beta-Catenin Directly Induces Expression of the Siamois Gene, and Can Initiate Signaling Indirectly via a Membrane-Tethered Forma

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Abstract
Beta-catenin is shown to directly induce the expression of siamois (a homeobox-containing gene involved in axial patterning) in a cell-autonomous, protein synthesis independent manner. Siamois can thus be considered a direct target of beta-catenin signaling in Xenopus. Expression of a portion of the armadillo repeat region of beta-catenin via a membrane-tethered fusion protein is shown to give similar levels of siamois induction and axis duplication as a free, untethered form. Reduction of endogenous free beta-catenin levels by overexpression of C-cadherin leads to complete inhibition of signaling by the membrane-tethered repeat region. Since the membrane-tethered repeat region is unlikely to be bound up by C-cadherin, these results show that the membrane-tethered beta-catenin relies on endogenous beta-catenin for signaling. We propose that the membrane-tethered construct acts by titrating a cytoplasmic inhibitor of beta-catenin signaling.