Separable and redundant regulatory determinants in Cactus mediate its dorsal group dependent degradation

Dorsal-ventral polarity within the Drosophila syncytial blastoderm embryo i s determined by the maternally encoded dorsal group signal transduction pat hway that regulates nuclear localization of the transcription factor Dorsal . Nuclear uptake of Dorsal, a Rel/NF kappaB homolog, is controlled by the i nteraction with its cognate I kappaB inhibitor protein Cactus, which is deg raded on the ventral side of the embryo in response to dorsal group signali ng. Previous studies have suggested that an N-terminally located kinase tar get motif similar to that found in I kappaB proteins is involved in the spa tially controlled degradation of Cactus. We report studies of the in vivo f unction and distribution of fusion proteins comprising segments of Cactus a ttached to Escherichia coli beta -galactosidase (lacZ). Full-length Cactus- lacZ expressed in vivo normalizes the ventralized phenotype of embryos that lack Cactus and faithfully reconstitutes dorsal group-regulated degradatio n, while fusion protein constructs that lack the first 125 amino acids of C actus escape dorsal group-dependent degradation. Furthermore, Cactus-lacZ c onstructs that lack only the putative I kappaB-dependent kinase target-like motif can nevertheless undergo spatially regulated dorsal group-dependent degradation and we have identified the regulatory determinant responsible f or dorsal group-dependent degradation of Cactus in the absence of this moti f. Taken together, our studies indicate the presence of two distinct redund antly acting determinants in the N terminus of Cactus that direct dorsal gr oup-dependent degradation. Strikingly, the regulatory domain of human I kap paB alpha can also direct polarized degradation of Cactus-lacZ fusion prote in.