Alternaria produces the AAL toxin, which activates a plant cell death program in tomato (Solanum lycopersicum) leaves.

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#Unit 4. Cell Communication and Cell Signaling

In the absence of Wnts,

“free” β-catenin are targeted for degradation by a cytosolic destruction complex that is held together by the tumor suppressor protein Axin.

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#Unit 4. Cell Communication and Cell Signaling

β-catenin Destruction Complex contains:

Tumor suppressor APC

Dvl protein

Constitutively active protein kinases GSK 3β and CK1

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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

E3 βTrCP ubiquitin ligase then binds to two phosphorylated residues on β catenin, leading to ubiquitinylation and degradation in proteasomes.

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In the absence of Wnt,

TCF is associated with transcriptional repressor proteins Groucho (Gro) that inhibit gene transcription in nucleus

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#Unit 4. Cell Communication and Cell Signaling