Abstract DmSmt3, Cactus and Dorsal of Drosophila are homologs of vertebrate SUMO-1, IкB£\ and NF-кB. Cactus/IкB£\ is an inhibitor of Dorsal/ NF-кB, the rel family of transcription factor. Conjugation of SUMO-1 to IкB£\ is thought to stabilize IкB£\. The stabilized IкB£\ inhibits NF-кB to enter the nucleus so that inhibits NF-кB gene expression activation. It was reported that DmSmt3 can bind to Dorsal and allow dorsal to enter the nucleus and activate transcription. It implies that the mechanism of Drosophila is different from vertebrate. We wonder if DmSmt3 can conjugate to Cactus just like in the vertebrate system. DmSmt3, Cactus and Dmubc9 are cloned and expressed. A series of three deletion of Cactus are constructed (cac-1,cac-2,cac-3). We use reticulocyte lysate and Drosophila larvae extract to perform the in vitro covalent bonding experiment. We find that DmSmt3 does not bind to Cactus. It indicates that though cactus is homologus to IкB£\ , the interaction may be different among different species. Otherwise, we find that Cactus degrades in the DmSmt3 covalent bonding experiment in the Drosophila larvae extract. Another finding is that cac-2 can bind to lots of proteins of the reticulocyte lysate and Drosophila larvae extract. But after adding of DmSmt3 in the Drosophila larvae extract, the cac-2 binding proteins are disappeared. We think that DmSmt3 can induce protein degradation mechanism of cac-2 binding proteins, or DmSmt3 degrades the auxiliary proteins which aid cac-2 binding to its target proteins, or DmSmt3 bind to the auxiliary proteins so that auxiliary proteins can not bind to cac-2.