Control of Drosophila embryonic dorsal -ventral polarity by the Cactus protein

Abstract

Dorsal-ventral polarity within the Drosophila syncytial blastoderm embryo is determined by the maternally-encoded dorsal-group signal transduction pathway that regulates nuclear localization of the transcription factor Dorsal. Nuclear uptake of Dorsal, a Rel/NFkappaB homolog, is controlled by the interaction with its cognate IkappaB inhibitor protein Cactus, which is degraded on the ventral side of the embryo in response to dorsal-group signaling. Previous studies have suggested that an N-terminally located kinase target motif similar to that found in IkappaB proteins is involved in the spatially controlled degradation of Cactus. To assess regions of Cactus required for spatially correct degradation, I constructed chimeric Cactus-LacZ fusions in embryos using either deleted segments of the N-terminus or the N-terminal domain carrying site-directed alterations. Full-length Cactus-LacZ expressed in vivo normalizes the ventralized phenotype of embryos lacking Cactus and faithfully reconstitutes dorsal-group regulated degradation, while fusion protein constructs lacking the first 125 amino acids of Cactus escape dorsal-group-dependent degradation. Furthermore, Cactus-LacZ constructs lacking only the putative IkappaB-dependent kinase target-like motif can nevertheless undergo spatially regulated dorsal-group-dependent degradation and we have identified the regulatory determinant responsible for dorsal-group dependent degradation of Cactus in the absence of this motif. Taken together, my studies indicate the presence of two distinct redundantly acting determinants in the amino terminus of Cactus that direct dorsal-group-dependent degradation. Strikingly, the regulatory domain of human IkappaBalpha can also direct polarized degradation of Cactus-LacZ fusion protein. In addition, I demonstrate that the regulated degradation of Cactus requires the presence of Dorsal and assess the involvement of the ubiquitin proteosome pathway in this process.