E3 ubiquitin-protein ligase which is a component of the N-end rule pathway. Recognizes and binds to proteins bearing specific N-terminal residues that are destabilizing according to the N-end rule, leading to their ubiquitination and subsequent degradation. May be involved in pancreatic homeostasis. Binds leucine and is a negative regulator of the leucine-mTOR signaling pathway, thereby controlling cell growth. Defects in UBR1 are a cause of Johanson-Blizzard syndrome (JBS). This disorder includes congenital exocrine pancreatic insufficiency, multiple malformations such as nasal wing aplasia, and frequent mental retardation. Pancreas of individuals with JBS do not express UBR1 and show intrauterine- onset destructive pancreatitis. Belongs to the UBR1 family. 2 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.