Editors' ChoiceBiochemistry

A New E1 Activation System for Ubiquitin

STKE  03 Jul 2007:
Vol. 2007, Issue 393, pp. tw238
DOI: 10.1126/stke.3932007tw238

Ubiquitination is an important process in the regulation of protein function and degradation and in the control of cell signaling. A sequence of events results in the modification of a protein with ubiquitin or ubiquitin-like proteins (UBLs) such as Nedd8 or the small ubiquitin-like modifier protein SUMO. The E1 ubiquitin-activating enzyme initiates ubiquitination by modifying ubiquitin (or a UBL) (through adenylation and thioesterification reactions) and then transferring it to the E2-conjugating enzyme (a process known as charging). E2 functions together with the ubiquitin ligase E3 to transfer ubiquitin to the target protein. The current paradigm states that within each species, single E1 enzymes are responsible for charging E2s with UBLs. In humans, this E1 is known as Ube1. Jin et al. performed a screen of the human genome for genes encoding proteins that contain a characteristic motif found in E1s and identified a previously uncharacterized protein, Uba6. Uba6 has 40% identity with Ube1, and orthologs of Uba6 are found in vertebrates and in the sea urchin but not in other organisms such as fungi and plants. Western blot analysis showed that Uba6 was as widely expressed as Ube1 in most human tissues and cell types. In vitro experiments with recombinant Uba6 and glutathione fusion proteins of UBLs demonstrated that Uba6 had similar ubiquitin activation properties as Ube1 but did not activate other UBLs. Lysates of 293T cells transfected with epitope-tagged Uba6 were analyzed by Western blotting and mass spectrometry, and Uba6 bound to ubiquitin but not to other UBLs. The authors then screened a panel of recombinant E2-conjugating enzymes for their ability to be charged in vitro by either Ube1 or Uba6 and found one previously uncharacterized E2 (Use1) that was charged by Uba6 but not by Ube1. Western blot analysis of lysates of transfected HeLa cells (which were treated with Ube1- or Uba6-specific siRNAs) confirmed that Use1 was charged by Uba6 but not Ube1. Taken together, these data demonstrate the existence of a previously unknown Ube1-independent mechanism for ubiquitin activation and charging of E2-conjugating enzymes such as Use1. Given that Ube1 and Uba6 are coexpressed in many tissues (although Ube1 is more abundant that Uba6), these two enzymes may act in concert or in sequence to affect various signaling pathways.

J. Jin, X. Li, S. P. Gygi, J. W. Harper, Dual E1 activation systems for ubiquitin differentially regulate E2 enzyme charging. Nature 447, 1135-1138 (2007). [PubMed]