HRD Complex Self-Remodeling Enables a Novel Route of Membrane Protein Retrotranslocation.

ABSTRACT

ER-associated degradation (ERAD) targets misfolded ER proteins for degradation. Retrotranslocation, a key feature of ERAD, entails removal of ubiquitinated substrates into the cytosol for proteasomal destruction. Recently, it has been shown that the Hrd1 E3 ligase forms a retrotranslocation channel for luminal (ERAD-L) substrates. Conversely, our studies found that integral membrane (ERAD-M) substrates exit the ER through a distinct pathway mediated by the Dfm1 rhomboid protein. Those studies also revealed a second, Hrd1-dependent pathway of ERAD-M retrotranslocation can arise in dfm1Δ null. Here we show that, in the dfm1Δ null, the HRD complex undergoes remodeling to a form that mediates ERAD-M retrotranslocation. Specifically, Hrd1's normally present stochiometric partner Hrd3 is efficiently removed during suppressive remodeling, allowing Hrd1 to function in this novel capacity. Neither Hrd1 autoubiquitination nor its cytosolic domain is required for suppressive ERAD-M retrotranslocation. Thus, the HRD complex displays remarkable functional flexibility in response to ER stress.