In cells not exposed to the Wnt signal (left), Arm levels are kept low through interactions with the protein kinase zw3 (GSK-3b), APC and Axin (Behrens J, et al., Itoh K et al., Hart MJ et al.). In the absence of the Wnt signal,TCF acts as a repressor of Wnt/Wg target genes (Brannon 1997, Bienz 1998 . Riese 1997; also in C. elegans Lin 1998).
The Wnt signal (right) leads, through its receptor (members of the frizzled (Fz) family) and dishevelled (Dsh) to inactivation of Zw3. As a consequence, Arm increases in concentration and can bind to TCF1/Lef-1/pangolin. This complex can activate nuclear target genes.
(reviewed in Nusse R, 1997; Cadigan 1997; included here are only later references)