Hedgehog 'on' state (Homo sapiens)
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Description
Hedgehog is a secreted morphogen that has evolutionarily conserved roles in body organization by regulating the activity of the Ci/Gli transcription factor family. In Drosophila in the absence of Hh signaling, full-length Ci is partially degraded by the proteasome to generate a truncated repressor form that translocates to the nucleus to represses Hh-responsive genes. Binding of Hh ligand to the Patched (PTC) receptor allows the 7-pass transmembrane protein Smoothened (SMO) to be activated in an unknown manner, disrupting the partial proteolysis of Ci and allowing the full length activator form to accumulate (reviewed in Ingham et al, 2011; Briscoe and Therond, 2013).
While many of the core components of Hh signaling are conserved from flies to humans, the pathways do show points of significant divergence. Notably, the human genome encodes three Ci homologues, GLI1, 2 and 3 that each play slightly different roles in regulating Hh responsive genes. GLI3 is the primary repressor of Hh signaling in vertebrates, and is converted to the truncated GLI3R repressor form in the absence of Hh. GLI2 is a potent activator of transcription in the presence of Hh but contributes only minimally to the repression function. While a minor fraction of GLI2 protein is processed into the repressor form in the absence of Hh, the majority is either fully degraded by the proteasome or sequestered in the full-length form in the cytosol by protein-protein interactions. GLI1 lacks the repression domain and appears to be an obligate transcriptional activator (reviewed in Briscoe and Therond, 2013).
Vertebrate but not fly Hh signaling also depends on the movement of pathway components through the primary cilium. The primary cilium is a non-motile microtubule based structure whose construction and maintenance depends on intraflagellar transport (IFT). Anterograde IFT moves molecules from the ciliary base along the axoneme to the ciliary tip in a manner that requires the microtubule-plus-end directed kinesin KIF3 motor complex and the IFT-B protein complex, while retrograde IFT back to the ciliary base depends on the minus-end directed dynein motor and the IFT-A complex. Genetic screens have identified a number of cilia-related proteins that are required both to maintain Hh in the 'off' state and to transduce the signal when the pathway is activated (reviewed in Hui and Angers, 2011; Goetz and Anderson, 2010). View original pathway at:Reactome.
While many of the core components of Hh signaling are conserved from flies to humans, the pathways do show points of significant divergence. Notably, the human genome encodes three Ci homologues, GLI1, 2 and 3 that each play slightly different roles in regulating Hh responsive genes. GLI3 is the primary repressor of Hh signaling in vertebrates, and is converted to the truncated GLI3R repressor form in the absence of Hh. GLI2 is a potent activator of transcription in the presence of Hh but contributes only minimally to the repression function. While a minor fraction of GLI2 protein is processed into the repressor form in the absence of Hh, the majority is either fully degraded by the proteasome or sequestered in the full-length form in the cytosol by protein-protein interactions. GLI1 lacks the repression domain and appears to be an obligate transcriptional activator (reviewed in Briscoe and Therond, 2013).
Vertebrate but not fly Hh signaling also depends on the movement of pathway components through the primary cilium. The primary cilium is a non-motile microtubule based structure whose construction and maintenance depends on intraflagellar transport (IFT). Anterograde IFT moves molecules from the ciliary base along the axoneme to the ciliary tip in a manner that requires the microtubule-plus-end directed kinesin KIF3 motor complex and the IFT-B protein complex, while retrograde IFT back to the ciliary base depends on the minus-end directed dynein motor and the IFT-A complex. Genetic screens have identified a number of cilia-related proteins that are required both to maintain Hh in the 'off' state and to transduce the signal when the pathway is activated (reviewed in Hui and Angers, 2011; Goetz and Anderson, 2010). View original pathway at:Reactome.
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DataNodes
gene,PTCH2 gene,BOC
geneT-SMO
dimer:CSNK1A1:ADRBK1S590, T593, S595-SMO
dimer:CSNK1A1:ADRBK1S595-SMO
dimer:CSNK1A1Annotated Interactions
gene,PTCH2 gene,BOC
geneOnce in the nucleus, phosphorylated GLI transcription factors bind to promoters of Hh-responsive genes such as PTCH1, PTCH2, GLI1 and HHIP to activate transcription (Vokes et al, 2007; Vokes et al 2007; Lee et al, 2010; reviewed in Briscoe and Therond, 2013). The full-length transcriptionally active GLI proteins are labile and subject to SPOP-dependent proteolysis (Chen et al, 2009; Zhang et al, 2009; Wen et al, 2010).
The stability of SPOP itself is regulated in an unknown manner by DZIP1, a regulator of Hh signaling best characterized in zebrafish for its positive role in promoting ciliogenesis (Sekimizu et al, 2004; Wolff et al, 2004; Glazer et al, 2010; Kim et al, 2010; Tay et al, 2010; Wang et al, 2013). More recently, DZIP1 has also been shown to act as a negative regulator of Hh signaling by preventing the ubiquitin- and proteasome-dependent degradation of SPOP, and in this way increasing the turnover of activated GLI proteins (Jin et al, 2011; Schwend et al, 2013).
T-SMO
dimer:CSNK1A1:ADRBK1T-SMO
dimer:CSNK1A1:ADRBK1S590, T593, S595-SMO
dimer:CSNK1A1:ADRBK1S590, T593, S595-SMO
dimer:CSNK1A1:ADRBK1S590, T593, S595-SMO
dimer:CSNK1A1:ADRBK1S595-SMO
dimer:CSNK1A1S595-SMO
dimer:CSNK1A1