CRL ligase activities are controlled by several complex regulation mechanisms, including activation by conjugation with the protein NEDD8 (neural precursor cell expressed, developmentally downregulated 8) and inhibition by binding of CAND1 (cullin-associated and neddylation-dissociated 1).
It has been hypothesized that by preventing binding of substrate receptors to cullin proteins, CAND1 prevents substrate receptor autoubiquitination in the absence of bound E3 ligase substrate [1], [2].
According to one proposed model, CAND1 functions by binding to and inactivating of cullin proteins in the absence of ubiquitination substrates.
Thus, taken together, these results provide further evidence that CAND1 does not function by binding and sequestering inactive cullin proteins, but suggests that CAND1 binding to cullin proteins in vivo is regulated via other mechanisms.
For instance, CAND1 binding to cullin proteins may be regulated by posttranslational modifications in either of the proteins or by other interacting proteins.
Structural characterization of BTB Cul3 complexes has revealed a number of critical assembly mechanisms, including the binding of an N-terminal Cullin extension to a bihelical '3-box' at the C-terminus of the BTB domain.
The binding of CandA to the cullin leads to disassembly of the SCF complex, and by a subsequent neddylation step, the complex can be reassembled with an exchanged F-box protein.
Analysis of the V5 immunoprecipitates with CAND1 antibody revealed specific binding of endogenous CAND1 to all cullin proteins, although the interaction with Cul5 was somewhat weaker and the interaction with Cul2 markedly reduced compared to Cul1, Cul3 and Cul4a (Fig. 1c).
We argue that the UIM motif contributes to the direct binding of UBXN7 to neddylated-cullins.
De-neddylation results in the binding of inhibitory protein CAND1 to Cullin.
