A new pre-print from Caroline Bertozzi’s lab shows that some RNA molecules are glycosylated. At least some of these glyco-RNA molecules might reside inside the ER lumen.
There are four major types of bio-molecules: proteins, nucleic acids, lipids and sugars. Apart from energy source, sugars or glycans are also attached to proteins and lipids, and these modifications are important for their function, related mostly to membrane surface, or secretion.
Using metabolic labeling of human cells with a glycan precursor, Ryan Flynn, a postdoc at Caroline Bertozzi’s lab, found that RNA is also labeled by this precursor.
This is really exciting since RNA was never shown to be glycosylated before, so this opens a whole new level of regulation on RNA. In their pre-print they proved, convincingly I think, that this is a bona-fide glycosylation on RNA molecules. That it occurs only on guanine residues, and that at least some of the enzymes required for protein glycosylation are involved. They further show that the bulk of glycosylated RNAs are Y RNAs and small nucleolar RNAs, in particular Y5 RNA and U3 snoRNA. Finally, they perform some crud cell fractionation with a biochemical assay to show that the glycoRNA is found in the membrane fraction, and at least some of it is in the lumen of membrane organelles, most likely the ER, but they don’t prove that. This will require more in depth fractionation to see the golgi, endosomes, lysosomes etc…
What does the glycoRNA do on the surface or in the lumen of these organelles? Which of the glycoRNA species is found where? Is this related to the fact that Y RNAs are found in exosomes or secreted as free RNP? Is the glycoRNA actually found in biofluids?
Flynn et al show that in a CRISPR’ed human cell line that is knocked-out of Y5 RNA there is reduction in total glycoRNA – indicating that this RNA is one of the major glycoRNA molecules. The cells grow fine so its not essential. But I think a better question will be what will happen if you mutate the glycosylated G’s (but make compensatory mutation to keep the structure). Will this affect the known function of the Y RNA as an RNP regulator or in DNA replication? Will it reveal defects in secretion? in ER function? One can then pull-down the Y RNA and find the associated proteins, and then compare that to the unglycosylated mutant. This will probably help in answering questions about the function.
The authors suggests that maybe the glycosyl moiety helps the RNA to associate with membranes, maybe even go through them. I have always wondered how RNA in exosomes gets out after the exosome is engulfed by acceptor cells, and typically goes through the endo-lysosomal pathway. Maybe that’s how the RNA leaves the exosome to do whatever it is doing in acceptor cells. I wonder if adding a GFP-mimic RNA aptamer like Mango could help in determining the localization of the Y RNA (or an unglycosylated mutant).
Overall i think that this is a ground breaking discovery and I’m sure that we will find more glycoRNAs in different cell types, in different organisms, with a variety of functions.
And then maybe we will find phosphorylated RNA, ubiquitynated RNA, and who knows what else…
Mammalian Y RNAs are modified at discrete guanosine residues with N-glycans
bioRxiv
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