E3 protein ligase that mediates ufmylation, the covalent attachment of the ubiquitin-like modifier UFM1 to lysine residues on target proteins, and which plays a key role in reticulophagy (also called ER-phagy) induced in response to endoplasmic reticulum stress. In response to endoplasmic reticulum stress, recruited to the endoplasmic reticulum membrane by DDRGK1, and mediates ufmylation of proteins such as RPN1 and RPL26/uL24, thereby promoting reticulophagy of endoplasmic reticulum sheets. Ufmylation-dependent reticulophagy inhibits the unfolded protein response (UPR) via ERN1/IRE1-alpha. Ufmylation in response to endoplasmic reticulum stress is essential for processes such as hematopoiesis, blood vessel morphogenesis or inflammatory response (By similarity). Regulates inflammation in response to endoplasmic reticulum stress by promoting reticulophagy, leading to inhibit the activity of the NF-kappa-B transcription factor (By similarity). Mediates ufmylation of DDRGK1 and CDK5RAP3; the role of these modifications is however unclear: as both DDRGK1 and CDK5RAP3 act as substrate adapters for ufmylation, it is uncertain whether ufmylation of these proteins is a collateral effect or is required for ufmylation (By similarity). Catalyzes ufmylation of various subunits of the ribosomal complex or associated components, such as RPS3/uS3, RPS20/uS10, RPL10/uL16, RPL26/uL24 and EIF6 (By similarity). Anchors CDK5RAP3 in the cytoplasm, preventing its translocation to the nucleus which allows expression of the CCND1 cyclin and progression of cells through the G1/S transition. Also involved in the response to DNA damage: recruited to double-strand break sites following DNA damage and mediates monoufmylation of histone H4. Catalyzes ufmylation of TRIP4, thereby playing a role in nuclear receptor-mediated transcription (By similarity). Required for hematopoietic stem cell function and hematopoiesis. Required for cardiac homeostasis (By similarity). Interacts with DDRGK1 (via PCI domain) (By similarity). Interacts with UFC1 (By similarity). Interacts with RELA (By similarity). Interacts with TRIP4 (By similarity). Interacts with CDK5RAP3; the interaction is direct (PubMed:20531390). Interacts with NBN; promoting recruitment to double-strand breaks following DNA damage (By similarity). Recruited to double-strand breaks by the MRE11-RAD50-NBN (MRN) complex following DNA damage. Ubiquitously expressed with expression detected in brain, skeletal muscle, lung, heart, gall bladder, liver, small intestine, pancreas, spleen and kidney (at protein level) (PubMed:20228063). At 8 weeks after birth, high expression in the Purkinje cell layer of the cerebellum (PubMed:20531390). Ubiquitinated, leading to its degradation by the proteasome. Interaction with CDK5RAP3 protects both proteins against ubiquitination and degradation via the proteasome. Phosphorylated at Ser-462 by ATM, enhancing protein ligase activity and promoting ATM activation in a positive feedback loop. Belongs to the UFL1 family.