| Symbol = SCXB
| AltSymbols =
| EntrezGene = 642658
| OMIM =
| RefSeq = XM_926116
| UniProt =
| PDB =
| ECnumber =
| Chromosome = 8
| Arm = q
| Band = 24.3
| LocusSupplementaryData =
}}

The '''scleraxis''' [[protein]] is a member of the [[basic helix-loop-helix]] (bHLH) superfamily of [[transcription factor]]s.<ref name="pmid7743923">{{cite journal | vauthors = Cserjesi P, Brown D, Ligon KL, Lyons GE, Copeland NG, Gilbert DJ, Jenkins NA, Olson EN | title = Scleraxis: a basic helix-loop-helix protein that prefigures skeletal formation during mouse embryogenesis | journal = Development | volume = 121 | issue = 4 | pages = 1099–110 | date = April 1995 | doi = 10.1242/dev.121.4.1099 | pmid = 7743923 | url = http://dev.biologists.org/cgi/content/abstract/121/4/1099 | url-access = subscription }}</ref> Currently two genes ({{gene|SCXA}} and {{gene|SCXB}} respectively) have been identified to code for identical scleraxis proteins.

== Function ==
It is thought that early scleraxis-expressing [[progenitor cell]]s lead to the eventual formation of [[tendon]] tissue and other muscle attachments.<ref name="pmid7743923"/> Scleraxis is involved in [[mesoderm]] formation and is expressed in the syndetome (a collection of embryonic tissue that develops into tendon and blood vessels) of developing [[somite]]s (primitive segments or compartments of embryos).<ref name="pmid12705871">{{cite journal | vauthors = Brent AE, Schweitzer R, Tabin CJ | title = A somitic compartment of tendon progenitors | journal = Cell | volume = 113 | issue = 2 | pages = 235–48 | date = April 2003 | pmid = 12705871 | doi = 10.1016/S0092-8674(03)00268-X | s2cid = 16291509 | doi-access = free }}</ref>

== Inducing scleraxis expression ==
The syndetome location within the somite is determined by [[Fibroblast growth factor|FGF]] secreted from the center of the myotome (a collection of embryonic tissue that develops into [[skeletal muscle]])- the FGF then induces the adjacent [[Anatomical terms of location|anterior]] and [[Anatomical terms of location|posterior]] sclerotome (a collection of embryonic tissue that develops into the [[axial skeleton]]) to adopt a tendon cell fate. This ultimately places future scleraxis-expressing cells between the two tissue types they will ultimately join.<ref name="Brent_2004">{{cite journal | vauthors = Brent AE, Tabin CJ | title = FGF acts directly on the somitic tendon progenitors through the Ets transcription factors Pea3 and Erm to regulate scleraxis expression | journal = Development | volume = 131 | issue = 16 | pages = 3885–96 | date = August 2004 | pmid = 15253939 | doi = 10.1242/dev.01275 | doi-access = free }}</ref>

Scleraxis expression will be seen throughout the entire sclerotome (rather than just the sclerotome directly anterior and posterior to the myotome) with an overexpression of [[FGF8]], demonstrating that all sclerotome cells are capable of expressing scleraxis in response to FGF signaling. While the FGF interaction has been shown to be necessary for scleraxis expression, it is still unclear as to whether the FGF signaling pathway directly induces the syndetome to secrete scleraxis, or indirectly through a secondary signaling pathway. Most likely, the syndetomal cells, through careful reading of the FGF [[concentration]] (coming from the myotome), can precisely determine their location and begin expressing scleraxis.<ref name="Brent_2004" /> Much of embryonic development follows this model of inducing specific cell fates through the reading of surrounding signaling molecule concentration gradients.

== Background ==
bHLH transcription factors have been shown to have a wide array of functions in [[developmental biology|developmental]] processes.<ref name="pmid8424906">{{cite journal | vauthors = Kadesch T | title = Consequences of heteromeric interactions among helix-loop-helix proteins | journal = Cell Growth & Differentiation | volume = 4 | issue = 1 | pages = 49–55 | date = January 1993 | pmid = 8424906 | url = http://dev.biologists.org/cgi/content/abstract/121/4/1099 }}</ref> More precisely, they have critical roles in the control of [[cellular differentiation]], [[cell growth|proliferation]] and regulation of [[oncogenesis]].<ref name="pmid8424906"/><ref name="pmid8288123">{{cite journal | vauthors = Olson EN, Klein WH | title = bHLH factors in muscle development: dead lines and commitments, what to leave in and what to leave out | journal = Genes & Development | volume = 8 | issue = 1 | pages = 1–8 | date = January 1994 | pmid = 8288123 | doi = 10.1101/gad.8.1.1 | doi-access = free }}</ref><ref name="pmid8378299">{{cite journal | vauthors = Jan YN, Jan LY | title = Functional gene cassettes in development | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 90 | issue = 18 | pages = 8305–7 | date = September 1993 | pmid = 8378299 | pmc = 47343 | doi = 10.1073/pnas.90.18.8305 | bibcode = 1993PNAS...90.8305J | doi-access = free }}</ref> To date, 242 [[eukaryote|eukaryotic]] proteins belonging to the HLH superfamily have been reported. They have varied expression patterns in all eukaryotes from yeast to humans.<ref name="pmid9144210">{{cite journal | vauthors = Atchley WR, Fitch WM | title = A natural classification of the basic helix-loop-helix class of transcription factors | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 94 | issue = 10 | pages = 5172–6 | date = May 1997 | pmid = 9144210 | pmc = 24651 | doi = 10.1073/pnas.94.10.5172 | bibcode = 1997PNAS...94.5172A | doi-access = free }}</ref>

Structurally, bHLH proteins are characterised by a “highly conserved domain containing a stretch of basic [[amino acid]]s adjacent to two [[amphiphile|amphipathic]] [[alpha helix|α-helices]] separated by a loop”.<ref name="pmid15226298">{{cite journal | vauthors = Wilson-Rawls J, Rhee JM, Rawls A | title = Paraxis is a basic helix-loop-helix protein that positively regulates transcription through binding to specific E-box elements | journal = The Journal of Biological Chemistry | volume = 279 | issue = 36 | pages = 37685–92 | date = September 2004 | pmid = 15226298 | doi = 10.1074/jbc.M401319200 | doi-access = free }}</ref><ref name="pmid7926781">{{cite journal | vauthors = Ellenberger T, Fass D, Arnaud M, Harrison SC | title = Crystal structure of transcription factor E47: E-box recognition by a basic region helix-loop-helix dimer | journal = Genes & Development | volume = 8 | issue = 8 | pages = 970–80 | date = April 1994 | pmid = 7926781 | doi = 10.1101/gad.8.8.970 | doi-access = free }}</ref>

These helices have important functional properties, forming part of the DNA binding and transcription activating domains. With respect to scleraxis, the bHLH region spans amino acid residues 78 to 131. A proline rich region is also predicted to lie between residues 161–170. A stretch of basic residues, which aids in DNA binding, is found closer to the N terminal end of scleraxis.<ref name="pmid7743923"/><ref name="pmid1840517">{{cite journal | vauthors = Wolf C, Thisse C, Stoetzel C, Thisse B, Gerlinger P, Perrin-Schmitt F | title = The M-twist gene of Mus is expressed in subsets of mesodermal cells and is closely related to the Xenopus X-twi and the Drosophila twist genes | journal = Developmental Biology | volume = 143 | issue = 2 | pages = 363–73 | date = February 1991 | pmid = 1840517 | doi = 10.1016/0012-1606(91)90086-I }}</ref>

HLH proteins that lack this basic domain have been shown to negatively regulate the activities of bHLH proteins and are called inhibitors of differentiation (Id).<ref name="pmid8922523">{{cite journal | vauthors = Jen Y, Manova K, Benezra R | title = Expression patterns of Id1, Id2, and Id3 are highly related but distinct from that of Id4 during mouse embryogenesis | journal = Developmental Dynamics | volume = 207 | issue = 3 | pages = 235–52 | date = November 1996 | pmid = 8922523 | doi = 10.1002/(SICI)1097-0177(199611)207:3<235::AID-AJA1>3.0.CO;2-I | doi-access = free }}</ref> Basic HLH proteins function normally as dimers and bind to a specific hexanucleotide DNA sequence (CAANTG) known as an [[E-box]] thus switching on the expression of various genes involved in cellular development and survival.

== References ==
{{Reflist|33em}}

{{Transcription factors|g1}}

[[Category:Genetics]]
[[Category:Gene expression]]
[[Category:Transcription factors]]

Scleraxis - История изменений

Admin: 1 версия импортирована

ru>Not-cheesewhisk3rs: Fix reference spacing per MOS:REFPUNCT