SUMMARY The goal of this project is to elucidate the mechanism by which the gp78 E3 ubiquitin (Ub) ligase suppresses the progression of hepatocellular carcinoma (HCC). Liver cancer is among the most common causes of cancer death worldwide. According to the statistics from American Cancer Society, an estimated 42,810 new cases of liver cancer will be diagnosed, and 30,160 people will die of this disease in the United States in 2020. HCC is the predominant form of primary liver cancer and is extremely difficult to treat because most patients are diagnosed in a late stage and few can benefit from surgery (liver resection and liver transplantation). Sorafenib remains the only drug available for HCC and increases survival time by mere 3 months. Thus, it is imperative to develop new and efficient approaches for the treatment and diagnosis of HCC. However, this task has been hampered by poor understanding of the molecular mechanism behind the progression of HCC. Our recent study demonstrated that the gp78 E3 Ub ligase is a tumor suppressor of HCC. As an E3 Ub ligase, the main function of gp78 is to target protein substrates for degradation by the Ub-proteasome system. Loss of gp78 likely leads to accumulation of substrate proteins, thereby contributing to the pathogenesis of HCC. To gain insight into the role of gp78 in suppressing HCC, we have initiated a proteomic analysis for liver proteins that are degraded in a gp78- dependent manner. Here we report the identification of MYBL2 as a novel gp78 substrate. MYBL2 is a transcription factor that activates multiple genes whose protein products promote cell cycle progression and cell proliferation. Overexpression of MYBL2 has been observed in liver cancer and is associated with poor survival of HCC patients. Thus, MYBL2 is a potential liver oncoprotein. We hypothesize that gp78-mediated degradation of MYBL2 may play an important role in suppressing HCC. To test this hypothesis, we will investigate the mechanism underlying gp78-mediated ubiquitylation of MYBL2 (Aim 1) and demonstrate the effects of blocking MYBL2 degradation on liver cell tumorigenesis (Aim 2). The proposed research will not only shed light on the role of gp78 in suppressing HCC, but potentially identify a target for the treatment and diagnosis of this dreadful disease.