The Drosophila ankyrin repeat protein cactus has a predominantly α‐helical secondary structure

The cactus protein is the Drosophila homologue of the mammalian I K B family of cytoplasmic anchor proteins. We have expressed in E. coli and purified a cactus fusion protein, CACT‐Bgl. CACT‐Bgl protein contains the six ankyrin repeat sequences which are necessary for specific binding to the Drosophila rel family transcription factor dorsal. We show that the purified CACT‐Bgl protein can bind specifically to dorsal and, using circular dichroism spectroscopy, that the protein adopts a largely α‐helical secondary structure. A further analysis of the ankyrin repeat domains of cactus, using an improved secondary structure prediction program indicates that the N‐terminal of the repeat will form into a loop structure and the C‐terminal section into an interrupted, amphipathic α‐helix. On the basis of these findings we propose that the ankyrin repeats of cactus fold together into helical bundles interconnected by diverged loops.