BTB protein, dKLHL18/CG3571, serves as an adaptor subunit for a dCul3 ubiquitin ligase complex

The BTB domain is a highly conserved protein-protein interaction motif and functions in diverse cellular processes, including transcriptional regulation, ion channel assembly, cytoskeleton dynamics and apoptosis. Recently, it was reported that some BTB domain-containing proteins associate with Cullin-3 (Cul3), an E3 ubiquitin ligase, and act as an adaptor for Cul3 recognition of its substrate. However, the target substrates for the Cul3/BTB protein E3 ubiquitin ligase complex are largely unknown. Here, we report the characterization of a novel Drosophila BTB protein, dKLHL18/CG3571. By purification of a dKLHL18-associated complex, we identified CG10324, CG5808, l(2)37Cb and dCul3/guftagu. Indeed, the physical association of dKLHL18 with these proteins was observed in insect S2 cells, and genetic interactions among the identified factors were also observed in thorax development. Moreover, transient overexpression of dKLHL18 increased the ubiquitinated protein levels of CG10324 and CG5808. These findings suggest that dKLHL18 is an adaptor for a dCul3 E3 ubiquitin ligase to accommodate CG10324, CG5808 and l(2)37Cb proteins for ubiquitination.