Structures of the SUMO E1 provide mechanistic insights into SUMO activation and E2 recruitment to E1

E1 enzymes facilitate conjugation of ubiquitin and ubiquitin-like proteins through adenylation, thioester transfer within E1, and thioester transfer from E1 to E2 conjugating proteins. Structures of human heterodimeric Sae1/Sae2-Mg. ATP and Sae1/Sae2-SUMO-1-Mg. ATP complexes were determined at 2.2 and 2.75 Angstrom resolution, respectively. Despite the presence of Mg. ATP, the Sae1/Sae2-SUMO-1Mg. ATP structure reveals a substrate complex insomuch as the SUMO C-terminus remains unmodified within the adenylation site and 35 Angstrom from the catalytic cysteine, suggesting that additional changes within the adenylation site may be required to facilitate chemistry prior to adenylation and thioester transfer. A mechanism for E2 recruitment to E1 is suggested by biochemical and genetic data, each of which supports a direct role for the E1 C-terminal ubiquitin-like domain for E2 recruitment during conjugation.