3. The role of the cytoskeleton in Hedgehog signaling (actin rearrangements, microtubules)a. The Smoothened protein localizes to the primary cilium of the target cells. Suppose there is amutation that impairs a protein involved in basal body formation. How do you think would thisaffect signaling through Smoothened? Make sure to define basal body, explain its function, andrelate it to Smoothened signaling.b. Cos2 and Fu bind to microtubules, localizing Ci/Gli to the cytoplasm. How do you think a mutationin a microtubule associated protein (MAP) that functions to stabilize microtubules would affectboth the microtubules and Hedgehog signaling? Make sure to describe the role of the protein,regulation of microtubule dynamics, and how this might affect cells when Hedgehog is bothpresent and absent. The mechanism of Ci/Gli cleavage to release the repressor and ubiquitin-mediated proteolysis of the transcriptionactivator subunit is also unclear. In the absence of Hedgehog signaling, the Ci/Gli protein is localized to microtubulesin the cytosol by two other proteins, called Fu and Cos2. A set of kinases phosphorylate proteins in the complex,which allows Ci/Gli to be recognized by the ubiquitin ligase (called Slimb in Drosophila). Proteolysis which onlydestroys the transcription activator subunit of Ci/Gli, releasing Ci/GliR (the transcription repressor). Activation ofSmoothened allows Fu and Cos2 to bind to Smoothened on the plasma membrane, somehow freeing Ci/Gli fromphosphorylation and ubiquitylation. Ci/Gli then travels to the nucleus.Because this signaling pathway is complicated and has a lot of unknowns, you can find Dr. Slenn's more detaileddrawing of the canonical signaling pathway below.Hedge hogplasma membraneSmoothenedFu Cos 2Does not happen when Smoothened is active,potenti ally because Smoothened binds to Fu and CasaSlimbvariouskinasesCIRFo Cos2microtubule(CiRCiRproteasomeCiRCiRtranscription"of tor getgenesenhancerHedgehog can also signal through non-canonical pathways that do not involve Ci/Gli. Smoothened is technically aGPCR, though its activating ligand is unknown, and the best studied result of Smoothened activation does notdepend on G-proteins. However, Smoothened does activate G-proteins in some cells. The activated G-proteinbinds to phospholipase C, turning it on. Through a series of steps that includes the opening of calcium channelson the endoplasmic reticulum and regulation of several other proteins, the actin cytoskeleton rearranges, leadingto cell movement and changes in cell shape. Additionally, Patched can activate other non-canonical intracellularsignaling pathways that do not involve Smoothened at all (not discussed on this project).312Patched Background information:In class, we divided cell signaling pathways into four main categories while acknowledging that not every signalingpathway fits into one of the four categories. For example, we discussed Notch signaling in animals and ethylenesignaling in plants. This project focuses on another new signaling pathway that does not fit into one ofour clear categories.The Hedgehog signaling pathway is involved in the embryonic development of animals. The primary outcomes ofHedgehog signaling are cell differentiation and proliferation due to changes in gene expression. These changes arenecessary for cell specialization during embryonic development.Signaling cells produce Hedgehog proteins. Hedgehog proteins activate their receptor, called Patched, onnearby target cells. The signal does not travel very far, and target cells that are closer to the signaling cell receivea stronger signal. In the target cell, signal transduction begins on the plasma membrane of the primary cilium, aspecialized part of the cell you may not have learned much about. There is no need to look up this structure (andif you choose to, Wikipedia is fine here); you should know what cilia are from(A) Absence of Hedgehog(B) Presence of Hedgehogclass.Outside of cellHedgehogPatchedSmoothenedPlasmamembraneIn the absence of Hedgehog signaling (left figure), Patched inhibits anothermembrane protein called Smoothened. In the cytosol, one subunit of atranscription factor called Ci in Drosophila and Gli in humans (Ci/Gli) isdestroyed via ubiquitin-mediated proteolysis. The remaining subunit of Ci/Gliacts as a transcription repressor (Ci/GliR), blocking transcription of genesCytosolCi/GliCi/GliРКАCK1GSK-3UbiquitylationCleavageinvolved in differentiation and cell division.RepressorWhen Hedgehog is present (right figure), it blocks Patched from inhibitingSmoothened. Smoothened is therefore activated, and through a series ofsteps, Smoothened activation results in the stabilization of Ci/Gli. Ci/Gli cantherefore travel to the nucleus and activate transcription of target genes.NucleusRepressorO Transcriptionrepressed- TranscriptionactivatedTHE CELL: A MOLECULAR APPROACH 7e, Figure 16.41© 2016 Sinauer Associates, Inc.Despite decades of research, the mechanism through which Patched inhibits Smoothened is still a bit of a mystery.One hypothesis is that Patched might be a transport protein that pumps a small molecule inhibitor of Smoothenedinto the cell or that pumps an activator out of the cell. There is some experimental evidence that Patched functionsby preventing Smoothened access to cholesterol in the membrane, and when Hedgehog inhibitsSmoothened gains access to the cholesterol it needs to be activated. Patched might control cholesterol levelsthrough its transporter function.2Page 2/1112

To address the question first, I need to briefly explain about the cilium structure. Each cilium is…

Answered: The role of the cytoskeleton in…

9th Edition

ISBN:9781319114671

Author:Lubert Stryer, Jeremy M. Berg, John L. Tymoczko, Gregory J. Gatto Jr.

Publisher:Lubert Stryer, Jeremy M. Berg, John L. Tymoczko, Gregory J. Gatto Jr.

Chapter1: Biochemistry: An Evolving Science

Section: Chapter Questions

Problem 1P