Figure 5.

Proposed model for reversible D2 ubiquitination. D2 is an ER-resident selenoprotein that is inactivated by selective ubiquitination via a process that is accelerated by catalysis of T4 deiodination (step 1). D2 ubiquitination involves a ubiquitin-activating enzyme (E1) (4), a ubiquitin conjugase (UBC7) (5, 31), and an as-yet unidentified ubiquitin ligase (E3) (step 2). Ub-D2 can either be taken up by the proteasome and irreversibly degraded (step 3) or interact with (step 4) and be reactivated by (step 5) VDU1- or VDU2-mediated deubiquitination.