A functional interaction between dorsal and components of the Smt3 conjugation machinery.

To identify proteins that regulate the function of Dorsal, a Drosophila Rel family transcription factor, we employed a yeast two-hybrid screen to search for genes encoding Dorsal-interacting proteins. Six genes were identified, including two that encode previously known Dorsal-interacting proteins (Twist and Cactus), three that encode novel proteins, and one that encodes Drosophila Ubc9 (DmUbc9), a protein thought to conjugate the ubiquitin-like polypeptide Smt3 to protein substrates. We have found that DmUbc9 binds and conjugates Drosophila Smt3 (DmSmt3) to Dorsal. In cultured cells, DmUbc9 was found to relieve inhibition of Dorsal nuclear uptake by Cactus, allowing Dorsal to enter the nucleus and activate transcription. The effect of DmUbc9 on Dorsal activity was potentiated by the overexpression of DmSmt3. We have also identified a DmSmt3-activating enzyme, DmSAE1/DmSAE2 and found that it further potentiates Dorsal-mediated activation.