Hedgehog signalling Flashcards
What is the hedgehog mutant phenotype
Segment polarity phenotype - In each segment the naked part is lost creating an embryo full with denticles
What modifications are made to the ligand in the secretory pathway
After removal of the N terminus secretory pathway signal a autoproteolytic cleavage occurs, catalysed by the C terminus of the ligand
The N terminus then has a cholesterol group added to its C terminus
N terminus also has a palmitoyl group added (this is performed by skinny Hh in invertebrates and Hhat in vertebrates)
What is the result of the addition of cholesterol and palmitoyl groups
Both are strongly hydrophobic and render hedgehog insoluble in water
This means it is tethered to the membrane and would only be able to achieve juxtacrine signalling if not for the action of dispatched protein and Scube glycoprotein
What is the action of Scube and Dispatched proteins
May help to create multimeric hedgehog proteins that have their hydrophobic tails buried inside
OR
Help load Hh onto lipoprotein particles in the ECM HSPGs have also been shown to be required for long range diffusion
What are the receptors in the Hh signalling pathway
1) Patched is a 12TMD protein that can bind Hh, it acts in a similar way to dispatched as it acts to continuously inhibit Smoothened
2) Smoothened - 7TMD - When the Hh binds patched it somehow inhibits the inhibitory function
What is the relationship between smoothened and patched in drosophila
Patched is able to inhibit the activity of a large number of smoothened
Patched somehow regulates the subcellular localisation and stability of smoothened
In the absence of Hh - patched prevents smoothened making it to the cell surface
Its thought to regulate the trafficking of smoothened to a compartment where it can be degraded
What happens to smoothened when Hh binds patched
Both patched and Hh are internalised to be degraded. Smoothened is then trafficked to the cell surface
This results in relocation, accumulation and phosphorylation
What is the role of primary cilium in mammals and the signalling process
In the absence of Hh patched is localised to the cilium of the cell so smoothened is excluded from this territory
When Hh binds to patched it is removed from the cilium allowing smoothened to accumulate there and initiate signalling
What is the result of mutations in mices’ cilium
Disruption of hedgehog signalling pathway lololol
What are two examples of genes that Ptc has homology to
Prokaryotic permeases that can confer multi drug resistance by pumping out toxins
NPC1 - gene mutated in NPC1 disease - metabolic disorder in which harmful levels of fatty substances accumulate in the spleen liver and lungs
What is the role of hedgehog interacting protein
Vertebrate specific protein - most likely mops up the free Hh thereby preventing it from reaching the patched receptor
Outline the signalling pathway in the absence of Hh
2 complexes exist that keep TF Ci out of the nucleus
1) Costal2(scaffold protein) and fused are bound to Ci
2) Ci and suppressor of fused gene SuFu
1 allows for three other genes to be able to act on Ci
They form a complex consisting of CK1, PKA, GSK3
The Ci is processed under the influence of these genes into a shorter version vis Slimb
The short form translocates into the nucleus and acts as a transcriptional repressor
What occurs in the signalling pathway when there is a low conc of Hh
Thought that PKA,GSK3 and CK1 complex dissociates from the complex containing Ci and as a result, active repression by CiR is lost
What occurs in the signalling pathway when there is a high conc of Hh
Hh acts through both complexes to release a full length Ci that will actively promote transcription
This could involve phosphorylation but the exact nature of this is unknown
Thought that the phosphorylation of SuFu by fused promotes the formation of an active form of Ci
