Wnt signaling plays a key role in cell proliferation and development. Recently, casein kinase I (CKI) and protein phosphatase 2A (PP2A) have emerged as positive and negative regulators of the Wnt pathway, respectively. However, it is not clear how these two enzymes with opposing functions regulate Wnt signaling. Here we show that both CKI␦ and CKI interacted directly with Dvl-1, and that CKI phosphorylated multiple components of the Wnt-regulated -catenin degradation complex in vitro, including Dvl-1, adenomatous polyposis coli (APC), axin, and -catenin. Comparison of peptide maps from in vivo and in vitro phosphorylated -catenin and axin suggests that CKI phosphorylates these proteins in vivo as well. CKI abrogated -catenin degradation in Xenopus egg extracts. Notably, CKI decreased, whereas inhibition of CKI increased, the association of PP2A with the -catenin degradation complex in vitro. Additionally, inhibition of CKI in vivo stabilized the -catenin degradation complex, suggesting that CKI actively destabilizes the complex in vivo. The ability of CKI to induce secondary body axes in Xenopus embryos was reduced by the B56 regulatory subunit of PP2A, and kinase-dead CKI acted synergistically with B56 in inhibiting Wnt signaling. The data suggest that CKI phosphorylates and destabilizes the -catenin degradation complex, likely through the dissociation of PP2A, providing a mechanism by which CKI stabilizes -catenin and propagates the Wnt signal.T he Wnt signaling pathway is a critical regulator of embryonic development and cellular proliferation (1, 2). In the absence of Wnt, -catenin is phosphorylated by glycogen synthase kinase 3 (GSK3) in a complex that also contains adenomatous polyposis coli (APC) and axin, targeting -catenin for ubiquitination and degradation. When Wnt binds to the frizzled receptor, disheveled (Dvl͞Dsh) is hyperphosphorylated and activated. Dvl activation leads to inhibition of GSK3, decreased phosphorylation of axin, APC, and -catenin, and stabilization of -catenin. -catenin is then translocated to the nucleus where it binds to Lef͞Tcf proteins and stimulates expression of Wnt-responsive genes.Multiple proteins have recently been identified as regulators of -catenin signaling. Protein phosphatase 2A (PP2A), a heterotrimeric serine͞threonine phosphatase (3), interacts via its B56 regulatory subunit with APC, Dsh, and axin (4-7), whereas its catalytic C subunit interacts with axin (8, 9). Inhibition of PP2A by okadaic acid increases, whereas B56 expression decreases, -catenin abundance in 293 cells. In Xenopus, B56 reduces both Xwnt-8-induced secondary axes and siamois and Xnr-3 expression. PP2A:B56 is likely to be a component of the -catenin degradation complex because epistasis positions B56 downstream of GSK3 and axin but upstream of APC and -catenin, and axin coimmunoprecipitates PP2A:B56, C, and structural A subunits. PP2A C and A also rescue Wnt-induced secondary axes, and PP2A activity is required for -catenin degradation in Xenopus egg extracts (4, 7).Recent ...
