Dynamic UFMylation governs cellular fitness by coordinating multi-organelle proteostasis

Ubiquitin-fold modifier 1 (UFM1) is a ubiquitin-like protein (UBL) covalently attached to substrates through a dedicated enzymatic cascade (UFMylation) and removed by specific proteases. Despite a key role in endoplasmic reticulum (ER)-ribosome homeostasis, the basis by which this UBL supports cell fitness remains elusive, as the essentiality of UFMylation machinery varies widely across hundreds of cancer lines. Here, we trace a conditional dependence on the UFMylation pathway to the availability of alanine, an amino acid provided by human plasma-like medium but absent from most conventional synthetic media. We show that by facilitating the clearance of stalled ribosomes at the ER, dynamic UFMylation maintains cellular levels of glutamic-pyruvic transaminase 2 (GPT2), the primary enzyme responsible for de novo alanine synthesis in most human cancer lines. This buffering preserves the alanine pools required to sustain protein synthesis under alanine-restricted conditions. Beyond GPT2, UFM1 deficiency leads to widespread proteomic remodeling that spans diverse processes, including mitochondrial translation. Our results reveal that despite primarily targeting ER-localized ribosomes, the UFMylation system orchestrates a multi-organelle proteostasis network whose client composition and contributions to cell fitness are shaped by intrinsic factors and nutrient conditions.