CellSig15 - 28 Flashcards
What targets Hh to the PM?
N-terminal sequence
What removes the N-terminal sequence of Hh?
C-terminal end
What happens to the C-terminal end of Hh after cleaving off the N-terminus?
Cholesterol coupling
What occurs to the N-terminal end of Hh after cleavage?
Palmitoylation
What adds palmitoyl to Hh?
Ski in Drosophila, Hhat in vertebrates
What do the modifications of Hh make it?
Highly hydrophobic - a problem for it trying to leave the membrane
What overcomes the hydrophobicity of Hh?
Dispatched and Scube - create multimeric Hh particles, or load Hh onto lipoprotein particles
What is required for long term transmission of Hh?
HSPGs
Characterise Ptc
12TM protein, similar to dispatched
What does Ptc do?
Inhibits Smo
What occurs when Hh binds Ptc?
Ptc inhibits its inhibition, so Smo is activated
What is the relationship stoichiometrically between Ptc and Smo?
Many Smo for each Ptc
What effect does Ptc have on Smo trafficking?
In absence of Hh, Smo is internalised
Outline how Ptc effects Smo trafficking in mammals
In the primary cilium - no Hh, Ptc localised to cilium; Hh, Ptc leaves and Smo can move in
Why is Ptc thought to be a pump?
Homology to prokaryotic permeases (pump out toxins); is related to NPC1, mutations in which cause uptake of fatty deposits on organs; C. elegans has Ptc-like proteins, but no Hh.
Molecule that can interfere with Smo
Cyclopamine
Who proposed the current model of Hh signalling, and when?
Hooper and Scott, 2005
What transcription factor is involved in Hh signalling?
Ci - cubitus interruptus
What occurs in the absence of Hh?
Two complexes keep Ci out of the nucleus - Cos2/Fused/Ci (bound to Smo) and SUFU/Ci. CKI/PKA/GSK3 acts on Ci, bringing Slimb which shortens Ci to CiR. CiR represses Hh target genes
What occurs in the presence of Hh?
Hh releases Ci from the two precursor complexes
How does the CKI/PKA/GSK3 complex prep Ci for Slimb interaction?
PKA phosphorylates first, priming for phosphorylation by GSK3 and CK1
What is Slimb?
F box protein - component of SCF unbiquitin E3 ligase complex
Positive regulators of Hh signalling?
CDO/BOC/GAS-1
What does Hhip do?
Binds Hh, limiting diffusion
What is interesting about Gli1?
Feedsforward - can only activate the Hh pathway
What are the mammalian homologues of Ci?
Gli1-3
What does Hh form in the developing limb?
Zone of polarizing activity - conferring posterior identity
What major conditions is Hh implicated in?
Holosprosenchephaly, syndactyly, cancers, Gorlin syndrome
