Protein glycosylation is usually relegated to the cell surface and intracellular compartments. In a fascinating exception to this rule that was first observed in the 1980's, A GlcNAc monosaccharide can be added to serine and threonine residues of cytosolic proteins. Many labs are trying to understand the dynamic regulation of the addition and removal of this sugar that seemingly has a hand in every cellular process and disease state known to man. More and more examples are being found to suggest that this modification and phosphorylation regulate each other, as if they weren't already complicated enough on their own.
There are a handful of ways to detect O-GlcNAc, which have helped build the laundry list by telling us which proteins are modified. Now, in a recent Nature Chemical Biology paper from Linda Hsieh-Wilson's lab at CalTech, they show us a useful new method that reveals what proportion of any particular protein is modified (2%? 80%), and of those that are modified, exactly how many GlcNAc residues there are per protein. They do this by adding a great big heavy tag (polyethylene glycol) to the modification, which makes the protein sluggish on a polyacrylamide gel. Proteins that have been modified then resolve as separate slower-moving bands on the gel according to how many GlcNAc residues adorn them. I made the illustration above for a highlight of the paper that will appear in the Functional Glycomics Update, a collaboration between the Consortium for Functional Glycomics and the Nature Publishing Group. The image will be much smaller than you see here, which is why I took kind of a loose style for the protein, playing around with the paint brush tools in Illustrator. I haven't actually decided whether I like it or not, but this is where I am with it.