The tubulin subunit is a heterodimer formed from two closely related globular proteins called a-tubulin and b-tubulin.

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#Unit 4. Cell Communication and Cell Signaling

“canonical” Wnt signaling pathway uses a second transmembrane protein, LRP that associates with Frizzled in a Wnt signal– dependent manner.

 

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#Unit 4. Cell Communication and Cell Signaling

Homologs

LRP in vertebrates = Arrow in Drosophila.

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#Unit 4. Cell Communication and Cell Signaling

In the absence of Wnts,

β-catenin is bound in a complex with Axin (a scaffold protein), APC, and kinases CK1 and GSK3.

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#Unit 4. Cell Communication and Cell Signaling

Kinases CK1 and GSK3,

sequentially phosphorylate β-catenin at multiple serine and threonine residues.

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#Unit 4. Cell Communication and Cell Signaling

The E3 TrCP ubiquitin ligase then binds to two phosphorylated β-catenin residues, leading to β-catenin ubiquitinylation and degradation in proteasomes.

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#Unit 4. Cell Communication and Cell Signaling

Binding of Wnt to its receptor Frizzled (Fz) and to the LRP co-receptor triggers phosphorylation of LRP by GSK3 and CK1, enables Axin to bind LRP & Dsh.