USP18 Isoform Switching Points to a Distinct Role in Early Myogenesis
USP18 has long been tied to type I interferon biology, but this preprint places the protein in a different setting: skeletal muscle development. The work centers on two proteoforms of USP18 — a canonical long isoform and a short isoform created by N-terminal truncation — and asks whether that structural difference changes how the protein behaves inside muscle cells. The answer is yes, and the shift is not subtle. The short form showed different stability, different localization, and a broader network of protein interactions than the long form, suggesting that the N-terminus acts as more than a simple structural appendage.
The study matters because endogenous short USP18 accumulates in the nucleus during muscle differentiation, yet its function had remained unclear. Rather than treating the two isoforms as interchangeable versions of the same enzyme, the investigators compared them directly in muscle cells engineered to stably overexpress each form. That design allowed them to look for intrinsic differences in proteoform behavior, independent of the broader noise of differentiating tissue.
A Shorter N Terminus Changed Where USP18 Went and How Long It Lasted
The short isoform did not merely resemble a trimmed version of the long protein. Its distinct N-terminus tracked with marked differences in protein stability and subcellular distribution. In practical terms, the shorter protein appeared better suited to nuclear residence, aligning with the observation that endogenous short USP18 accumulates in the nucleus during muscle cell differentiation.
This matters because nuclear localization opens the door to roles beyond USP18’s better-known activity in interferon signaling. USP18 is usually discussed as an ISG15 deISGylase and as a non-catalytic brake on IFNAR signaling. Here, the focus shifts away from those canonical functions and toward a basal, non-immune role in muscle biology. The preprint supports the idea that the protein’s N terminus helps determine whether USP18 participates in immune signaling, protein homeostasis, or developmental transcriptional control.
The stability differences also carry biological weight. A protein that persists longer, or is handled differently by the cell’s quality-control machinery, can exert effects at the precise moment a myogenic program is being launched. Early differentiation is a narrow window, and proteoform-specific kinetics can decide which regulatory interactions occur before the cell commits to a muscle fate.
The Short Isoform Built a Larger Nuclear Interaction Network
Proteomics expanded the story further. The short isoform displayed a substantially broader interactome than the long isoform, with increased association with nuclear proteins, chaperones, and regulators of protein homeostasis. That pattern suggests the truncated proteoform is embedded in a different cellular neighborhood, one that is compatible with chromatin-linked regulation, protein folding oversight, and co-transcriptional or post-transcriptional control.
The enrichment for nuclear partners is especially interesting in the setting of muscle differentiation. Myogenesis demands synchronized changes in gene expression, RNA processing, and protein assembly. A proteoform that associates more heavily with nuclear machinery could influence all three. Chaperone and protein-homeostasis partners also hint that USP18 may be acting as a scaffold or modifier within a proteostasis network rather than only as a ubiquitin-like modifier enzyme.
One sentence captures the central biological shift: the shorter USP18 proteoform behaves like a different molecule, not just a shorter version of the same one.
Early Myogenic Differentiation Improved With the Short Isoform
Functionally, the short isoform enhanced an early stage of myogenic differentiation. The report does not present this as a broad, finished remodeling of muscle cells; instead, the effect appears early, at a stage when lineage commitment and preparatory transcriptional events are still taking shape. That timing aligns well with the protein’s nuclear enrichment and altered interaction profile.
Early differentiation is often where small regulatory changes have the biggest consequences. A protein that changes local RNA handling, stabilizes a set of regulatory partners, or shifts the balance of nuclear quality control can tilt the trajectory of a cell before downstream structural genes are fully activated. The preprint suggests that USP18 short isoform may sit inside that decision-making layer.
Alternative Splicing Linked the Isoform to Sarcomere Gene Programs
RNA sequencing added another layer: the short isoform was associated with changes in alternative splicing, including in key sarcomere genes. This is a notable finding because sarcomere assembly depends on precise isoform usage and coordinated expression of contractile components. If a proteoform alters splicing decisions in that space, the downstream effect could extend beyond differentiation markers to the architecture of the muscle fiber itself.
The link between a nuclear proteoform and splicing changes makes biological sense. Nuclear proteins that interact with RNA-processing machinery can influence exon inclusion, transcript maturation, and the production of protein variants required during muscle development. In this case, the data suggest that N-terminal truncation may convert USP18 from a protein best known for interferon control into one that helps shape the transcriptome of differentiating muscle cells.
Why the N Terminus Matters for USP18 Biology
The broader implication is straightforward: the N-terminus is a determinant of non-canonical USP18 function and proteoform diversity. That conclusion matters because it reframes how the protein should be studied in tissues outside the interferon response. If the same gene produces proteoforms with different localization, stability, and interaction partners, then lumping them together risks missing tissue-specific biology.
For muscle research, the work points to a new regulatory node during early myogenesis. For USP18 biology more generally, it raises the possibility that truncation events generate functionally specialized proteins with distinct cellular roles. The findings also sit comfortably with the data availability resources released alongside the paper: mass spectrometry data are deposited in PRIDE, and RNA sequencing data are available in EGA, giving other groups a route to re-examine the proteoform-linked networks and splicing effects.
The study remains a preprint, so the claims have not yet been certified by peer review. Even so, the central message is clear from the data presented: shortening USP18 changes the protein’s behavior enough to produce a distinct nuclear, differentiation-associated signature in muscle cells. What still needs to be established is how this proteoform behaves in vivo, whether the same patterns appear in human muscle development, and which direct molecular interactions drive the observed splicing changes in sarcomere genes.