HRD1 is a non-glycosylated ER protein with a cytosolic RING-H2 finger domain. In vitro it shows ability to form Lys48-polymerized multiubiquitin chains in the presence of UBC7. Human HRD1 is involved in the basal degradation of HMGR but not in the sterol-regulated degradation. HRD1 is also involved in the elimination of model ER-associated degradation substrates TCR-alpha and CD3-delta .
Doolman R, Leichner GS, Avner R, Roitelman J. Ubiquitin is conjugated by membrane ubiquitin ligase to three sites, including the N terminus, in transmembrane region of mammalian 3-hydroxy-3-methylglutaryl coenzyme A reductase: implications for sterol-regulated enzyme degradation. J Biol Chem. 279(37):38184-93 (2004). 
Kikkert M, Doolman R, Dai M, Avner R, Hassink G, van Voorden S, Thanedar S, Roitelman J, Chau V, Wiertz E. Human HRD1 is an E3 ubiquitin ligase involved in degradation of proteins from the endoplasmic reticulum. J Biol Chem. 279(5):3525-34 (2004).