Figure 1.

Assembly and fates of various forms of the ubiquitin domain. The E1 activating enzyme adenylates the carboxyl terminus of ubiquitin (Ub) and then forms a thiolester with the E2 conjugating enzymes that act as mobile carriers of activated Ub. Ub ligases are responsible for the specificity of attachment of Ub to the target protein through the recruitment of both an E2 thiolester and a specific substrate. Modification by a single Ub domain regulates localization and/or activity of conjugated proteins. PolyUb chains can be formed with different linkages and these direct proteins to different fates, presumably requiring chain-specific receptors. Note that more than one E2 can work with a given E3 and that several E3s can use a single E2. BRCA1, breast cancer 1; UEV, ubiquitin-conjugating enzyme E2 variant protein.