Fig. 3. Schematic representation of the BUL1 mutants. BUL1 contains an N-terminal C2 domain (black boxes), two WW domains (grey boxes) and a C-terminal HECT domain (dashed boxes). All constructs were expressed as proteins containing a Myc-tag at the N-terminus. The cysteine residue at position 1530 is critical for the ubiquitin ligase activity of HECT domain proteins (Anan et al., 1998; Goulet et al., 1998; Harvey and Kumar, 1999).