An E3 Ubiquitin Ligase, Synoviolin, Is Involved in the Degradation of Homocysteine-Inducible Endoplasmic Reticulum Protein.

Homocysteine-inducible endoplasmic reticulum (ER) protein (Herp) is an ER stress-inducible membrane protein involved in ER-associated degradation. Herp expression is maintained at low levels through a strict regulatory mechanism, but the details of this mechanism and the reasons why Herp expression is restricted in this manner remain unclear. Here, we show that Herp degradation involves synoviolin, an ER-resident E3 ubiquitin ligase. Herp protein levels were found to be markedly elevated in synoviolin-null cells, and Herp expression decreased when synoviolin was overexpressed. However, the lysine residues of Herp, which are ubiquitinated by E3 ubiquitin ligase, were not sufficient for regulation of Herp degradation. These results suggest that Herp degradation is mediated via synoviolin and that Herp ubiquitination involves amino acids other than lysine.