| Cullins are proteins that function as molecular scaffolds for modular ubiquitin ligases typified by the SCF (Skp1-CUL1-F-box) complex. 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 such as Skp2. CUL3 has been shown to be required for embryonic development in mammals and Caenorhabditis elegans but until recently, its substrate specificity adaptor had yet to be elucidated. It is now recognized that substrate adaptors for CUL3-based ubiquitin ligase complexes contain a conserved BTB/POZ (Pox virus and Zinc finger) domain. This domain, which was initially identified in the Drosophila transcriptional repressors broad complex, tramtrack, and bric-a-brac is present in more than 190 human proteins. BTB proteins contain a variety of putative protein-protein interaction domains, including MATH domains, zinc finger repeats, and kelch repeats. There are several lines of evidence suggesting that Kelch-like 12 protein (KLHL12) is a substrate-specific adaptor for the CUL3-based ubiquitin ligase complex. Analysis of the amino acid sequence of KLHL12 reveals an amino-terminal BTB motif, a central linker region, and a carboxy-terminal kelch domain composed of kelch repeats. Furthermore, KLHL12 has been shown to negatively regulate Wnt signaling by binding Disheveled and targeting it for ubiquitin-dependent proteasomal degradation. More recently, KLHL12 was shown to drive the assembly of large COPII vesicles by promoting the monoubiquitination of the COPII component Sec31. As a result, CUL3-KLHL12-dependent ubiquitination is essential for collagen export, a step that is required for integrin-dependent mouse embryonic stem cell division. |
