A yeast model system for functional analysis of β-catenin signaling

We have developed a novel Saccharomyces cerevisiae model system to dissect the molecular events of β-catenin (β-cat) signaling. Coexpression of mammalian β-cat with TCF4 or LEF1 results in nuclear accumulation of these proteins and a functional complex that activates reporter gene transcription from constructs containing leukocyte enhancer factor (LEF)/T cell factor (TCF) response elements. Reporter transcription is constitutive, requires expression of both β-cat and TCF4 or LEF1, and is not supported by mutated LEF/TCF binding elements or by TCF4 or LEF1 mutants. A cytoplasmic domain of E-cadherin or a functional fragment of adenomatous polyposis coli (APC) protein (APC-25) complexes with β-cat, reduces β-cat binding to TCF4, and leads to increased cytoplasmic localization of β-cat and a reduction in reporter activation. Systematic mutation of putative nuclear export signal sequences in APC-25 decreases APC-25 binding to β-cat and restores reporter gene transcription. Additional β-cat signaling components, Axin and glycogen synthase kinase 3β, form a multisubunit complex similar to that found in mammalian cells. Coexpression of the F-box protein β-transducin repeat-containing protein reduces the stability of β-cat and decreases reporter activation. Thus, we have reconstituted a functional β-cat signal transduction pathway in yeast and show that β-cat signaling can be regulated at multiple levels, including protein subcellular localization, protein complex formation, and protein stability.