Fig. 8. The N terminus of IB can substitute for the N terminus of Cactus to direct regulated degradation. A Cactus-lacZ fusion derivative in which the first 71 amino acids of human IB were fused to the N-terminus of 144Cactus-lacZ was expressed under alpha-tubulin 67C transcriptional control and its distribution determined by lacZ activity. This construct exhibited polarized degradation (A) and the ability to rescue DV polarity in the otherwise ventralized progeny of cact⁰¹¹/ cact⁰¹¹ mutant females (C). In contrast, a derivative in which the IKK target site serines 32 and 36 had been converted to alanine exhibited a uniform distribution (B) and led to the dominant dorsalization of what would have otherwise been normal hatching progeny from cact⁰¹¹/+ females (D).