42. In response to a Wnt signal, β-catenin enters the nucleus and binds to the LEF1/TCF proteins by displacing which one of the following proteins?
    (1) CBP (2) GSK3
    (3) Groucho  (4) NEMO

The correct answer is:

(3) Groucho

Explanation:

In response to a Wnt signal, β-catenin enters the nucleus and binds to the LEF1/TCF family of transcription factors. This binding displaces the corepressor protein Groucho (also known as Gro/TLE) from the LEF1/TCF proteins. Groucho typically acts to repress transcription in the absence of β-catenin. When β-catenin binds, it displaces Groucho, allowing transcriptional activation of Wnt target genes.

• CBP is a histone acetyltransferase coactivator and is not displaced by β-catenin in this context.

• GSK3 is a kinase involved in targeting β-catenin for degradation in the cytoplasm; it is not part of nuclear LEF1/TCF complexes.

• NEMO is involved in NF-kB signaling and unrelated here.

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Summary

• Wnt signals stabilize β-catenin, allowing its translocation to the nucleus.

• Nuclear β-catenin binds LEF1/TCF transcription factors on DNA.

• β-catenin displaces Groucho, a repressor protein, from LEF1/TCF complexes.

• Displacement relieves transcriptional repression and activates gene expression.

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Answer: (3) Groucho is displaced by β-catenin from LEF1/TCF proteins during Wnt signaling to activate transcription.