The N-end rule pathway is a proteolytic system in which single N-terminal amino acids of proteins act as a class of degrons (N-degrons) that determine the half-lives of proteins. We have previously identified a family of mammals N-recognins whose conserved UBR boxes bind N-degrons to facilitate substrate ubiquitination, leading to proteolysis via the ubiquitin proteasome system or autophagy. Amongst these N-recognins, UBR4 binds to both type-1 and type-2 residues without known ubiquitylation domain. N-terminal Arg is one of the principal degrons that it can be generated through post-translational conjugation of L-Arg from Arg-tRNAArg to N-terminal Asp or Glu, which is mediated by ATE1-encoded Arg-tRNA transferases. In this dissertation study, we addressed the roles of UBR4 in mammalian development and ATE1 in innate immune response, respectively.
First, we generated UBR4-deficient mice in which the UBR box of UBR4 was deleted and characterized the null phenotypes. UBR4-deficient mice exhibit severe embryonic lethality and pleiotropic abnormalities, including accumulated autophagic vacuoles in the yolk sac endoderm. UBR4 also modulates early endosomal maturation and the trafficking through the interaction with Ca2+-bound calmodulin. UBR4-/- embryos have multiple developmental defects including neurogenesis and cardiovascular system, which is at least in part attributed to the impairment in cell adhesion and depletion of cell surface proteins. Collectively, these data reveal that the N-recognin UBR4 plays important roles in multiple developmental processes associated with angiogenesis, neurogenesis and cardiovascular system. The developmental processes are most commonly involved in non-proteolytic processes such as endosomal maturation, trafficking and cellular adhesion.
We show that cytosolic foreign DNA induces N-terminal arginylation of ER chaperons, which is required for host defense system for IFN-β mediated gene induction and IRF3 phosphorylation. Cytosolic dsDNA facilitates relocation of ATE1 to the ER, on which ATE1 is colocalized with STING. Interference of Nt-arginylation that is important for host defense induces the production of virion. Our results suggest that N-terminal arginylation is essential for cellular immune response against foreign DNA and viral infections.
This work provides meaningful evidences demonstrating that the N-end rule pathway plays a pivotal role in mammalian development and innate immunity beside proteolytic function
