The most common mutation associated with cystic fibrosis is the deletion of phenylalanine 508 of cystic fibrosis transmembrane conductance regulator (CFTR Delta F508). This mutation renders otherwise functional protein susceptible to ER-associated degradation (ERAD) and prevents CFTR from exiting the ER and trafficking to the plasma membrane. In this study, we demonstrate that RNAi-mediated silencing of gp78, an established ubiquitin ligase (E3) involved in ERAD, leads to accumulation of CFTR Delta F508 protein in cells. gp78 facilitates the degradation of CFTR Delta F508 by enhancing both its ubiquitination and interaction with p97/VCP. SVIP, which is the inhibitor of gp78, causes accumulation of CFTR Delta F508. We showed that endogenous gp78 co-immunoprecipitates with Hrd1. Furthermore, the results indicate that silencing the expression of another ERAD E3, Hrd1, leads to stabilization of gp78 and decline in gp78 ubiquitination; thereby enhancing CFTR Delta F508 degradation. The results support that gp78 is an E3 targeting CFTR Delta F508 for degradation and Hrd1 inhibits CFTR Delta F508 degradation by acting as an E3 for gp78. (C) 2009 Elsevier Ltd. All rights reserved.