UFMylation: A Conserved Modifier at the Centre of Immune Regulation and Checkpoint Control

Ubiquitin-like modifiers are small proteins that attach to lysine residues and change what the modified protein does. UFM1 was identified in 2004 as one of the later additions to that family, and for years its functions looked narrow. Work over the past decade has widened the picture considerably, placing the conjugation system inside antiviral signalling, inflammatory control and checkpoint regulation.

The Core Machinery and Its Reversibility

The system is built from a small set of dedicated enzymes. UFM1 is the modifier, UBA5 is the activating enzyme, UFC1 is the conjugating enzyme and UFL1 is the ligase that attaches UFM1 to substrate lysines. Two proteases, UFSP1 and UFSP2, remove the modification, and adaptor proteins DDRGK1, also called UFBP1, and C53 determine which substrates are engaged.

Attachment and removal form a cycle rather than a switch. The steady-state level of any modified protein reflects the balance between ligase and protease activity. A perturbation that changes one side can therefore produce the same readout as a change in substrate abundance. This system is conserved across eukaryotes with the notable exception of budding yeast.

The consequences of losing that balance reach basic cell biology. UFMylation supports endoplasmic reticulum homeostasis, genome stability and progression through the cell cycle. Loss of function in core components is linked to developmental disorders that include hip dysplasia, infantile encephalopathy and microcephaly. That spectrum shows how little room the system has for error.

Antiviral Signalling: Three Different Outcomes

The antiviral literature illustrates how context decides the direction of the effect. During herpes simplex virus type 1 infection, UFL1 associates with STING and blocks its ubiquitin-dependent degradation, which sustains the antiviral signal. The modification therefore protects a sensor from disposal and lengthens the response.

Seneca Valley virus tells a similar story through a different route. Here UFL1 works with the 14-3-3 epsilon protein to promote activation of the RIG-I pathway, which accelerates interferon production. Two unrelated sensors converge on the same outcome, and in both cases the ligase acts as a positive regulator.

Epstein-Barr virus reverses the logic. Its protein BILF1 hijacks the system and drives UFMylation of MAVS, which sends the adaptor to lysosomes for degradation. The result is immune evasion built from the host's own machinery, and it explains why the same modification can be protective in one infection and permissive in another.

Braking Inflammation

In macrophages, an intact system restrains activation triggered by lipopolysaccharide or interferon gamma. Loss of function releases the brake, and the cells secrete more IL-6 and TNF-alpha than controls. The physiological purpose looks like damage limitation: an inflammatory response that clears a pathogen without destroying the tissue around it.

That framing matters for drug development. An inhibitor of the ligase would be expected to amplify inflammation, which is useful in oncology and risky in inflammatory disease. An inhibitor of the opposing protease would do the reverse. The two enzyme classes therefore point at opposite therapeutic directions even though they act on the same cycle.

Two Arms of Checkpoint Control

The tumour immunology findings are the most striking part of the picture. UFL1 promotes UFMylation of PD-L1, which accelerates its ubiquitin-dependent degradation and therefore strengthens antitumour immunity. The same ligase modifies PD-1, where the modification suppresses degradation and weakens CD8 T cell activity.

Those two effects pull in opposite directions. Removing PD-L1 from the tumour surface helps, while stabilising PD-1 on T cells hurts, and both are driven by the same enzymatic activity. The therapeutic hypothesis that follows is combination rather than monotherapy: a modifier-directed compound plus a checkpoint antibody, so that one arm is amplified while the other is blocked.

Preclinical support exists for that design. A UFSP2 inhibitor combined with a PD-1 antibody suppressed breast cancer growth more effectively than either agent alone in the reported model. The result does not prove that the combination will transfer to patients, but it does establish that the two mechanisms can be engaged at once without cancelling out.

Humoral immunity depends on the system as well. The adaptor UFBP1 restrains the PERK branch of the unfolded protein response, and that restraint is required for plasma cell development and antibody secretion. Animals lacking the adaptor fail to mount normal antibody responses, which places UFMylation upstream of a cell type that oncology combinations rarely consider.

Endocrine Cross-Talk and Metabolic Stress

The system also sits at the interface between hormone signalling and immunity. It regulates endoplasmic reticulum export of adrenergic receptors and so helps keep stress responses balanced. It stabilises the oestrogen receptor, which links the modification to reproductive immunology. Pancreatic beta cells express the machinery at high levels, and an intact system protects them from apoptosis driven by endoplasmic reticulum stress.

These observations broaden the therapeutic question. A compound aimed at tumour immunity will also act on endocrine tissue, and the therapeutic window may depend on how much of the effect is tissue-specific. That is one reason the field has moved cautiously from genetics to small molecules.

Where Inhibitors Stand

Small-molecule inhibitors of the system, including the compound known as Compound 8, have reached preclinical testing. They target the enzymatic steps rather than the modifier itself, which offers selectivity across the ubiquitin-like family. Whether the same molecule can serve both oncology and antiviral indications remains unresolved, because the required direction of effect differs.

Core UFM1 conjugation machinery, the stress inputs that engage it and the disease settings linked to its loss

Reagents for Following Both Arms

Testing these mechanisms requires reagents that report on each arm separately, because a change in total PD-1 or PD-L1 says nothing about which side of the pathway moved. Fluorochrome-conjugated PD-1 antibodies support surface phenotyping of human and mouse cells, while unconjugated clones support blocking and western blot work in the same models.

The table below lists reagents for the checkpoint arm and for mitochondrial complex I subunits, which report on the metabolic and endoplasmic reticulum stress side of the pathway. The two groups are used together when a modifier perturbation is expected to shift both immune and metabolic readouts.

What the Next Few Years Need to Settle

Three questions dominate. Which substrates explain the immune phenotype, since the proteomic lists are long and the functional tests are few. Whether inhibition of the protease or of the ligase gives the better therapeutic index in tumours. And how the system behaves in primary human immune cells, where most of the mechanistic work so far has used immortalised lines or mouse models.

A 2025 review in a clinical and translational medicine journal, with an impact factor near 7.9, pulled these strands together. Its argument is that the modifier enzymes are druggable nodes rather than housekeeping genes. That claim is now testable, and the reagents for testing it are largely available.

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