RNF 45 (GP78) oli aiemmin jo todettu ubikitiiniligaasiksi, joka ubikitinoi ja johti silppuriin HMG-CoA-reduktaasin steroli-induktiosta. Tutkijat etsivät muita ubikitiiniligaaseja, jotka suorittavat samaa ja havaitsivat RNF145, josta on vähemmän artikkeleita saatavilla.
To identify other ubiquitin ligase(s) that may function together with gp78 in triggering HMGCR degradation,
J Biol Chem. 2018 Mar 16;293(11):4047-4055. doi: 10.1074/jbc.RA117.001260. Epub 2018 Jan 26. Ring finger protein 145 (RNF145) is a ubiquitin ligase for sterol-induced degradation of HMG-CoA reductase. Jiang LY1, Jiang W1, Tian N1, Xiong YN1, Liu J1, Wei J1, Wu KY1, Luo J1, Shi XJ2, Song BL3.
Cholesterol biosynthesis is tightly regulated in the cell. For example, high sterol concentrations can stimulate degradation of the rate-limiting cholesterol biosynthetic enzyme 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase, HMGCR). HMGCR is broken down by the endoplasmic reticulum membrane-associated (ERAD) protein complexes consisting of insulin-induced genes (Insigs) and the E3 ubiquitin ligase gp78. Here we found that HMGCR degradation is partially blunted in Chinese hamster ovary (CHO) cells lacking gp78 (gp78-KO). To identify other ubiquitin ligase(s) that may function together with gp78 in triggering HMGCR degradation, we performed a small-scale short hairpin RNA-based screening targeting endoplasmic reticulum-localized E3s. We found that knockdown of both ring finger protein 145 (Rnf145) and gp78 (Rnf45) genes abrogates sterol-induced degradation of HMGCR in CHO cells. We also observed that RNF145 interacts with Insig-1 and -2 proteins and ubiquitinates HMGCR. Moreover, the tetrapeptide sequence YLYF in the sterol-sensing domain and the Cys-537 residue in the RING finger domain were essential for RNF145 binding to Insigs and RNF145 E3 activity, respectively. Of note, amino acid substitutions in the YLYF or of Cys-537 completely abolished RNF145-mediated HMGCR degradation. In summary, our study reveals that RNF145, along with gp78 (RNF45) , promotes HMGCR degradation in response to elevated sterol levels and identifies residues essential for RNF145 function.