Mechanism of ubiquitin activation revealed by the structure of a bacterialMoeB-MoaD complex

The activation of ubiquitin and related protein modifiers(1,2) is catalysed by members of the E1 enzyme family that use ATP for the covalent self-atta chment of the modifiers to a conserved cysteine. The Escherichia coli prote ins MoeB and MoaD are involved in molybdenum cofactor (Moco) biosynthesis, an evolutionarily conserved pathway(3,4). The MoeB- and E1-catalysed reacti ons are mechanistically similar, and despite a lack of sequence similarity, MoaD and ubiquitin display the same fold including a conserved carboxy-ter minal Gly-Gly motif(5). Similar to the E1 enzymes, MoeB activates the C ter minus of MoaD to form an acyl-adenylate. Subsequently, a sulphurtransferase converts the MoaD acyl-adenylate to a thiocarboxylate that acts as the sul phur donor during Moco biosynthesis(6,7). These findings suggest that ubiqu itin and E1 are derived from two ancestral genes closely related to moaD an d moeB(3,5). Here we present the crystal structures of the MoeB-MoaD comple x in its apo, ATP-bound, and MoaD-adenylate forms, and highlight the functi onal similarities between the MoeB- and E1-substrate complexes. These struc tures provide a molecular framework for understanding the activation of ubi quitin, Rub, SUMO and the sulphur incorporation step during Moco and thiami ne biosynthesis.