INTERACTIONS OF A REL PROTEIN WITH ITS INHIBITOR

Cactus, a Drosophila homologue of I kappa B, binds to and inhibits Dor sal, a homologue of the p50 and p65 components of NF-kappa B. We descr ibe experiments in yeast with various Dorsal and Cactus derivatives sh owing that Cactus blocks the DNA binding and nuclear localization func tions of Dorsal. In contrast, Dorsal's transcriptional activating regi on is functional in the Dorsal-Cactus complex. We identify two Dorsal mutants, Dorsal C233R and Dorsal S234P, that escape Cactus inhibition in vivo, and we show that these mutants fail to interact with Cactus i n vitro. From this and data of others, we identify the likely surface of Dorsal that binds Cactus. We also describe a modified PCR mutagenes is procedure, easier to use than conventional methods, that produces a library of high complexity.