Figure 9. Blocking potential phosphorylation at 2 conserved serines in B disrupts APC2 function in the fly.
APC2DD and APC2AA (Figure 8B) were expressed with the endogenous APC2 promoter in APC2 APC1 maternal/zygotic double mutants. (A) APC2 APC1 maternal/zygotic double mutants die as embryos (50% of embryos are zygotically rescued). APC2DD rescues embryonic viability as well as wildtype APC2. In contrast, APC2AA has only weak rescue ability. (B) Cuticles. (B1) Wildtype. Note pattern of anterior denticles (Wnt inactive) and posterior naked cuticle (Wnt active). (B2) Loss of APC2 and APC1 leads to denticle loss and expanded naked cuticle. (B3) Wildtype APC2 fully restores Wnt regulated cell fates of alternating denticles and naked cuticle. (B4) APC2DD similarly restores cell fates. (B5–B7) APC2AA largely fails to restore Wnt-regulated cell fates, and thus most cells secrete naked cuticle. Images = range of rescue ability. (C–I) βcat (fly Armadillo (Arm)) levels. Stage 9–10 embryos. (J–N) Close-ups of C–I. (C,J) Wildtype. Striped pattern of βcat indicative of Wg (fly Wnt) active and Wg inactive regions. (D) Loss of APC1 and APC2 leads to uniform very high levels of βcat. (E) WT APC2 restores normal βcat regulation, with higher levels in cells receiving Wg signal, and lower levels in other cells. However, Wg signal does not elevate βcat levels to those seen in embryos lacking functional APC. (F and K) APC2DD also rescues normal βcat regulation. (G–I and L–N) APC2AA restores some Wnt responsiveness, but βcat levels are elevated in all cells and especially elevated in a subset of cells receiving Wnt signal. (O) Quantification, embryos blind-scored.