1. Department of Pathology, Harvard Medical School, 77 Avenue Louis Pasteur, Boston, Massachusetts 02115, USA
2. Department of Cell Biology, Harvard Medical School, 200 Longwood Drive, Boston, Massachusetts 02115, USA

Correspondence to: J. Wade Harper¹ Correspondence and requests for materials should be addressed to J.W.H. (Email: wade_harper@hms.harvard.edu).

Modification of proteins with ubiquitin or ubiquitin-like proteins (UBLs) by means of an E1–E2–E3 cascade controls many signalling networks^{1, 2, 3}. Ubiquitin conjugation involves adenylation and thioesterification of the carboxy-terminal carboxylate of ubiquitin by the E1-activating enzyme Ube1 (Uba1 in yeast), followed by ubiquitin transfer to an E2-conjugating enzyme through a transthiolation reaction^{4, 5, 6, 7}. Charged E2s function with E3s to ubiquitinate substrates¹. It is currently thought that Ube1/Uba1 is the sole E1 for charging of E2s with ubiquitin in animals and fungi^{1, 8}. Here we identify a divergent E1 in vertebrates and sea urchin, Uba6, which specifically activates ubiquitin but not other UBLs in vitro and in vivo. Human Uba6 and Ube1 have distinct preferences for E2 charging in vitro, and their specificity depends in part on their C-terminal ubiquitin-fold domains, which recruit E2s. In tissue culture cells, Uba6 is required for charging a previously uncharacterized Uba6-specific E2 (Use1), whereas Ube1 is required for charging the cell-cycle E2s Cdc34A and Cdc34B. Our data reveal unexpected complexity in the pathways that control the conjugation of ubiquitin, in which dual E1s orchestrate the charging of distinct cohorts of E2s.

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