For decades, the textbook view of glycosylation has been clear: glycans are added to proteins in the secretory pathway and end up on the cell surface or in the extracellular space. The one known exception was O-GlcNAc, a small single-sugar modification found on cytoplasmic and nuclear proteins and well-studied in its own right. Beyond that, the nucleus was considered glycan-free territory. We thought this was worth challenging.
Our new preprint, led by Jon Lundstrøm from the lab, and including Ujala Bashir (plus excellent collaborators at the University of Gothenburg and the University of Alberta), presents comprehensive evidence that extended O-GalNAc glycans (the kind you would normally only expect to find on secreted or membrane proteins) are in fact common on proteins inside mammalian nuclei. The paper is available on bioRxiv.
What we found
Using a combination of glycosylation-enriched proteomics, metabolic labeling, knock-out cell lines, and imaging, we show that extended O-glycans are present on intranuclear proteins across multiple mammalian cell lines and primary cells. This is not a cell-line artifact or a contamination issue, as we went to considerable lengths to rule those out. The glycans are genuine, site-specific, and functionally relevant.
One of the more satisfying parts of the work was figuring out where these glycans actually come from. The secretory pathway makes them, but how do they get into the nucleus? Using cells knocked out for key vesicular transport components, we show that nuclear glycans depend on active vesicular transport. So the glycans are made in the ER/Golgi as normal, and then a subset of glycoproteins gets shuttled into the nucleus via vesicles rather than being routed to the surface. This is an unconventional trafficking route, but there is precedent for vesicle-mediated nuclear entry in other contexts.
Who carries these glycans
Several of the intranuclear glycoproteins we identified are RNA-binding proteins, including KHSRP/FUBP2, RBM12, and RPP30. That last one, RPP30, is a component of RNase P, the ribonucleoprotein complex responsible for tRNA processing. We looked at this more carefully and found that RPP30 carries extended O-GalNAc glycans specifically at serine 55. When we mutate that site (S55A), RPP30 glycosylation drops substantially, and the protein’s ability to bind tRNAs is significantly reduced. The downstream consequence is a measurable decrease in global protein synthesis, which we detected using the SUnSET assay. So the glycosylation here is not just decorative, it is part of how RPP30 works.
Why this matters
Glycobiology has spent a lot of energy on surface glycans and secreted glycoproteins, for good reasons: they are accessible, they mediate cell-cell interactions, and they are directly druggable in many disease contexts. But if extended glycans are also operating inside the nucleus on RNA-binding proteins and other nuclear factors, that opens up a whole new layer of post-translational regulation that has been essentially invisible until now.
There are obvious implications for disease. Many nuclear RNA-binding proteins are dysregulated in cancer, and glycosylation already influences protein function in multiple ways. Whether nuclear glycosylation is altered in tumor cells, and whether that contributes to the gene expression and translation changes that drive cancer, is something we are actively pursuing. Our ERC-funded SweetSwap project is built around exactly these questions.
More broadly, this finding suggests the scope of glycobiology as a field is larger than we thought. Any nuclear protein that passes through or contacts vesicles in transit to the nucleus is a candidate for carrying glycans. The number of proteins this might apply to, and what it means for their function, is an open question that deserves a lot more attention.
What comes next
We are continuing to map the nuclear glycoproteome more comprehensively, characterizing additional functional targets beyond RPP30, and developing tools to specifically perturb nuclear glycosylation in living cells. If you work on RNA biology, nuclear transport, or translation regulation and think there might be overlap with our findings, we would genuinely love to hear from you.
Preprint: Lundstrøm et al. (2026). Extended nuclear glycosylation regulates RNA processing. bioRxiv.
Engineering Life 