Disulfide cross-linking in crude embryonic lysates reveals three complexes of the Drosophila morphogen dorsal and its inhibitor cactus.

In Drosophila embryos dorsoventral polarity is determined by a concentration gradient of dorsal (dl) protein in the nuclei formed by the differential regulation of nuclear localization of dl protein. cactus (cact) represses the nuclear localization of dl protein. By introducing intermolecular disulfide bonds in homogenates of embryos, we detected three complexes of dl and/or cact proteins. Complex 1 (190 kDa) is a dl protein homodimer (dl2). Complex 2 (270 kDa) consists of one complex 1 and one cact molecule (dl2cact). Complex 3 (200 kDa) is a cact protein complex that does not contain dl protein. In wild-type embryos dl2cact was detected as the major form of dl protein, and dl2 was minor. With this assay virtually no dl monomer is detected. Analysis of the dl protein complexes in ventralized and dorsalized mutant embryos indicates that dl2cact is a cytoplasmic form, whereas dl2 is localized mainly in the nuclei. It seems that a small amount of dl2 is also present in the cytoplasm.