BTB proteins are substrate-specific adaptors in an SCF-like modular ubiquitin ligase containing CUL-3

Programmed destruction of regulatory proteins through the ubiquitin-proteasome system is a widely used mechanism for controlling signalling pathways(1,2). Cullins(3) are proteins that function as scaffolds for modular ubiquitin ligases typified by the SCF (Skp1-Cul1-F-box) complex(4-6). The substrate selectivity of these E3 ligases is dictated by a specificity module that binds cullins. In the SCF complex, this module is composed of Skp1, which binds directly to Cul1, and a member of the F-box family of proteins(4-7). F-box proteins bind Skp1 through the F-box motif(7), and substrates by means of carboxy-terminal protein interaction domains(1,2,5). Similarly, Cul2 and Cul5 interact with BC-box-containing specificity factors through the Skp1-like protein elongin C-2. Cul3 is required for embryonic development in mammals and Caenorhabditis elegans(8-10) but its specificity module is unknown. Here we report the identification of a large family of BTB-domain proteins as substrate-specific adaptors for C. elegans CUL-3. Biochemical studies using the BTB protein MEL-26 and its genetic target MEI-1 (refs 12, 13) indicate that BTB proteins merge the functional properties of Skp1 and F-box proteins into a single polypeptide.