Axin-independent phosphorylation of APC controls β-catenin signaling via cytoplasmic retention of β-catenin

It has been shown that accumulation of free β-catenin leads to mobility shift of adenomatous polyposis coli (APC) protein and that Axin facilitates this process. Here we show that the β-catenin-mediated mobility shift of APC is due to phosphorylation of two domains of APC by casein kinase 1ε/glycogen synthase kinase 3β and unknown kinase(s), respectively. Interestingly, our results suggest that this process does not require Axin. The phosphorylated APC showed higher affinity to β-catenin in vivo, and fragments of APC containing the phosphorylated domains can inhibit β-catenin/Tcf-mediated reporter activity regardless of their ability to reduce the level of β-catenin. From our data we propose a new role of APC: accumulation of excessive cytoplasmic β-catenin induces phosphorylation of APC and the phosphorylated APC retains β-catenin in cytoplasm to prevent excessive β-catenin signaling. The retained β-catenin in cytoplasm by APC may be down-regulated by Axin 2, which is induced by β-catenin/Tcf signaling.