Identification And Function Of A De-ubiquitinase That Interacts With MST1

MST1 is a kinase that is associated with the development of many cancers,such as liver cancer,breast cancer,gastric cancer and pancreatic cancer,by participating in the Hippo signaling pathway.Researchers have found that MST1 plays a critical role in controlling liver cancer development.Liver cancer is one of the common types of human tumors that can caused by alcohol consumption,viral hepatitis,and genetic factors.The high incidence and mortality rate of liver cancer pose a huge threat to human life and health and there are millions of patients dying from liver cancer each year.Therefore,exploring the molecular mechanism of the occurrence and development of liver cancer is of great significance for preventing and treating liver cancer.Ubiquitination is a post translational modification process in which a small protein called ubiquitin can be connected to other proteins.The addition of ubiquitin to target proteins will lead to the degradation of those target proteins by the cell’s proteasome or altering their activity,localization,or interaction with other proteins.This process is crucial for many cellular functions,including protein degradation,signal transduction,DNA repair,and immune response.Deubiquitination is the reverse process of ubiquitination,which is accomplished by the de-ubiquitination enzymes(DUBs)that removes ubiquitin molecules from the target proteins.DUBs can rescue ubiquitinated proteins from proteasomal degradation or alter their physiological functions.According to the TCGA database and other studies,the expression of MST1 is low in liver cancer and MST1 plays a tumor suppressor role in liver cancer.MST1 can be regulated at different level.The ubiquitination is one of the mechanism to regulate the expression of MST1.However,little is known of its reverse process,de-ubiquitination.In our study,we found that USP5,one of the ubiquitin-specific peptidase,can interacts with MST1.Meanwhile,immunofluorescence experiments shown that USP5 and MST1 are co-localized in the cytoplasm.The interaction regions between USP5 and MST1 were determined by immune-precipitation and Western Blot through using different deletion mutants of USP5 and MST1.We found that the interaction sites of USP5 and MST1 were located in the Zn F-UBP domain of USP5 and the kinase domain of MST1.Contrary to the classical function of de-ubiquitinase,USP5 did not promote the expression of MST1,but rather inhibited its expression.Preliminary functional studies found that USP5 promoted the proliferation and cell clone formation of liver cancer cells.We speculated that USP5 may played a role as an oncogene in liver cancer cells by inhibiting the expression of MST1.